robot/include/boost/process/v2/environment.hpp

//
// process/environment.hpp
// ~~~~~~~~~~~~~~~~~~~~~~~~
//
// Copyright (c) 2021 Klemens D. Morgenstern (klemens dot morgenstern at gmx dot net)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

#ifndef BOOST_PROCESS_V2_ENVIRONMENT_HPP
#define BOOST_PROCESS_V2_ENVIRONMENT_HPP

#include <boost/process/v2/detail/config.hpp>
#include <boost/process/v2/cstring_ref.hpp>
#include <boost/process/v2/detail/utf8.hpp>

#include <boost/type_traits.hpp>

#include <functional>
#include <memory>
#include <numeric>
#include <vector>

#if !defined(GENERATING_DOCUMENTATION)
#if defined(BOOST_PROCESS_V2_WINDOWS)
#include <boost/process/v2/detail/environment_win.hpp>
#else
#include <boost/process/v2/detail/environment_posix.hpp>
#endif
#endif

BOOST_PROCESS_V2_BEGIN_NAMESPACE

/// Namespace for information and functions regarding the calling process.
namespace environment
{

#if defined(GENERATING_DOCUMENTATION)

/// A char traits type that reflects the OS rules for string representing environment keys.
/** Can be an alias of std::char_traits. May only be defined for `char` and `wchar_t`.
 * 
 * Windows treats keys as case-insensitive yet perserving. The char traits are made to reflect 
 * that behaviour.
*/
tempalte<typename Char>
using key_char_traits = implementation_defined ;

/// A char traits type that reflects the OS rules for string representing environment values.
/** Can be an alias of std::char_traits. May only be defined for `char` and `wchar_t`.
*/
tempalte<typename Char>
using value_char_traits = implementation_defined ;

/// The character type used by the environment. Either `char` or `wchar_t`.
using char_type = implementation_defined ;

/// The equal character in an environment string used to separate key and value.
constexpr char_type equality_sign = implementation_defined;

/// The delimiter in environemtn lists. Commonly used by the `PATH` variable.
constexpr char_type delimiter = implementation_defined;

/// The native handle of an environment. Note that this can be an owning pointer and is generally not thread safe.
using native_handle = implementation_defined;


#endif


/// The iterator used by a value or value_view to iterator through environments that are lists.
struct value_iterator
{
    using string_view_type  = basic_string_view<char_type, value_char_traits<char_type>>;
    using difference_type   = int;
    using reference         = string_view_type;
    using iterator_category = std::forward_iterator_tag;

    value_iterator & operator++()
    {
        const auto delim = view_.find(delimiter);
        if (delim != string_view_type::npos)
          view_ = view_.substr(delim + 1);
        else
          view_ = view_.substr(view_.size());
        return *this;
    }

    value_iterator operator++(int)
    {
        auto last = *this;
        ++(*this);
        return last;
    }
    string_view_type operator*() const
    {
      const auto delim = view_.find(delimiter);
      if (delim == string_view_type::npos)
        return view_;
      else
        return view_.substr(0, delim);
    }

    value_iterator() = default;
    value_iterator(const value_iterator & ) = default;
    value_iterator(string_view_type view, std::size_t offset = 0u) : view_(view.substr(offset))
    {
    }

    friend bool operator==(const value_iterator & l, const value_iterator & r) { return l.view_ == r.view_; }
    friend bool operator!=(const value_iterator & l, const value_iterator & r) { return l.view_ != r.view_; }

  private:
    string_view_type view_;
};

/// A view type for a key of an environment
struct key_view
{
    using value_type       = char_type;
    using traits_type      = key_char_traits<char_type>;
    using string_view_type = basic_string_view<char_type, traits_type>;
    using string_type      = std::basic_string<char_type, key_char_traits<char_type>>;

    key_view() noexcept = default;
    key_view( const key_view& p ) = default;
    key_view( key_view&& p ) noexcept = default;
    template<typename Source, 
             typename = typename std::enable_if<is_constructible<string_view_type, Source>::value>::type>
    key_view( const Source& source ) : value_(source) {}
    key_view( const char_type * p) : value_(p) {}
    key_view(       char_type * p) : value_(p) {}

    ~key_view() = default;

    key_view& operator=( const key_view& p ) = default;
    key_view& operator=( key_view&& p ) noexcept = default;
    key_view& operator=( string_view_type source )
    {
        value_ = source;
        return *this;
    }

    void swap( key_view& other ) noexcept
    {
        std::swap(value_, other.value_);
    }

    string_view_type native() const noexcept {return value_;}

    operator string_view_type() const {return native();}

    int compare( const key_view& p ) const noexcept {return value_.compare(p.value_);}
    int compare( string_view_type str ) const {return value_.compare(str);}
    int compare( const value_type* s ) const {return value_.compare(s);}

    template< class CharT, class Traits = std::char_traits<CharT>,
            class Alloc = std::allocator<CharT> >
    std::basic_string<CharT,Traits,Alloc>
    basic_string( const Alloc& alloc = Alloc()) const
    {
        return BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<CharT, Traits>(
            value_.data(), value_.size(), alloc);
    }

    std::string       string() const {return basic_string<char>();}
    std::wstring     wstring() const {return basic_string<wchar_t>();}

    string_type native_string() const
    {
        return basic_string<char_type, key_char_traits<char_type>>();
    }

    friend bool operator==(key_view l, key_view r) { return l.value_ == r.value_; }
    friend bool operator!=(key_view l, key_view r) { return l.value_ != r.value_; }
    friend bool operator<=(key_view l, key_view r) { return l.value_ <= r.value_; }
    friend bool operator>=(key_view l, key_view r) { return l.value_ >= r.value_; }
    friend bool operator< (key_view l, key_view r) { return l.value_ <  r.value_; }
    friend bool operator> (key_view l, key_view r) { return l.value_ >  r.value_; }

    bool empty() const {return value_.empty(); }

    template< class CharT, class Traits >
    friend std::basic_ostream<CharT,Traits>&
    operator<<( std::basic_ostream<CharT,Traits>& os, const key_view& p )
    {
        os << BOOST_PROCESS_V2_NAMESPACE::quoted(p.basic_string<CharT,Traits>());
        return os;
    }

    template< class CharT, class Traits >
    friend std::basic_istream<CharT,Traits>&
    operator>>( std::basic_istream<CharT,Traits>& is, key_view& p )
    {
        std::basic_string<CharT, Traits> t;
        is >> BOOST_PROCESS_V2_NAMESPACE::quoted(t);
        p = t;
        return is;
    }
    const value_type * data() const {return value_.data(); }
    std::size_t size() const {return value_.size(); }
  private:
    string_view_type value_;
};

/// A view for a value in an environment
struct value_view
{
    using value_type       = char_type;
    using string_view_type = basic_cstring_ref<char_type, value_char_traits<char_type>>;
    using string_type      = std::basic_string<char_type, value_char_traits<char_type>>;
    using traits_type      = value_char_traits<char_type>;

    value_view() noexcept = default;
    value_view( const value_view& p ) = default;
    value_view( value_view&& p ) noexcept = default;
    template<typename Source, typename = typename std::enable_if<is_constructible<string_view_type, Source>::value>::type>
    value_view( const Source& source ) : value_(source) {}
    value_view( const char_type * p) : value_(p) {}
    value_view(       char_type * p) : value_(p) {}

    ~value_view() = default;

    value_view& operator=( const value_view& p ) = default;
    value_view& operator=( value_view&& p ) noexcept = default;
    value_view& operator=( string_view_type source )
    {
        value_ = source;
        return *this;
    }

    void swap( value_view& other ) noexcept
    {
        std::swap(value_, other.value_);
    }

    string_view_type native() const noexcept {return value_;}

    operator string_view_type() const {return native();}
    operator typename string_view_type::string_view_type() const {return value_; }

    int compare( const value_view& p ) const noexcept {return value_.compare(p.value_);}
    int compare( string_view_type str ) const {return value_.compare(str);}
    int compare( const value_type* s ) const {return value_.compare(s);}

    template< class CharT, class Traits = std::char_traits<CharT>,
            class Alloc = std::allocator<CharT> >
    std::basic_string<CharT,Traits,Alloc>
    basic_string( const Alloc& alloc = Alloc() ) const
    {
        return BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<CharT, Traits>(
                value_.data(), value_.size(),  alloc);
    }

    std::string string() const       {return basic_string<char>();}
    std::wstring wstring() const     {return basic_string<wchar_t>();}

    string_type native_string() const
    {
        return basic_string<char_type, value_char_traits<char_type>>();
    }

    bool empty() const {return value_.empty(); }

    friend bool operator==(value_view l, value_view r) { return l.value_ == r.value_; }
    friend bool operator!=(value_view l, value_view r) { return l.value_ != r.value_; }
    friend bool operator<=(value_view l, value_view r) { return l.value_ <= r.value_; }
    friend bool operator>=(value_view l, value_view r) { return l.value_ >= r.value_; }
    friend bool operator< (value_view l, value_view r) { return l.value_ <  r.value_; }
    friend bool operator> (value_view l, value_view r) { return l.value_ >  r.value_; }


    template< class CharT, class Traits >
    friend std::basic_ostream<CharT,Traits>&
    operator<<( std::basic_ostream<CharT,Traits>& os, const value_view& p )
    {
        os << BOOST_PROCESS_V2_NAMESPACE::quoted(p.basic_string<CharT,Traits>());
        return os;
    }

    template< class CharT, class Traits >
    friend std::basic_istream<CharT,Traits>&
    operator>>( std::basic_istream<CharT,Traits>& is, value_view& p )
    {
        std::basic_string<CharT, Traits> t;
        is >> BOOST_PROCESS_V2_NAMESPACE::quoted(t);
        p = t;
        return is;
    }
    value_iterator begin() const {return value_iterator(value_.data());}
    value_iterator   end() const {return value_iterator(value_.data() , value_.size());}

    const char_type * c_str() {return value_.c_str(); }
    const value_type * data() const {return value_.data(); }
    std::size_t size() const {return value_.size(); }

  private:
    string_view_type value_;
};

/// A view for a key value pair in an environment
struct key_value_pair_view
{
  using value_type       = char_type;
  using string_type      = std::basic_string<char_type>;
  using string_view_type = basic_cstring_ref<char_type>;
  using traits_type      = std::char_traits<char_type>;

  key_value_pair_view() noexcept = default;
  key_value_pair_view( const key_value_pair_view& p ) = default;
  key_value_pair_view( key_value_pair_view&& p ) noexcept = default;
  template<typename Source,
           typename = typename std::enable_if<is_constructible<string_view_type, Source>::value>::type>
  key_value_pair_view( const Source& source ) : value_(source) {}

  key_value_pair_view( const char_type * p) : value_(p) {}
  key_value_pair_view(       char_type * p) : value_(p) {}


  ~key_value_pair_view() = default;

  key_value_pair_view& operator=( const key_value_pair_view& p ) = default;
  key_value_pair_view& operator=( key_value_pair_view&& p ) noexcept = default;

  void swap( key_value_pair_view& other ) noexcept
  {
      std::swap(value_, other.value_);
  }

  string_view_type native() const noexcept {return value_;}

  operator string_view_type() const {return native();}
  operator typename string_view_type::string_view_type() const {return value_; }

  int compare( key_value_pair_view p ) const noexcept 
  {
      const auto c = key().compare(p.key());
      if (c != 0)
            return c;
      return value().compare(p.value());
  }
  int compare( const string_type& str ) const
  {
      return compare(key_value_pair_view(str));
  }
  int compare( string_view_type str ) const 
  {
      string_type st(str.data(), str.size());
      return compare(st);
  }
  int compare( const value_type* s ) const
  {
      return compare(key_value_pair_view(s));
  }

  template< class CharT, class Traits = std::char_traits<CharT>, class Alloc = std::allocator<CharT> >
  std::basic_string<CharT,Traits,Alloc>
  basic_string( const Alloc& alloc = Alloc()) const
  {
      return BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<CharT, Traits>(value_.begin(), value_.size(), alloc);
  }

  std::string string() const       {return basic_string<char>();}
  std::wstring wstring() const     {return basic_string<wchar_t>();}

  string_type native_string() const
  {
    return basic_string<char_type>();
  }

  bool empty() const {return value_.empty(); }

  key_view key() const
  {
      auto eq = value_.find(equality_sign);
      if (eq == 0)
      {
          auto eq2 = value_.find(equality_sign, 1);
          if (eq2 != string_type::npos)
              eq = eq2;
      }
      const auto res = native().substr(0,  eq == string_view_type::npos ? value_.size() : eq);
      return key_view::string_view_type(res.data(), res.size());
  }
  value_view value() const
  {
      auto eq = value_.find(equality_sign);
      if (eq == 0)
      {
          auto eq2 = value_.find(equality_sign, 1);
          if (eq2 != string_type::npos)
              eq = eq2;
      }
      return environment::value_view(native().substr(eq + 1));
  }

  friend bool operator==(key_value_pair_view l, key_value_pair_view r) { return l.compare(r) == 0; }
  friend bool operator!=(key_value_pair_view l, key_value_pair_view r) { return l.compare(r) != 0; }
  friend bool operator<=(key_value_pair_view l, key_value_pair_view r) { return l.compare(r) <= 0; }
  friend bool operator>=(key_value_pair_view l, key_value_pair_view r) { return l.compare(r) >= 0; }
  friend bool operator< (key_value_pair_view l, key_value_pair_view r) { return l.compare(r) <  0; }
  friend bool operator> (key_value_pair_view l, key_value_pair_view r) { return l.compare(r) >  0; }


  template< class CharT, class Traits >
  friend std::basic_ostream<CharT,Traits>&
  operator<<( std::basic_ostream<CharT,Traits>& os, const key_value_pair_view& p )
  {
      os << BOOST_PROCESS_V2_NAMESPACE::quoted(p.basic_string<CharT,Traits>());
      return os;
  }

  template< class CharT, class Traits >
  friend std::basic_istream<CharT,Traits>&
  operator>>( std::basic_istream<CharT,Traits>& is, key_value_pair_view& p )
  {
      std::basic_string<CharT, Traits> t;
      is >> BOOST_PROCESS_V2_NAMESPACE::quoted(t);
      p = t;
      return is;
  }

  template<std::size_t Idx>
  inline auto get() const -> typename conditional<Idx == 0u, BOOST_PROCESS_V2_NAMESPACE::environment::key_view,
                                                             BOOST_PROCESS_V2_NAMESPACE::environment::value_view>::type;
  const value_type * c_str() const noexcept
  {
    return value_.data();
  }
  const value_type * data() const {return value_.data(); }
  std::size_t size() const {return value_.size(); }

 private:

  string_view_type value_;
};

template<>
inline key_view key_value_pair_view::get<0u>() const
{
    return key();
}

template<>
inline value_view key_value_pair_view::get<1u>() const
{
    return value();
}

namespace detail
{

template<typename Char>
std::size_t hash_step(std::size_t prev, Char c, std::char_traits<Char>)
{
    return prev ^ (c << 1);
}

}

inline std::size_t hash_value(const key_view & value)
{
    std::size_t hash = 0u;
    for (auto c = value.data(); *c != *v2::detail::null_char_(*c); c++)
        hash = detail::hash_step(hash, *c, key_view::traits_type{});
    return hash ;
}


inline std::size_t hash_value(const BOOST_PROCESS_V2_NAMESPACE::environment::value_view & value)
{
    std::size_t hash = 0u;
    for (auto c = value.data(); *c != *v2::detail::null_char_(*c); c++)
        hash = detail::hash_step(hash, *c, value_view::traits_type{});
    return hash ;
}


inline std::size_t hash_value(const BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair_view & value)
{
    std::size_t hash = 0u;
    for (auto c = value.data(); *c != *v2::detail::null_char_(*c); c++)
        hash = detail::hash_step(hash, *c, key_value_pair_view::traits_type{});
    return hash ;
}

/// A class representing a key within an environment.
struct key
{
    using value_type       = char_type;
    using traits_type      = key_char_traits<char_type>;
    using string_type      = std::basic_string<char_type, traits_type>;
    using string_view_type = basic_string_view<char_type, traits_type>;

    key() {}
    key( const key& p ) = default;
    key( key&& p ) noexcept = default;
    key( const string_type& source ) : value_(source) {}
    key( string_type&& source ) : value_(std::move(source)) {}
    key( const value_type * raw ) : value_(raw) {}
    key(       value_type * raw ) : value_(raw) {}

    explicit key(key_view kv) : value_(kv.native_string()) {}


    template< class Source >
    key( const Source& source,
        decltype(std::declval<Source>().data()) = nullptr,
        decltype(std::declval<Source>().size()) = 0u)
        : value_(
             BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<char_type, traits_type>(
                source.data(), source.size()))
    {
    }

    key(const typename conditional<is_same<value_type, char>::value, wchar_t, char>::type  * raw)
        : value_(BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<char_type, traits_type>(
                raw, std::char_traits<std::decay<std::remove_pointer<decltype(raw)>::type>::type>::length(raw)))
    {
    }

    template< class InputIt >
    key( InputIt first, InputIt last)
        : key(std::basic_string<typename std::iterator_traits<typename std::decay<InputIt>::type>::value_type>(first, last))
    {
    }

    ~key() = default;

    key& operator=( const key& p ) = default;
    key& operator=( key&& p )
    {
      value_ = std::move(p.value_);
      return *this;
    }
    key& operator=( string_type&& source )
    {
        value_ = std::move(source);
        return *this;
    }
    template< class Source >
    key& operator=( const Source& source )
    {
        value_ = BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<char_type, traits_type>(source.data(), source.size());
        return *this;
    }

    key& assign( string_type&& source )
    {
        value_ = std::move(source);
        return *this;
    }
    template< class Source >
    key& assign( const Source& source )
    {
        value_ = BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<char_type, traits_type>(source.data(), source.size());
        return *this;
    }

    template< class InputIt >
    key& assign( InputIt first, InputIt last )
    {

        return assign(std::basic_string<typename std::iterator_traits<typename std::decay<InputIt>::type>::value_type>(first, last));
    }

    void clear() {value_.clear();}

    void swap( key& other ) noexcept
    {
        std::swap(value_, other.value_);
    }

    const value_type* c_str() const noexcept {return value_.c_str();}
    const string_type& native() const noexcept {return value_;}
    string_view_type native_view() const noexcept {return value_;}

    operator string_type() const {return native();}
    operator string_view_type() const {return native_view();}

    int compare( const key& p ) const noexcept {return value_.compare(p.value_);}
    int compare( const string_type& str ) const {return value_.compare(str);}
    int compare( string_view_type str ) const {return -str.compare(value_);}
    int compare( const value_type* s ) const {return value_.compare(s);}

    template< class CharT, class Traits = std::char_traits<CharT>,
        class Alloc = std::allocator<CharT> >
    std::basic_string<CharT,Traits,Alloc>
    basic_string( const Alloc& alloc = Alloc()) const
    {
        return BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<CharT, Traits>(
            value_.data(), value_.size(), alloc);
    }


    std::string string() const       {return basic_string<char>();}
    std::wstring wstring() const     {return basic_string<wchar_t>();}

    const string_type & native_string() const
    {
        return value_;
    }

    bool empty() const {return value_.empty(); }

    friend bool operator==(const key & l, const key & r) { return l.value_ == r.value_; }
    friend bool operator!=(const key & l, const key & r) { return l.value_ != r.value_; }
    friend bool operator<=(const key & l, const key & r) { return l.value_ <= r.value_; }
    friend bool operator>=(const key & l, const key & r) { return l.value_ >= r.value_; }
    friend bool operator< (const key & l, const key & r) { return l.value_ <  r.value_; }
    friend bool operator> (const key & l, const key & r) { return l.value_ >  r.value_; }

    template< class CharT, class Traits >
    friend std::basic_ostream<CharT,Traits>&
    operator<<( std::basic_ostream<CharT,Traits>& os, const key& p )
    {
        os << BOOST_PROCESS_V2_NAMESPACE::quoted(p.basic_string<CharT,Traits>());
        return os;
    }

    template< class CharT, class Traits >
    friend std::basic_istream<CharT,Traits>&
    operator>>( std::basic_istream<CharT,Traits>& is, key& p )
    {
        std::basic_string<CharT, Traits> t;
        is >> BOOST_PROCESS_V2_NAMESPACE::quoted(t);
        p = t;
        return is;
    }
    const value_type * data() const {return value_.data(); }
    std::size_t size() const {return value_.size(); }

  private:
    string_type value_;
};

#if !defined(GENERATING_DOCUMENTATION)

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, key>::value || std::is_same<T, key_view>::value) &&
         std::is_convertible<const U &, key_view>::value)
        ||
        ((std::is_same<U, key>::value || std::is_same<U, key_view>::value) &&
         std::is_convertible<const T & , key_view>::value),
        bool>::type
operator==(const T  &l, const U & r) { return key_view(l) == key_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, key>::value || std::is_same<T, key_view>::value) &&
         std::is_convertible<const U &, key_view>::value)
        ||
        ((std::is_same<U, key>::value || std::is_same<U, key_view>::value) &&
         std::is_convertible<const T & , key_view>::value),
        bool>::type
operator!=(const T  &l, const U & r) { return key_view(l) != key_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, key>::value || std::is_same<T, key_view>::value) &&
         std::is_convertible<const U &, key_view>::value)
        ||
        ((std::is_same<U, key>::value || std::is_same<U, key_view>::value) &&
         std::is_convertible<const T & , key_view>::value),
        bool>::type
operator<=(const T  &l, const U & r) { return key_view(l) <= key_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, key>::value || std::is_same<T, key_view>::value) &&
         std::is_convertible<const U &, key_view>::value)
        ||
        ((std::is_same<U, key>::value || std::is_same<U, key_view>::value) &&
         std::is_convertible<const T & , key_view>::value),
        bool>::type
operator <(const T  &l, const U & r) { return key_view(l)  < key_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, key>::value || std::is_same<T, key_view>::value) &&
         std::is_convertible<const U &, key_view>::value)
        ||
        ((std::is_same<U, key>::value || std::is_same<U, key_view>::value) &&
         std::is_convertible<const T & , key_view>::value),
        bool>::type
operator>=(const T  &l, const U & r) { return key_view(l) >= key_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, key>::value || std::is_same<T, key_view>::value) &&
         std::is_convertible<const U &, key_view>::value)
        ||
        ((std::is_same<U, key>::value || std::is_same<U, key_view>::value) &&
         std::is_convertible<const T & , key_view>::value),
        bool>::type
operator >(const T  &l, const U & r) { return key_view(l) >  key_view(r); }

#else


bool operator==(const value_view &, const value_view);
bool operator!=(const value_view &, const value_view);
bool operator<=(const value_view &, const value_view);
bool operator< (const value_view &, const value_view);
bool operator> (const value_view &, const value_view);
bool operator>=(const value_view &, const value_view);

#endif


struct value
{
    using value_type       = char_type;
    using traits_type      = value_char_traits<char_type>;
    using string_type      = std::basic_string<char_type, traits_type>;
    using string_view_type = basic_cstring_ref<char_type, traits_type>;

    value() {}
    value( const value& p ) = default;

    value( const string_type& source ) : value_(source) {}
    value( string_type&& source ) : value_(std::move(source)) {}
    value( const value_type * raw ) : value_(raw) {}
    value(       value_type * raw ) : value_(raw) {}

    explicit value(value_view kv) : value_(kv.c_str()) {}

    template< class Source >
    value( const Source& source,
           decltype(std::declval<Source>().data()) = nullptr,
    decltype(std::declval<Source>().size()) = 0u)
    : value_(BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<char_type, traits_type>(
        source.data(), source.size()))
    {
    }

    value(const typename conditional<is_same<value_type, char>::value, wchar_t, char>::type  * raw)
            : value_(BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<char_type, traits_type>(
            raw, std::char_traits<std::decay<std::remove_pointer<decltype(raw)>::type>::type>::length(raw)))
    {
    }

    template< class InputIt >
    value( InputIt first, InputIt last)
            : value(std::basic_string<typename std::iterator_traits<typename std::decay<InputIt>::type>::value_type>(first, last))
    {
    }

    ~value() = default;

    value& operator=( const value& p ) = default;
    value& operator=( value&& p )
    {
      value_ = std::move(p.value_);
      return *this;
    }
    value& operator=( string_type&& source )
    {
        value_ = std::move(source);
        return *this;
    }
    template< class Source >
    value& operator=( const Source& source )
    {
        value_ = BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<char_type, traits_type>(
            source.data(), source.size);
        return *this;
    }

    value& assign( string_type&& source )
    {
        value_ = std::move(source);
        return *this;
    }
    template< class Source >
    value& assign( const Source& source )
    {
      value_ = BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<char_type, traits_type>(
          source.data(), source.size());
      return *this;
    }

    template< class InputIt >
    value& assign( InputIt first, InputIt last )
    {
        return assign(std::basic_string<typename std::iterator_traits<typename std::decay<InputIt>::type>::value_type>(first, last));
    }

    void push_back(const value & sv)
    {
        (value_ += delimiter) += sv;
    }

    void clear() {value_.clear();}

    void swap( value& other ) noexcept
    {
        std::swap(value_, other.value_);
    }

    const value_type* c_str() const noexcept {return value_.c_str();}
    const string_type& native() const noexcept {return value_;}
    string_view_type native_view() const noexcept {return value_;}

    operator string_type() const {return native();}
    operator string_view_type() const {return native_view();}
    operator typename string_view_type::string_view_type() const {return value_; }

    int compare( const value& p ) const noexcept {return value_.compare(p.value_);}
    int compare( const string_type& str ) const {return value_.compare(str);}
    int compare( string_view_type str ) const {return -str.compare(value_);}
    int compare( const value_type* s ) const {return value_.compare(s);}

    template< class CharT, class Traits = std::char_traits<CharT>,
            class Alloc = std::allocator<CharT> >
    std::basic_string<CharT,Traits,Alloc>
    basic_string( const Alloc& alloc = Alloc()) const
    {
        return BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<CharT, Traits>(
            value_.data(), value_.size(),alloc);
    }

    std::string string() const       {return basic_string<char>();}
    std::wstring wstring() const     {return basic_string<wchar_t>();}


    const string_type & native_string() const
    {
        return value_;
    }

    bool empty() const {return value_.empty(); }

    friend bool operator==(const value & l, const value & r) { return l.value_ == r.value_; }
    friend bool operator!=(const value & l, const value & r) { return l.value_ != r.value_; }
    friend bool operator<=(const value & l, const value & r) { return l.value_ <= r.value_; }
    friend bool operator>=(const value & l, const value & r) { return l.value_ >= r.value_; }
    friend bool operator< (const value & l, const value & r) { return l.value_ <  r.value_; }
    friend bool operator> (const value & l, const value & r) { return l.value_ >  r.value_; }

    template< class CharT, class Traits >
    friend std::basic_ostream<CharT,Traits>&
    operator<<( std::basic_ostream<CharT,Traits>& os, const value& p )
    {
        os << BOOST_PROCESS_V2_NAMESPACE::quoted(p.basic_string<CharT,Traits>());
        return os;
    }

    template< class CharT, class Traits >
    friend std::basic_istream<CharT,Traits>&
    operator>>( std::basic_istream<CharT,Traits>& is, value& p )
    {
        std::basic_string<CharT, Traits> t;
        is >> BOOST_PROCESS_V2_NAMESPACE::quoted(t);
        p = t;
        return is;
    }

    value_iterator begin() const {return value_iterator(value_.data());}
    value_iterator   end() const {return value_iterator(value_.data(), value_.size());}
    const value_type * data() const {return value_.data(); }
    std::size_t size() const {return value_.size(); }

  private:
    string_type value_;
};


#if !defined(GENERATING_DOCUMENTATION)

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, value>::value || std::is_same<T, value_view>::value) &&
         std::is_convertible<const U &, value_view>::value)
        ||
        ((std::is_same<U, value>::value || std::is_same<U, value_view>::value) &&
         std::is_convertible<const T & , value_view>::value),
        bool>::type
operator==(const T  &l, const U & r) { return value_view(l) == value_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, value>::value || std::is_same<T, value_view>::value) &&
         std::is_convertible<const U &, value_view>::value)
        ||
        ((std::is_same<U, value>::value || std::is_same<U, value_view>::value) &&
         std::is_convertible<const T & , value_view>::value),
        bool>::type
operator!=(const T  &l, const U & r) { return value_view(l) != value_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, value>::value || std::is_same<T, value_view>::value) &&
         std::is_convertible<const U &, value_view>::value)
        ||
        ((std::is_same<U, value>::value || std::is_same<U, value_view>::value) &&
         std::is_convertible<const T & , value_view>::value),
        bool>::type
operator<=(const T  &l, const U & r) { return value_view(l) <= value_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, value>::value || std::is_same<T, value_view>::value) &&
         std::is_convertible<const U &, value_view>::value)
        ||
        ((std::is_same<U, value>::value || std::is_same<U, value_view>::value) &&
         std::is_convertible<const T & , value_view>::value),
        bool>::type
operator <(const T  &l, const U & r) { return value_view(l)  < value_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, value>::value || std::is_same<T, value_view>::value) &&
         std::is_convertible<const U &, value_view>::value)
        ||
        ((std::is_same<U, value>::value || std::is_same<U, value_view>::value) &&
         std::is_convertible<const T & , value_view>::value),
        bool>::type
operator>=(const T  &l, const U & r) { return value_view(l) >= value_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, value>::value || std::is_same<T, value_view>::value) &&
         std::is_convertible<const U &, value_view>::value)
        ||
        ((std::is_same<U, value>::value || std::is_same<U, value_view>::value) &&
         std::is_convertible<const T & , value_view>::value),
        bool>::type
operator >(const T  &l, const U & r) { return value_view(l) >  value_view(r); }

#else

bool operator==(const value_view &, const value_view);
bool operator!=(const value_view &, const value_view);
bool operator<=(const value_view &, const value_view);
bool operator< (const value_view &, const value_view);
bool operator> (const value_view &, const value_view);
bool operator>=(const value_view &, const value_view);

#endif

struct key_value_pair
{
    using value_type       = char_type;
    using traits_type      = std::char_traits<char_type>;
    using string_type      = std::basic_string<char_type>;
    using string_view_type = basic_cstring_ref<char_type>;

    key_value_pair() {}
    key_value_pair( const key_value_pair& p ) = default;
    key_value_pair( key_value_pair&& p ) noexcept = default;
    key_value_pair(key_view key, value_view value) : value_(key.basic_string<char_type, traits_type>() + equality_sign + 
                                                            value.basic_string<char_type, traits_type>()) {}

    key_value_pair(key_view key, std::initializer_list<basic_string_view<char_type>> values)
    {
        const auto sz = std::accumulate(values.begin(), values.end(),
                                        key.size(), [](std::size_t sz, const basic_string_view<char_type> & str) { return sz + str.size() + 1;});

        value_.reserve(sz);
        value_.append(key.data(), key.size());
        value_ += equality_sign;
        for (auto & value : values)
        {
            if (value_.back() != equality_sign)
                value_ += delimiter;
            value_.append(value.data(), value.size());
        }
    }

    key_value_pair( const string_type& source ) : value_(source) {}
    key_value_pair( string_type&& source ) : value_(std::move(source)) {}
    key_value_pair( const value_type * raw ) : value_(raw) {}
    key_value_pair(       value_type * raw ) : value_(raw) {}

    explicit key_value_pair(key_value_pair_view kv) : value_(kv.c_str()) {}

    template< class Source >
    key_value_pair( const Source& source,
           decltype(std::declval<Source>().data()) = nullptr,
           decltype(std::declval<Source>().size()) = 0u)
            : value_(BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<char_type, traits_type>(
                source.data(), source.size()))
    {
    }

    template< typename Key, 
              typename Value >
    key_value_pair(
         const std::pair<Key, Value> & kv/*,
         typename std::enable_if<std::is_constructible<struct key,   Key >::value && 
                                 std::is_constructible<struct value, Value>::value
                >::type = 0*/) : value_(((struct key)(kv.first)).basic_string<char_type, traits_type>() + equality_sign 
                                      + ((struct value)(kv.second)).basic_string<char_type, traits_type>())
    {}

    key_value_pair(const typename conditional<is_same<value_type, char>::value, wchar_t, char>::type  * raw)
            : value_(BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<char_type, traits_type>(
                     raw,
                     std::char_traits<std::decay<std::remove_pointer<decltype(raw)>::type>::type>::length(raw)))
    {
    }

    template< class InputIt , typename std::iterator_traits<InputIt>::iterator_category>
    key_value_pair( InputIt first, InputIt last )
            : key_value_pair(std::basic_string<typename std::iterator_traits<typename std::decay<InputIt>::type>::value_type>(first, last))
    {
    }

    ~key_value_pair() = default;

    key_value_pair& operator=( const key_value_pair& p ) = default;
    key_value_pair& operator=( key_value_pair&& p )
    {
      value_ = std::move(p.value_);
      return *this;
    }
    key_value_pair& operator=( string_type&& source )
    {
        value_ = std::move(source);
        return *this;
    }
    template< class Source >
    key_value_pair& operator=( const Source& source )
    {
        value_ = BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<char_type, traits_type>(
            source.data(), source.size());
        return *this;
    }

    key_value_pair& assign( string_type&& source )
    {
        value_ = std::move(source);
        return *this;
    }
    template< class Source >
    key_value_pair& assign( const Source& source )
    {
        value_ = BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<char_type, traits_type>(
            source.data(), source.size());
        return *this;
    }


    template< class InputIt >
    key_value_pair& assign( InputIt first, InputIt last )
    {
        return assign(std::basic_string<typename std::iterator_traits<typename std::decay<InputIt>::type>::value_type>(first, last));
    }

    void clear() {value_.clear();}

    void swap( key_value_pair& other ) noexcept
    {
        std::swap(value_, other.value_);
    }

    const value_type* c_str() const noexcept {return value_.c_str();}
    const string_type& native() const noexcept {return value_;}
    string_view_type native_view() const noexcept {return value_;}

    operator string_type() const {return native();}
    operator string_view_type() const {return native_view();}
    operator typename string_view_type::string_view_type() const {return native_view();}
    operator key_value_pair_view() const {return native_view();}

    int compare( const key_value_pair& p ) const noexcept 
    {
        return key_value_pair_view(*this).compare(key_value_pair_view(p));
    }
    
    int compare( const string_type& str ) const 
    {
        return key_value_pair_view(*this).compare(str);
    }
    int compare( string_view_type str ) const   
    {
        return key_value_pair_view(*this).compare(str);
    }
    int compare( const value_type* s ) const
    {
        return key_value_pair_view(*this).compare(s);
    }

    template< class CharT, class Traits = std::char_traits<CharT>, class Alloc = std::allocator<CharT> >
    std::basic_string<CharT,Traits,Alloc>
    basic_string( const Alloc& alloc = Alloc() ) const
    {
        return BOOST_PROCESS_V2_NAMESPACE::detail::conv_string<CharT, Traits>(value_.data(), value_.size(), alloc);
    }

    std::string string() const       {return basic_string<char>();}
    std::wstring wstring() const     {return basic_string<wchar_t>();}

    const string_type & native_string() const
    {
        return value_;
    }

    friend bool operator==(const key_value_pair & l, const key_value_pair & r) { return l.compare(r) == 0; }
    friend bool operator!=(const key_value_pair & l, const key_value_pair & r) { return l.compare(r) != 0; }
    friend bool operator<=(const key_value_pair & l, const key_value_pair & r) { return l.compare(r) <= 0; }
    friend bool operator>=(const key_value_pair & l, const key_value_pair & r) { return l.compare(r) >= 0; }
    friend bool operator< (const key_value_pair & l, const key_value_pair & r) { return l.compare(r) <  0; }
    friend bool operator> (const key_value_pair & l, const key_value_pair & r) { return l.compare(r) >  0; }

    bool empty() const {return value_.empty(); }

    struct key_view key() const
    {
        auto eq = value_.find(equality_sign);
        if (eq == 0)
        {
            auto eq2 = value_.find(equality_sign, 1);
            if (eq2 != string_type::npos)
              eq = eq2;
        }
        const auto k = native_view().substr(0, eq);

        return BOOST_PROCESS_V2_NAMESPACE::environment::key_view::string_view_type (k.data(), k.size());
    }
    struct value_view value() const
    {
        auto eq = value_.find(equality_sign);
        if (eq == 0)
        {
            auto eq2 = value_.find(equality_sign, 1);
            if (eq2 != string_type::npos)
               eq = eq2;
        }
        return value_view::string_view_type(native_view().substr(eq + 1));
    }

    template< class CharT, class Traits >
    friend std::basic_ostream<CharT,Traits>&
    operator<<( std::basic_ostream<CharT,Traits>& os, const key_value_pair& p )
    {
        os << BOOST_PROCESS_V2_NAMESPACE::quoted(p.basic_string<CharT,Traits>());
        return os;
    }

    template< class CharT, class Traits >
    friend std::basic_istream<CharT,Traits>&
    operator>>( std::basic_istream<CharT,Traits>& is, key_value_pair& p )
    {
        is >> BOOST_PROCESS_V2_NAMESPACE::quoted(p.value_);
        return is;
    }

    const value_type * data() const {return value_.data(); }
    std::size_t size() const {return value_.size(); }

    template<std::size_t Idx>
    inline auto get() const 
            -> typename conditional<Idx == 0u, BOOST_PROCESS_V2_NAMESPACE::environment::key_view,
                                            BOOST_PROCESS_V2_NAMESPACE::environment::value_view>::type;

private:
    string_type value_;
};

#if !defined(GENERATING_DOCUMENTATION)

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, key_value_pair>::value || std::is_same<T, key_value_pair_view>::value) &&
         std::is_convertible<const U &, key_value_pair_view>::value)
        ||
        ((std::is_same<U, key_value_pair>::value || std::is_same<U, key_value_pair_view>::value) &&
         std::is_convertible<const T & , key_value_pair_view>::value),
        bool>::type
operator==(const T  &l, const U & r) { return key_value_pair_view(l) == key_value_pair_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, key_value_pair>::value || std::is_same<T, key_value_pair_view>::value) &&
         std::is_convertible<const U &, key_value_pair_view>::value)
        ||
        ((std::is_same<U, key_value_pair>::value || std::is_same<U, key_value_pair_view>::value) &&
         std::is_convertible<const T & , key_value_pair_view>::value),
        bool>::type
operator!=(const T  &l, const U & r) { return key_value_pair_view(l) != key_value_pair_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, key_value_pair>::value || std::is_same<T, key_value_pair_view>::value) &&
         std::is_convertible<const U &, key_value_pair_view>::value)
        ||
        ((std::is_same<U, key_value_pair>::value || std::is_same<U, key_value_pair_view>::value) &&
         std::is_convertible<const T & , key_value_pair_view>::value),
        bool>::type
operator<=(const T  &l, const U & r) { return key_value_pair_view(l) <= key_value_pair_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, key_value_pair>::value || std::is_same<T, key_value_pair_view>::value) &&
         std::is_convertible<const U &, key_value_pair_view>::value)
        ||
        ((std::is_same<U, key_value_pair>::value || std::is_same<U, key_value_pair_view>::value) &&
         std::is_convertible<const T & , key_value_pair_view>::value),
        bool>::type
operator <(const T  &l, const U & r) { return key_value_pair_view(l)  < key_value_pair_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, key_value_pair>::value || std::is_same<T, key_value_pair_view>::value) &&
         std::is_convertible<const U &, key_value_pair_view>::value)
        ||
        ((std::is_same<U, key_value_pair>::value || std::is_same<U, key_value_pair_view>::value) &&
         std::is_convertible<const T & , key_value_pair_view>::value),
        bool>::type
operator>=(const T  &l, const U & r) { return key_value_pair_view(l) >= key_value_pair_view(r); }

template<typename T, typename U>
typename std::enable_if<
        ((std::is_same<T, key_value_pair>::value || std::is_same<T, key_value_pair_view>::value) &&
         std::is_convertible<const U &, key_value_pair_view>::value)
        ||
        ((std::is_same<U, key_value_pair>::value || std::is_same<U, key_value_pair_view>::value) &&
         std::is_convertible<const T & , key_value_pair_view>::value),
        bool>::type
operator >(const T  &l, const U & r) { return key_value_pair_view(l) >  key_value_pair_view(r); }

#else

bool operator==(const key_value_pair_view &, const key_value_pair_view);
bool operator!=(const key_value_pair_view &, const key_value_pair_view);
bool operator<=(const key_value_pair_view &, const key_value_pair_view);
bool operator< (const key_value_pair_view &, const key_value_pair_view);
bool operator> (const key_value_pair_view &, const key_value_pair_view);
bool operator>=(const key_value_pair_view &, const key_value_pair_view);

#endif


template<>
inline key_view key_value_pair::get<0u>() const
{
    return key();
}

template<>
inline value_view key_value_pair::get<1u>() const
{
    return value();
}

}
BOOST_PROCESS_V2_END_NAMESPACE

namespace std
{

template<>
class tuple_size<BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair> : integral_constant<std::size_t, 2u> {};

template<>
class tuple_element<0u, BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair> 
{
  public: 
    using type = BOOST_PROCESS_V2_NAMESPACE::environment::key_view;
};

template<>
class tuple_element<1u, BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair> 
{
  public: 
    using type = BOOST_PROCESS_V2_NAMESPACE::environment::value_view;
};

template<std::size_t Idx>
inline auto get(const BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair & kvp) 
        -> typename std::tuple_element<Idx, BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair>::type
{
    return kvp.get<Idx>();
}

template<>
class tuple_size<BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair_view> : integral_constant<std::size_t, 2u> {};

template<>
class tuple_element<0u, BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair_view>
{
  public:
    using type = BOOST_PROCESS_V2_NAMESPACE::environment::key_view;
};

template<>
class tuple_element<1u, BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair_view>
{
  public:
    using type = BOOST_PROCESS_V2_NAMESPACE::environment::value_view;
};

template<std::size_t Idx>
inline auto get(BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair_view kvp) 
        -> typename std::tuple_element<Idx, BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair_view>::type
{
    return kvp.get<Idx>();
}

}

BOOST_PROCESS_V2_BEGIN_NAMESPACE
namespace environment 
{


/// A view object for the current environment of this process.
/**
 * The view might (windows) or might not (posix) be owning;
 * if it owns it will deallocate the on destruction, like a unique_ptr.
 * 
 * Note that accessing the environment in this way is not thread-safe.
 * 
 * @code
 * 
 * void dump_my_env(current_view env = current())
 * {
 *    for (auto  & [k, v] : env)
 *        std::cout << k.string() << " = "  << v.string() << std::endl;
 * }
 * 
 * @endcode
 * 
 * 
 */ 
struct current_view
{
    using native_handle_type = environment::native_handle_type;
    using value_type = key_value_pair_view;

    current_view() = default;
    current_view(current_view && nt) = default;

    native_handle_type  native_handle() { return handle_.get(); }

    struct iterator
    {
        using value_type        = key_value_pair_view;
        using difference_type   = int;
        using reference         = key_value_pair_view;
        using pointer           = key_value_pair_view;
        using iterator_category = std::forward_iterator_tag;

        iterator() = default;
        iterator(const iterator & ) = default;
        iterator(const native_iterator &native_handle) : iterator_(native_handle) {}

        iterator & operator++()
        {
            iterator_ = detail::next(iterator_);
            return *this;
        }

        iterator operator++(int)
        {
            auto last = *this;
            iterator_ = detail::next(iterator_);
            return last;
        }
        key_value_pair_view operator*() const
        {
            return detail::dereference(iterator_);
        }

        friend bool operator==(const iterator & l, const iterator & r) {return l.iterator_ == r.iterator_;}
        friend bool operator!=(const iterator & l, const iterator & r) {return l.iterator_ != r.iterator_;}

      private:
        environment::native_iterator iterator_;
    };

    iterator begin() const {return iterator(handle_.get());}
    iterator   end() const {return iterator(detail::find_end(handle_.get()));}

 private:

  std::unique_ptr<typename remove_pointer<native_handle_type>::type,
                    detail::native_handle_deleter> handle_{environment::detail::load_native_handle()};
};

/// Obtain a handle to the current environment
inline current_view current() {return current_view();}

namespace detail
{

template<typename Environment>
auto find_key(Environment & env, key_view ky) 
    -> typename std::enable_if<std::is_convertible<decltype(*std::begin(env)), key_value_pair_view>::value, value_view>::type
{
    const auto itr = std::find_if(std::begin(env), std::end(env),
                                  [&](key_value_pair_view vp)
                                  {
                                    auto tmp = std::get<0>(vp) == ky;
                                    if (tmp)
                                      return true;
                                    else
                                      return false;
                                  });
    
    if (itr != std::end(env))
      return key_value_pair_view(*itr).value();
    else
      return {};
}

template<typename Environment>
auto find_key(Environment & env, key_view ky) 
    -> typename std::enable_if<
    !std::is_convertible<decltype(*std::begin(env)), key_value_pair_view>::value && 
     std::is_convertible<decltype(*std::begin(env)), key_value_pair>::value, 
    value>::type
{
    const auto itr = std::find_if(std::begin(env), std::end(env),
                                  [&](key_value_pair vp)
                                  {
                                    auto tmp = std::get<0>(vp) == ky;
                                    if (tmp)
                                      return true;
                                    else
                                      return false;
                                  });
    if (itr != std::end(env))
      return key_value_pair(*itr).value();
    else
      return {};
}


}


/// Find the home folder in an environment-like type.
/** 
 * @param env The environment to search. Defaults to the current environment of this process
 * 
 * The environment type passed in must be a range with value T that fulfills the following requirements:
 * 
 *  For `T value`
 *
 *  - std::get<0>(value) must return a type comparable to `key_view`.
 *  - std::get<1>(value) must return a type convertible to filesystem::path.
 * 
 * @return A filesystem::path to the home directory or an empty path if it cannot be found.
 * 
 */
template<typename Environment = current_view>
inline filesystem::path home(Environment && env = current())
{
#if defined(BOOST_PROCESS_V2_WINDOWS)
  return detail::find_key(env, L"HOMEDRIVE").native_string() 
       + detail::find_key(env, L"HOMEPATH").native_string();
#else
  return detail::find_key(env, "HOME").native_string();
#endif
}

/// Find the executable `name` in an environment-like type.
/** 
 * @param env The environment to search. Defaults to the current environment of this process
 * 
 * The environment type passed in must be a range with value T that fulfills the following requirements:
 * 
 *  For `T value`
 *
 *  - std::get<0>(value) must return a type comparable to `key_view`.
 *  - std::get<1>(value) must return a type convertible to `value_view`.
 * 
 * 
 * @return A filesystem::path to the executable or an empty path if it cannot be found.
 * 
 */
template<typename Environment = current_view>
inline BOOST_PROCESS_V2_NAMESPACE::filesystem::path find_executable(
                                             BOOST_PROCESS_V2_NAMESPACE::filesystem::path name,
                                             Environment && env = current())
{

#if defined(BOOST_PROCESS_V2_WINDOWS)
    auto path = detail::find_key(env, L"PATH");
    auto pathext = detail::find_key(env, L"PATHEXT");
    for (auto pp_view : path)
    {
        // first check if it has the extension already
        BOOST_PROCESS_V2_NAMESPACE::filesystem::path full_nm(name);
        BOOST_PROCESS_V2_NAMESPACE::filesystem::path pp(pp_view.begin(), pp_view.end());
        auto p = pp / full_nm;
        error_code ec;

        if (detail::is_executable(p, ec) && !ec)
            return p;

        for (auto ext : pathext)
        {
            ec.clear();
            BOOST_PROCESS_V2_NAMESPACE::filesystem::path nm(name);
            nm.concat(ext.begin(), ext.end());

            auto p = pp / nm;

            if (detail::is_executable(p, ec) && !ec)
                return p;
        }
    }
#else
    for (auto pp_view : detail::find_key(env, "PATH"))
    {
        auto p = BOOST_PROCESS_V2_NAMESPACE::filesystem::path(pp_view.begin(), pp_view.end()) / name;
        error_code ec;
        bool is_exec = detail::is_executable(p, ec);
        if (!ec && is_exec)
            return p;
    }
#endif
    return {};
}

/// Get an environment variable from the current process.
inline value get(const key & k, error_code & ec) { return detail::get(k.c_str(), ec);}
/// Throwing @overload value get(const key & k, error_code & ec)
inline value get(const key & k)
{
  error_code ec;
  auto tmp = detail::get(k.c_str(), ec);
  BOOST_PROCESS_V2_NAMESPACE::detail::throw_error(ec, "environment::get");
  return tmp;
}

/// Disambiguating @overload value get(const key & k, error_code & ec)
inline value get(basic_cstring_ref<char_type, key_char_traits<char_type>> k, error_code & ec)
{
  return detail::get(k, ec);
}
/// Disambiguating @overload value get(const key & k)
inline value get(basic_cstring_ref<char_type, key_char_traits<char_type>> k)
{
  error_code ec;
  auto tmp = detail::get(k, ec);
  BOOST_PROCESS_V2_NAMESPACE::detail::throw_error(ec, "environment::get");
  return tmp;
}


/// Disambiguating @overload value get(const key & k, error_code & ec)
inline value get(const char_type * c, error_code & ec) { return detail::get(c, ec);}
/// Disambiguating @overload value get(const key & k)
inline value get(const char_type * c)
{
  error_code ec;
  auto tmp = detail::get(c, ec);
  BOOST_PROCESS_V2_NAMESPACE::detail::throw_error(ec, "environment::get");
  return tmp;
}

/// Set an environment variable for the current process.
inline void set(const key & k, value_view vw, error_code & ec) { detail::set(k, vw, ec);}
/// Throwing @overload void set(const key & k, value_view vw, error_code & ec)
inline void set(const key & k, value_view vw)
{
  error_code ec;
  detail::set(k, vw, ec);
  BOOST_PROCESS_V2_NAMESPACE::detail::throw_error(ec, "environment::set");
}

/// Disambiguating @overload void set(const key & k, value_view vw, error_code & ec)
inline void set(basic_cstring_ref<char_type, key_char_traits<char_type>> k, value_view vw, error_code & ec) { detail::set(k, vw, ec);}
/// Disambiguating @overload void set(const key & k, value_view vw, error_code & ec)
inline void set(basic_cstring_ref<char_type, key_char_traits<char_type>> k, value_view vw)
{
  error_code ec;
  detail::set(k, vw, ec);
  BOOST_PROCESS_V2_NAMESPACE::detail::throw_error(ec, "environment::set");
}


/// Disambiguating @overload void set(const key & k, value_view vw, error_code & ec)
inline void set(const char_type * k, value_view vw, error_code & ec) { detail::set(k, vw, ec);}
/// Disambiguating @overload void set(const key & k, value_view vw, error_code & ec)
inline void set(const char_type * k, value_view vw)
{
  error_code ec;
  detail::set(k, vw, ec);
  BOOST_PROCESS_V2_NAMESPACE::detail::throw_error(ec, "environment::set");
}

/// Disambiguating @overload void set(const key & k, value_view vw, error_code & ec)
template<typename Char, typename = typename std::enable_if<!std::is_same<Char, char_type>::value>::type>
inline void set(const key & k, const Char * vw, error_code & ec)
{
    value val{vw};
    detail::set(k, val, ec);
}
/// Disambiguating @overload void set(const key & k, value_view vw, error_code & ec)
template<typename Char, typename = typename std::enable_if<!std::is_same<Char, char_type>::value>::type>
inline void set(const key & k, const Char * vw)
{
    error_code ec;
    value val{vw};
    detail::set(k, val, ec);
    BOOST_PROCESS_V2_NAMESPACE::detail::throw_error(ec, "environment::set");
}

/// Disambiguating @overload void set(const key & k, value_view vw, error_code & ec)
template<typename Char, typename = typename std::enable_if<!std::is_same<Char, char_type>::value>::type>
inline void set(basic_cstring_ref<char_type, key_char_traits<char_type>> k, const Char * vw, error_code & ec)
{
    value val{vw};
    detail::set(k, val, ec);
}

/// Disambiguating @overload void set(const key & k, value_view vw, error_code & ec)
template<typename Char, typename = typename std::enable_if<!std::is_same<Char, char_type>::value>::type>
inline void set(basic_cstring_ref<char_type, key_char_traits<char_type>> k, const Char * vw)
{
    error_code ec;
    value val{vw};
    detail::set(k, val, ec);
    BOOST_PROCESS_V2_NAMESPACE::detail::throw_error(ec, "environment::set");
}

/// Disambiguating @overload void set(const key & k, value_view vw, error_code & ec)
template<typename Char, typename = typename std::enable_if<!std::is_same<Char, char_type>::value>::type>
inline void set(const char_type * k, const Char * vw, error_code & ec)
{
    value val{vw};
    detail::set(k, val, ec);
}

/// Disambiguating @overload void set(const key & k, value_view vw, error_code & ec)
template<typename Char, typename = typename std::enable_if<!std::is_same<Char, char_type>::value>::type>
inline void set(const char_type * k, const Char * vw)
{
    error_code ec;
    value val{vw};
    detail::set(k, val, ec);
    BOOST_PROCESS_V2_NAMESPACE::detail::throw_error(ec, "environment::set");
}


/// Remove an environment variable from the current process.
inline void unset(const key & k, error_code & ec) { detail::unset(k, ec);}
/// Throwing @overload void unset(const key & k, error_code & ec)
inline void unset(const key & k)
{
  error_code ec;
  detail::unset(k, ec);
  BOOST_PROCESS_V2_NAMESPACE::detail::throw_error(ec, "environment::unset");
}

/// Disambiguating @overload void unset(const key & k, error_code & ec)
inline void unset(basic_cstring_ref<char_type, key_char_traits<char_type>> k, error_code & ec)
{
  detail::unset(k, ec);
}

/// Disambiguating @overload void unset(const key & k, error_code & ec)
inline void unset(basic_cstring_ref<char_type, key_char_traits<char_type>> k)
{
  error_code ec;
  detail::unset(k, ec);
  BOOST_PROCESS_V2_NAMESPACE::detail::throw_error(ec, "environment::unset");
}

/// Disambiguating @overload void unset(const key & k, error_code & ec)
inline void unset(const char_type * c, error_code & ec) { detail::unset(c, ec);}

/// Disambiguating @overload void unset(const key & k, error_code & ec)
inline void unset(const char_type * c)
{
  error_code ec;
  detail::unset(c, ec);
  BOOST_PROCESS_V2_NAMESPACE::detail::throw_error(ec, "environment::unset");
}
}

// sub process environment stuff

#if defined(BOOST_PROCESS_V2_WINDOWS)
namespace windows { struct default_launcher ;}
#else
namespace posix { struct default_launcher ;}
#endif 

/// Initializer for the environment of sub process.
/**
 * This will set the environment in a subprocess:
 * 
 * @code {.cpp}
 * 
 * process proc{executor, find_executable("printenv"), {"foo"}, process_environment{"foo=bar"}};
 * @endcode
 * 
 * The environment initializer will persist it's state, so that it can
 * be used multiple times. Do however note the the Operating System is
 * allowed to modify the internal state.
 * 
 * @code {.cpp}
 * auto exe = find_executable("printenv");
 * process_environment env = {"FOO=BAR", "BAR=FOO"};
 * 
 * process proc1(executor, exe, {"FOO"}, env);
 * process proc2(executor, exe, {"BAR"}, env);
 * @endcode
 * 
 * 
 */
struct process_environment
{

#if defined(BOOST_PROCESS_V2_WINDOWS)


  template<typename Args>
  static
  std::vector<wchar_t> build_env(Args && args,
                                      typename std::enable_if<
                                              std::is_convertible<
                                                      decltype(*std::begin(std::declval<Args>())),
                                                      wcstring_ref>::value>::type * = nullptr)
  {
    std::vector<wchar_t> res;
    std::size_t sz = 1;
    for (wcstring_ref cs : std::forward<Args>(args))
        sz =+ cs.size() + 1;
    res.reserve(sz);
    
    for (wcstring_ref cs : std::forward<Args>(args))
        res.insert(res.end(), cs.begin(), std::next(cs.end()));
        

    res.push_back(L'\0');
    return res;
  }

  template<typename Args>
  std::vector<wchar_t> build_env(Args && args,
                                      typename std::enable_if<
                                              !std::is_convertible<
                                                      decltype(*std::begin(std::declval<Args>())),
                                                      wcstring_ref>::value>::type * = nullptr)
  {
    for (auto && arg: std::forward<Args>(args))
      env_buffer.emplace_back(arg);
    return build_env(env_buffer);
  }

  process_environment(std::initializer_list<string_view> sv)  : unicode_env{build_env(sv)} {}
  process_environment(std::initializer_list<wstring_view> sv) : unicode_env{build_env(sv)} {}

  template<typename Args>
  process_environment(Args && args) : unicode_env{build_env(std::forward<Args>(args))}
  {
  }

  error_code error() {return ec;}
  error_code ec;
  std::vector<environment::key_value_pair> env_buffer;
  std::vector<wchar_t> unicode_env;

  BOOST_PROCESS_V2_DECL
  error_code on_setup(windows::default_launcher & launcher,
                      const filesystem::path &, const std::wstring &);

#else

  template<typename Args>
  static
  std::vector<const char *> build_env(Args && args,
                                      typename std::enable_if<
                                              std::is_convertible<
                                                      decltype(*std::begin(std::declval<Args>())),
                                                      cstring_ref>::value>::type * = nullptr)
  {
    std::vector<const char *> env;
    for (auto && e : args)
      env.push_back(e.c_str());

    env.push_back(nullptr);
    return env;
  }

  template<typename Args>
  std::vector<const char *> build_env(Args && args,
                                      typename std::enable_if<
                                              !std::is_convertible<
                                                      decltype(*std::begin(std::declval<Args>())),
                                                      cstring_ref>::value>::type * = nullptr)
  {
    std::vector<const char *> env;

    for (auto && arg: std::forward<Args>(args))
      env_buffer.emplace_back(arg);

    for (auto && e : env_buffer)
      env.push_back(e.c_str());
    env.push_back(nullptr);
    return env;
  }


  process_environment(std::initializer_list<string_view> sv) : env{build_env(sv)}  {  }

  template<typename Args>
  process_environment(Args && args) : env(build_env(std::forward<Args>(args)))
  {
  }


  BOOST_PROCESS_V2_DECL
  error_code on_setup(posix::default_launcher & launcher, 
                      const filesystem::path &, const char * const *);

  std::vector<environment::key_value_pair> env_buffer;
  std::vector<const char *> env;

#endif

};



BOOST_PROCESS_V2_END_NAMESPACE


namespace std
{

template<>
struct hash<BOOST_PROCESS_V2_NAMESPACE::environment::key_view>
{
    std::size_t operator()( BOOST_PROCESS_V2_NAMESPACE::environment::key_view kv) const noexcept
    {
        return BOOST_PROCESS_V2_NAMESPACE::environment::hash_value(kv);
    }
};


template<>
struct hash<BOOST_PROCESS_V2_NAMESPACE::environment::value_view>
{
    std::size_t operator()( BOOST_PROCESS_V2_NAMESPACE::environment::value_view kv) const noexcept
    {
        return BOOST_PROCESS_V2_NAMESPACE::environment::hash_value(kv);
    }
};

template<>
struct hash<BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair_view>
{
    std::size_t operator()( BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair_view kv) const noexcept
    {
        return BOOST_PROCESS_V2_NAMESPACE::environment::hash_value(kv);
    }
};


template<>
struct hash<BOOST_PROCESS_V2_NAMESPACE::environment::key>
{
    std::size_t operator()( BOOST_PROCESS_V2_NAMESPACE::environment::key_view kv) const noexcept
    {
        return BOOST_PROCESS_V2_NAMESPACE::environment::hash_value(kv);
    }
};


template<>
struct hash<BOOST_PROCESS_V2_NAMESPACE::environment::value>
{
    std::size_t operator()( BOOST_PROCESS_V2_NAMESPACE::environment::value_view kv) const noexcept
    {
        return BOOST_PROCESS_V2_NAMESPACE::environment::hash_value(kv);
    }
};

template<>
struct hash<BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair>
{
    std::size_t operator()( BOOST_PROCESS_V2_NAMESPACE::environment::key_value_pair_view kv) const noexcept
    {
        return BOOST_PROCESS_V2_NAMESPACE::environment::hash_value(kv);
    }
};

}


#endif //BOOST_PROCESS_V2_ENVIRONMENT_HPP