.. _program_listing_file_uconfig_format_impl_Env.ipp:

Program Listing for File Env.ipp
================================

|exhale_lsh| :ref:`Return to documentation for file <file_uconfig_format_impl_Env.ipp>` (``uconfig/format/impl/Env.ipp``)

.. |exhale_lsh| unicode:: U+021B0 .. UPWARDS ARROW WITH TIP LEFTWARDS

.. code-block:: cpp

   #pragma once
   
   #include <cstdlib>
   #include <limits>
   #include <sstream>
   
   namespace uconfig {
   
   template <typename T>
   std::optional<T> EnvFormat::Parse(const source_type*, const std::string& path) const
   {
       const char* env_var = std::getenv(path.c_str());
       if (!env_var) {
           return std::nullopt;
       }
       return FromString<T>(env_var);
   }
   
   template <typename T>
   void EnvFormat::Emit(dest_type* dest, const std::string& path, const T& value) const
   {
       dest->emplace(std::make_pair(path, ToString<T>(value)));
   }
   
   std::string EnvFormat::VectorElementPath(const std::string& vector_path, std::size_t index) const noexcept
   {
       return vector_path + "_" + std::to_string(index);
   }
   
   template <typename T>
   std::optional<T> EnvFormat::FromString(const std::string& str)
   {
       T result;
   
       std::istringstream ss(str);
       ss >> result;
       if (ss.fail() || ToString(result) != str) {
           return std::nullopt;
       }
   
       return result;
   }
   
   template <typename T>
   std::string EnvFormat::ToString(const T& value)
   {
       std::ostringstream ss;
       // by default stream print with .3 digits accuracy, so 123456.0 -> "1.234e+05" -> 123400;
       // but full precision (max_digits10) leads to: 11.0/10.0 -> 1.1000000000000001
       // [https://coliru.stacked-crooked.com/a/1681ce5326697585]
       if (std::numeric_limits<T>::max_digits10 > 0) {
           ss.precision(std::numeric_limits<T>::max_digits10 - 1);
       }
   
       ss << value;
       if (ss.fail()) {
           throw std::runtime_error("failed to print value");
       }
       return ss.str();
   }
   
   template <>
   inline std::optional<std::string> EnvFormat::FromString<std::string>(const std::string& str)
   {
       return str;
   }
   
   template <>
   inline std::string EnvFormat::ToString<std::string>(const std::string& value)
   {
       return value;
   }
   
   } // namespace uconfig