picojson の value, array, object の生成を簡単にするヘルパーライブラリーの実装例 ref: http://qiita.com/usagi/items/fa09b3881979b923e3a6

file0.txt

#pragma once

#include <picojson.h>
#include <string>

namespace usagi::json::picojson
{
  using object_type = ::picojson::object;
  using array_type  = ::picojson::array;
  using value_type  = ::picojson::value;
  using null_type   = ::picojson::null;
  using number_type = double;
  using string_type = std::string;
}

file1.txt

/// @file
/// @brief 任意の何かから value_type を"生成"する make_value さん群

#pragma once

#include "type.hxx"

namespace usagi::json::picojson
{
  static inline auto make_value(                       ) -> value_type { return value_type(); }
  static inline auto make_value( const null_type       ) -> value_type { return make_value(); }
  static inline auto make_value( const value_type&  in ) -> value_type { return value_type( in ); }
  static inline auto make_value( value_type&&       in ) -> value_type { return std::move( in ); }
  static inline auto make_value( const array_type&  in ) -> value_type { return value_type( in ); }
  static inline auto make_value( array_type&        in ) -> value_type { return value_type( std::move( in ) ); }
  static inline auto make_value( const object_type& in ) -> value_type { return value_type( in ); }
  static inline auto make_value( object_type&&      in ) -> value_type { return value_type( std::move( in ) ); }
  static inline auto make_value( const char*        in ) -> value_type { return value_type( in ); }
  static inline auto make_value( const std::string& in ) -> value_type { return value_type( in ); }
  static inline auto make_value( std::string&&      in ) -> value_type { return value_type( std::move( in ) ); }
  static inline auto make_value( const bool         in ) -> value_type { return value_type( in ); }

  template < typename T >
  static inline auto make_value
  ( const T v
  ) -> value_type
  { return value_type( static_cast< double >( v ) ); }
}

file2.txt

auto a = make_array( 1.23f, 1.23, 123, "hoge", "fuga"s, some_value, some_array, some_object, ... );
auto v = make_array_value( ... );

file3.txt

#pragma once

#include "make_value.hxx"

namespace usagi::json::picojson
{
  namespace detail
  {
    template < typename T >
    static inline decltype( auto ) emplace_back
    ( array_type& a, const T& v )
    {
      a.emplace_back( make_value( v ) );
      return a;
    }

    static inline decltype( auto ) make_array_internal( array_type& a )
    {
      return a;
    }

    template < typename HEAD, typename ... TAIL >
    static inline decltype( auto ) make_array_internal
    ( array_type& a, const HEAD& v, const TAIL& ... vs )
    {
      return make_array_internal( emplace_back( a, v ), vs ... );
    }
  }

  template < typename ... TS >
  static inline auto make_array
  ( const TS& ... vs )
  {
    array_type a;
    return detail::make_array_internal( a, vs ... );
  }

  template < typename ... TS >
  static inline auto make_array_value
  ( const TS& ... vs )
  {
    return value_type( make_array( vs ... ) );
  }
}

file4.txt

auto o =
  make_object
  ( "f", 1.23f
  , "d", 1.23
  , "i", 123
  , "c", "hoge"
  , "s", "fuga"s
  , "v", some_value
  , "a", some_array
  , "o", some_object
  , "some_key", something
  , ...
  );
auto v = make_object_value( ... );

file5.txt

#pragma once

#include "make_value.hxx"

namespace usagi::json::picojson
{
  namespace detail
  {
    template < typename T >
    static inline decltype( auto ) emplace
    ( object_type& o, const std::string& k, const T& v )
    {
      o.emplace( k, make_value( v ) );
      return o;
    }

    static inline decltype( auto ) make_object_internal( object_type& o )
    {
      return o;
    }

    template < typename HEAD, typename ... TAIL >
    static inline decltype( auto ) make_object_internal
    ( object_type& o, const std::string& k, const HEAD& v, const TAIL& ... vs )
    {
      return make_object_internal( emplace( o, k, v ), vs ... );
    }
  }

  template < typename ... TS >
  static inline auto make_object( const TS& ... vs )
  {
    object_type o;
    return detail::make_object_internal( o, vs ... );
  }

  template < typename ... TS >
  static inline auto make_object_value
  ( const TS& ... vs )
  {
    return value_type( make_object( vs ... ) );
  }
}

file6.txt

#include <usagi/json/picojson/make_array.hxx>
#include <usagi/json/picojson/make_object.hxx>

#include <iostream>

auto main() -> int
{
  using namespace std;
  using namespace usagi::json::picojson;

  std::cout
    << "f: " << make_value( 1.23f ) << '\n'
    << "d: " << make_value( 1.23 ) << '\n'
    << "i: " << make_value( 123 ) << '\n'
    << "n: " << make_value( ) << '\n'
    << "b: " << make_value( true ) << '\n'
    << "s: " << make_value( string( "hoge" ) ) << '\n'
    << "c: " << make_value( "hoge" ) << '\n'
    << "v: " << make_value( make_value( "hoge" ) ) << '\n'
    << "a: " << make_array_value( "hoge", string( "fuga" ), 1.23f, 1.23, 123, null_type(), make_array( 1,2,3 ) ) << '\n'
    << "o: " << make_object_value( "f", 1.23f, "d", 1.23, "i", 123, "n", null_type(), "a", make_array( 1,2,3 ), "o", make_object( "a", 1 ) ) << '\n'
    ;
}

file7.txt

f: 1.2300000190734863
d: 1.23
i: 123
n: null
b: true
s: "hoge"
c: "hoge"
v: "hoge"
a: ["hoge","fuga",1.2300000190734863,1.23,123,null,[1,2,3]]
o: {"a":[1,2,3],"d":1.23,"f":1.2300000190734863,"i":123,"n":null,"o":{"a":1}}