Config-Class that supports adding of config item member functions during compile time with macros and uses C++ member initialization order to read and release the document on construction

config_example.cpp

#include <iostream>
#include <string>


// uses the initialization ordering of c++ to construct members, based on previously created member (document)
// that will be released within the constructor body

// http://en.cppreference.com/w/cpp/language/initializer_list

// this is what should be released after initialization was done
struct Document 
{ 
    int extract_value() { return 1234; } 
};

#define ADD_CONFIG_ITEM(ITEM_TYPE, ITEM_NAME)\
private: ConfigItem<ITEM_TYPE> m_config_item_ ## ITEM_NAME = { *this };\
public: ITEM_TYPE get_ ## ITEM_NAME () const noexcept { return m_config_item_ ## ITEM_NAME.get_value(); }

class Config
{
private:
    // document to extract values from
    Document * m_document;
    
    template<typename T>
    struct ConfigItem
    {
        // constructor gets document member from Config and extracts it's value
        // the value will be stored internally and then it's available forever
        ConfigItem(Config const & conf) : value(conf.m_document->extract_value()) {}
        
        // add constructor with additional default value
        
        T const & get_value() const noexcept { return value; }
        T const value;
    };
    
    public:
    
    // maybe use shared_ptr or unique instead
    Config(Document * d) 
        : m_document(d) 
    {
        // members of values should be extracted through member default initialization
        // so delete should be safe because noone else is using it anymore
        delete d; 
        std::cout << "d was deleted" << std::endl;
    }
    
    // add config items here
    
    // member will be default initialized BEFORE constructor body of Config will be executed
    ConfigItem<int> m_mem = { *this }; // initialize with this to get the previously initialized m_document member
    ADD_CONFIG_ITEM(int, test_item1);
    ADD_CONFIG_ITEM(int, test_some_very_long_item1);

};

int main()
{

  auto d = new Document();
  Config c(d);
  std::cout << c.m_mem.get_value() << std::endl;
  std::cout << c.get_test_item1() << std::endl;
  std::cout << c.get_test_some_very_long_item1() << std::endl;
}