console.cpp

#include <boost/describe.hpp>
#include <boost/mp11.hpp>
#include <boost/json.hpp>
#include <boost/utility/string_view.hpp>
#include <string>
#include <stdexcept>
#include <iostream>

// get variable

template<class Scope> boost::json::value get( Scope& scope, boost::string_view name )
{
    using Md = boost::describe::describe_members<Scope, boost::describe::mod_public>;

    bool found = false;
    boost::json::value result;

    boost::mp11::mp_for_each<Md>([&](auto D) {

        if( !found && name == D.name )
        {
            result = boost::json::value_from( scope.*D.pointer );
            found = true;
        }

    });

    if( !found )
    {
        throw std::invalid_argument( std::string( "'" ) + std::string( name ) + "': no such variable" );
    }

    return result;
}

// set variable

template<class T> void set_impl( T & t, boost::string_view /*name*/, boost::json::value const& value )
{
    t = boost::json::value_to<T>( value );
}

template<class T> void set_impl( T const & /*t*/, boost::string_view name, boost::json::value const& /*value*/ )
{
    throw std::invalid_argument( std::string( "'" ) + std::string( name ) + "': variable is const" );
}

template<class Scope> boost::json::value set( Scope& scope, boost::string_view name, boost::json::value const& value )
{
    using Md = boost::describe::describe_members<Scope, boost::describe::mod_public>;

    bool found = false;
    boost::json::value result;

    boost::mp11::mp_for_each<Md>([&](auto D) {

        if( !found && name == D.name )
        {
            set_impl( scope.*D.pointer, name, value );
            found = true;
        }

    });

    if( !found )
    {
        throw std::invalid_argument( std::string( "'" ) + std::string( name ) + "': no such variable" );
    }

    return result;
}

//

struct globals
{
    std::string const help = "Enter a variable name ('x', 'y', 'v', or 'm') to print its value; enter variable=value to assign a new value to a variable. Values are in JSON format.";

    int x = 1;

    double y = 3.14;

    std::vector<int> v{ 1, 2, 3 };

    std::map<std::string, double> m{ { "BTC", 44898.68 }, { "ETH", 1386.57 } };
};

BOOST_DESCRIBE_STRUCT( globals, (), (help, x, y, v, m) )

int main()
{
    globals g_;

    for( ;; )
    {
        std::cout << "\n> ";

        std::string line;
        std::getline( std::cin, line );

        try
        {
            std::size_t i = line.find( '=' );

            if( i != std::string::npos )
            {
                set( g_, line.substr( 0, i ), boost::json::parse( line.substr( i + 1 ) ) );
            }
            else
            {
                std::cout << get( g_, line ) << std::endl;
            }
        }
        catch( std::exception const& x )
        {
            std::cout << "Error: " << x.what() << std::endl;
        }
    }
}