lua config example

#include <lua/lua.hpp>
#include <string>
#include <cstdio>

void config_loadfile( lua_State* L, char const* path )
{
    if( luaL_loadfile( L, "conf.lua" ) || lua_pcall( L, 0, 0, 0 ) )
    {
        fprintf( stderr, "error in config file \"%s\": %s\n",
            path, lua_tostring( L, -1 ) );
        std::terminate();
    }
}

void lua_getvalue( lua_State* L, int index, bool* result )
{
    *result = lua_toboolean( L, index );
}

void lua_getvalue( lua_State* L, int index, int* result )
{
    *result = lua_tointeger( L, index );
}

void lua_getvalue( lua_State* L, int index, double* result )
{
    *result = lua_tonumber( L, index );
}

void lua_getvalue( lua_State* L, int index, std::string* result )
{
    size_t len;
    char const* ch = lua_tolstring( L, index, &len );
    *result = std::string( ch, ch + len );
}

template< typename T >
T config_getvalue(
    lua_State* L, std::string const& expression, T const& defval )
{
    T result;
    std::string chunk = std::string( "return " ) + expression;
    if( luaL_loadstring( L, chunk.c_str() ) || lua_pcall( L, 0, 1, 0 ) ||
        lua_isnil( L, -1 ) )
    {
        result = defval;
    }
    else
    {
        lua_getvalue( L, -1, &result );
    }
    lua_pop( L, -1 );
    return result;
}

int main( int argc, char const** argv )
{
    char const* platform = argc >= 2 ? argv[ 1 ] : "default";
    lua_State* conf = luaL_newstate();
    lua_pushstring( conf, platform );
    lua_setglobal( conf, "platform" );
    config_loadfile( conf, "conf.lua" );
    int windowwidth = config_getvalue( conf, "display.width", 640 );
    int windowheight = config_getvalue( conf, "display.height", 480 );
    float gamma = config_getvalue( conf, "display.gamma", 2.2 );
    bool fullscreen = config_getvalue( conf, "display.fullscreen", false );
    std::string renderername = config_getvalue(
        conf, "display.renderer", std::string( "d3d9renderer" ) );
    printf( "window size = %i*%i\n", windowwidth, windowheight );
    printf( "gamma = %.5f\n", gamma );
    printf( "fullscreen = %s\n", fullscreen ? "true" : "false" );
    printf( "renderer library = %s.dll\n", renderername.c_str() );
    return 0;
}

//////////////////////////////////////////////////////////////////

display = {
    gamma = 1 / 2.2,
}
if platform == 'win' then
    display.renderer = 'd3drenderer'
    display.fullscreen = true
elseif platform == 'linux' then
    display.renderer = 'glrenderer'
    display.fullscreen = false
elseif platform == 'android' then
    display.renderer = 'glesrenderer'
    display.fullscreen = true
else
    display.renderer = 'softwarerenderer'
    display.fullscreen = false
end

if display.fullscreen then
    display.width, display.height = 1024, 768
else
    display.width, display.height = 640, 480
end