Question

I have the following templated function…

template< class T > T *create_object( lua_State *L )    
{
    // Get a raw block of memory, managed by Lua.
    void *mem = lua_newuserdata( L, sizeof( T ) );
    // Construct the object in the allocated memory.
    T *object = new (mem) T;
    // Do other stuff here...
    return object;
}

… that allocates and sets up a C++ object for use in the Lua scripting language. I’d like to extend this function so I can pass in arguments for the object’s constructor. It might look something like this:

template< class T > T *create_object( lua_State *L, ??? ctor_args )    
{
    void *mem = lua_newuserdata( L, sizeof( T ) );
    T *object = new (mem) T( ctor_args ); // Call correct constructor as determined by args.
    // ...
    return object;
}

… and work something like this:

class widget
{
    public:
        // Multiple constructors
        widget(); // #1
        widget( const widget &w ); // #2
        widget( int width, int height, float x, float y ); //#3
};

class font
{
    public:
        font( std::vector<uint8_t> );
}

// Other classes with multiple constructors.

// Example usage: (L = lua_State pointer)
create_object<widget>( L ); // Pass no arguments - use constructor #1
create_object<widget>( L, existing_widget ); // Pass one argument- use constructor #2
create_object<widget>( L, 128, 64, 100.0f, 100.0f ); // Pass 4 arguments - use construct #3
create_object<font>( L, buffer ); // Just to show it needs to work with many object types...
... and so on ...

Avoiding a variadic template that ends up like this:

create_object<widget, int, int, float, float >( L, 256, 512, 120.0f, 0.0f );

Would be nice.

Is this possible in c++11?

Update: I’m currently using gcc 4.6 with -pedantic turned on. Non compiler-specific solutions would be preferred.

Answer 1

Like this, provided you have proper support for variadic templates:

template< class T, typename... Args > 
T *create_object( lua_State *L, Args&&... args)    
{
    void *mem = lua_newuserdata( L, sizeof( T ) );
    T *object = new (mem) T(std::forward<Args>(args)...);
    // ...
    return object;
}

This will correctly forward references, even your constructor takes some of its arguments by (const or not) reference and other by value.