Variadic template function parameters and method pointers.

[Talaxy]

Hello there,

I'm new here, hope I have posted to the appropriate place. I have been experimenting with variadic templates with the aim of caching a call to a class method by storing away the object pointer, method pointer and parameters. I've actually had some reasonable success but have now hit a stumbling block. I now wish to wrap my parameters in a simple template class when I cache them. My success is as follows:

Using variadic template functions to store these pointers and paremeters;

• I'm able to pass a method pointer and unwrapped parameters
• I'm able to pass wrapped parameters on their own.
• I'm NOT able to pass a method pointer and wrapped parameters

I set up a little prototype project to demonstrate the issue and added comments above the function calls to indicate the compilation results. Here is the code:

#include "stdafx.h"

//////////////////////////////////////////////////
// Basic class with a simple method
//////////////////////////////////////////////////
class MyClass
{
public:
	char Method( int i, float f )
	{
		return 'A';
	}
};

//////////////////////////////////////////////////
// Simple template wrapper class
//////////////////////////////////////////////////
template < typename classType >
class MyTemplateWrapper
{
public:
	MyTemplateWrapper( const classType& initVal ) :
		mVal( initVal )
	{
	}

private:
	classType mVal;
};

//////////////////////////////////////////////////
// Template function that takes a method pointer
// and parameters.
// (Able to call)
//////////////////////////////////////////////////
template < typename returnType, typename classType, typename... argTypes >
void TakeMethodAndParams( returnType( classType::*fnPtr )( argTypes... ), argTypes... args )
{
	// Do nothing
}

//////////////////////////////////////////////////
// Template function that takes some wrapped
// params.
// (Able to call)
//////////////////////////////////////////////////
template < typename returnType, typename classType, typename... argTypes >
void TakeWrappedParams( MyTemplateWrapper<argTypes>... args )
{
	// Do nothing
}

//////////////////////////////////////////////////
// Template function that takes a method pointer
// and wrapped params.
// (Unable to call)
//////////////////////////////////////////////////
template < typename returnType, typename classType, typename... argTypes >
void TakeMethodAndWrappedParams( returnType( classType::*fnPtr )( argTypes... ), MyTemplateWrapper<argTypes>... args )
{
	// Do nothing
}

//////////////////////////////////////////////////
// Main
//////////////////////////////////////////////////
int _tmain(int argc, _TCHAR* argv[])
{
	// Compiles!
	TakeMethodAndParams< char, MyClass, int, float >( &MyClass::Method, 7, 123.0f );
	
	// Compiles!
	TakeWrappedParams< char, MyClass, int, float >( 123, 456.0f );

	// Error C2660: 'TakeMethodAndWrappedParams' : function does not take 3 arguments
	TakeMethodAndWrappedParams< char, MyClass, int, float >( &MyClass::Method, 7, 123.0f );

	// Error C2660: 'TakeMethodAndWrappedParams' : function does not take 3 arguments
	MyTemplateWrapper< int > intWrapper( 7 );
	MyTemplateWrapper< float > floatWrapper( 123.0f );
	TakeMethodAndWrappedParams< char, MyClass, int, float >( &MyClass::Method, intWrapper, floatWrapper );

	return 0;
}

As you can see, the last two function calls return the following error:
// Error C2660: 'TakeMethodAndWrappedParams' : function does not take 3 arguments

But I'm convinced it should take three arguments, the method pointer and two wrapped parameters. Visual studio even suggested it should as shown below:


So I'm quite confused by this as you can imagine...

Thanks very much for reading, any thoughts would be much appreciated

Talaxy