Variadic template function parameters and method pointers.
CodeGuru Home VC++ / MFC / C++ .NET / C# Visual Basic VB Forums Developer.com
Results 1 to 4 of 4

Thread: Variadic template function parameters and method pointers.

  1. #1
    Join Date
    Oct 2013
    Posts
    5

    Question Variadic template function parameters and method pointers.

    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:

    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:
    Name:  error.png
Views: 86
Size:  8.1 KB

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

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

    Talaxy

  2. #2
    Join Date
    Oct 2008
    Posts
    1,088

    Re: Variadic template function parameters and method pointers.

    I'd say it's a compiler bug ... anyway, it seems to me you don't want to automatically deduce the types, so, as a workaround, you can try:

    Code:
    template < typename returnType, typename classType, typename... argTypes >
    struct WorkAround{ typedef returnType( classType::*type )(argTypes...); };
    
    template < typename returnType, typename classType, typename... argTypes >
    void TakeMethodAndWrappedParams( typename WorkAround<returnType,classType,argTypes...>::type fnPtr, MyTemplateWrapper<argTypes>... args )
    ...
    the online vc++ compiler seems happy with this.

  3. #3
    Join Date
    Oct 2013
    Posts
    5

    Re: Variadic template function parameters and method pointers.

    That's correct, I am willing to specify the types per function function call. That work around is a nice idea, thanks for that I'll give it a try later!

    I eventually intend to wrap this whole call up in a more elegant macro. Hopefully I won't run into the same problem when I try to pass the method pointer and wrapped parameters to the macro, but I think it should work.

    Many Thanks
    Talaxy

  4. #4
    Join Date
    Oct 2013
    Posts
    5

    Re: Variadic template function parameters and method pointers.

    Bingo! It worked a treat. An by giving the "WorkAround" struct a constructor that takes the function pointer, I was actually able to just call the function as before, by directly passing the function pointer.

    Cheers!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  


Azure Activities Information Page

Windows Mobile Development Center


Click Here to Expand Forum to Full Width

This is a CodeGuru survey question.


Featured


HTML5 Development Center