get return type of function
CodeGuru Home VC++ / MFC / C++ .NET / C# Visual Basic VB Forums Developer.com
Results 1 to 5 of 5

Thread: get return type of function

  1. #1
    Join Date
    Sep 2010
    Posts
    39

    get return type of function

    Hi,
    Say I have overloaded functions with different arguments AND return types. Each set of argument has only one corresponding return type.
    Code:
    Vector grad(Scalar)
    Tensor grad(Vector)
    Later I have:
    Code:
    template <class T>
    void test(T x) {
        ...  Y = grad(x) 
    }
    Then how do I automatically declare the type of Y. Do I need to make the grad function a template and specialize each of them ?
    Thanks for any help

  2. #2
    Join Date
    Oct 2008
    Posts
    1,131

    Re: get return type of function

    in c++11, you can simply write

    "auto Y = grad(x);"
    "auto const& Y = grad(x);"
    ...

    or

    "typedef decltype(grad(x)) GradType;

    GradType Y = grad(x);"

    etc... depending on what's the expected type of Y.

    with c++2003 compilers, things get more complex ...

  3. #3
    Join Date
    Sep 2010
    Posts
    39

    Re: get return type of function

    Thanks a lot.
    Unfortunately I am not using c++11 so I can not use those solutions.
    I get 'error C3861: 'decltype': identifier not found' for the second solution.
    Do you have the solution that works for in Visual studio 2008?
    I appreciate your help.
    Last edited by dshawul; November 21st, 2012 at 04:13 AM.

  4. #4
    Join Date
    Oct 2008
    Posts
    1,131

    Re: get return type of function

    the easiest way is to write a trait class

    Code:
    template <class T>
    struct grad_type {};
    
    template <>
    struct grad_type<Scalar> { typedef Vector type; };
    
    template <>
    struct grad_type<Vector> { typedef Tensor type; };
    
    template <class T>
    void test(T x) {
        typename grad_type<Y>::type  Y = grad(x)
    }
    of course, you need to specialize the trait for each possible parameter type. It would be easier if Scalar,Vector,Tensor had some high-level info in them ( like a rank grading ), in this way the trait could compute the grad_type automatically at compile time.

    Anyway, I think emulating "auto"/"decltype" is possible in this case via some template machinery like SFINAE, but I wouldn't advise it if you don't know what you're doing ...

    moreover, another simple solution could be to refactor the code, if possible; something like

    Code:
    template <class T,class G>
    void test_impl(T& x,G y) {
    	// ...
    }
    
    template <class T>
    void test(T x) {
        test_impl( x, grad(x) );
    }

  5. #5
    Join Date
    Sep 2010
    Posts
    39

    Re: get return type of function

    You are a life saver. I will use the traits method for now.
    Scalar,vector and tensor are 1st,2nd and 3rd rank tensors but they are of completely different types represented by
    double, double[], and double[][]. So I don't think I can automatically compute them. But anyway it is important to know
    the higher or lower ranks in many places of the code so I will use that.
    Thanks again.
    Last edited by dshawul; November 21st, 2012 at 05:48 AM.

Posting Permissions

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


Windows Mobile Development Center


Click Here to Expand Forum to Full Width

This is a CodeGuru survey question.


Featured


HTML5 Development Center