check if member exists using enable_if

Question

Here\'s what I\'m trying to do:

template  struct Model
{
    vector vertices ;

    #if T has a .normal member
    void transform(         


        

Answer 1

This has become way easier with C++11.

template  struct Model
{
    vector vertices;

    void transform( Matrix m )
    {
        for(auto &&vertex : vertices)
        {
          vertex.pos = m * vertex.pos;
          modifyNormal(vertex, m, special_());
        }
    }

private:

    struct general_ {};
    struct special_ : general_ {};
    template struct int_ { typedef int type; };

    template::type = 0>
    void modifyNormal(Lhs &&lhs, Rhs &&rhs, special_) {
       lhs.normal = rhs * lhs.normal;
    }

    template
    void modifyNormal(Lhs &&lhs, Rhs &&rhs, general_) {
       // do nothing
    }
};

Things to note:

• You can name non-static data members in decltype and sizeof without needing an object.
• You can apply extended SFINAE. Basically any expression can be checked and if it is not valid when the arguments are substituted, the template is ignored.