check if member exists using enable_if

问题:

Here's what I'm trying to do:

template <typename T> struct Model
{
    vector<T> vertices ;

    #if T has a .normal member
    void transform( Matrix m )
    {
        each vertex in vertices
        {
          vertex.pos = m * vertex.pos ;
          vertex.normal = m * vertex.normal ;
        }
    }
    #endif

    #if T has NO .normal member
    void transform( Matrix m )
    {
        each vertex in vertices
        {
          vertex.pos = m * vertex.pos ;
        }
    }
    #endif
} ;

I've seen examples of using enable_if, but I cannot understand how to apply enable_if to this problem, or if it even can be applied.

回答1:

This has become way easier with C++11.

template <typename T> struct Model
{
    vector<T> 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<typename> struct int_ { typedef int type; };

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

    template<typename Lhs, typename Rhs>
    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.

回答2:

You need a meta function to detect your member so that you can use enable_if. The idiom to do this is called Member Detector. It's a bit tricky, but it can be done!

回答3:

This isn't an answer to your exact case, but it is an alternative answer to the question title and problem in general.

#include <iostream>
#include <vector>

struct Foo {
    size_t length() { return 5; }
};

struct Bar {
    void length();
};

template <typename R, bool result = std::is_same<decltype(((R*)nullptr)->length()), size_t>::value>
constexpr bool hasLengthHelper(int) { 
    return result;
}

template <typename R>
constexpr bool hasLengthHelper(...) { return false; }

template <typename R>
constexpr bool hasLength() {
    return hasLengthHelper<R>(0);
}

// function is only valid if `.length()` is present, with return type `size_t`
template <typename R>
typename std::enable_if<hasLength<R>(), size_t>::type lengthOf (R r) {
  return r.length();
}

int main() {
    std::cout << 
      hasLength<Foo>() << "; " <<
      hasLength<std::vector<int>>() << "; " <<
      hasLength<Bar>() << ";" <<
      lengthOf(Foo()) <<
      std::endl;
    // 1; 0; 0; 5

    return 0;
}

Relevant https://ideone.com/utZqjk.

Credits to dyreshark on the freenode IRC #c++.