使用enable_if检查成员是否存在 [英] check if member exists using enable_if

问题描述

这是我想要做的：

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
} ;

我看过 enable_if 的enable-if-seem-to-work-outside-a-class-but-not-inside？rq = 1>示例我不明白如何应用 enable_if 到这个问题，或者如果它甚至可以应用。

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.

推荐答案

这使C ++ 11变得更容易了。

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
    }
};

注意事项：

• 您可以在 decltype 和 sizeof 中命名非静态数据成员，而无需对象。


• 您可以应用扩展SFINAE。基本上可以检查任何表达式，并且如果在替换参数时该表达式无效，则会忽略该模板。

• 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.

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