enable_if模板参数为lambda(具有特定签名) [英] enable_if template param is lambda (with particular signature)

问题描述

我有这样的东西:

template<typename T>
class Image {
    Image(int w, int h, T defaultVal){
        for(int i=0; i<h; i++)
            for(int j=0; j<w; j++)
                pixel(j, i) = defaultVal;
    }
    template<typename F>
    Image(int w, int h, F initializer){
        for(int i=0; i<h; i++)
            for(int j=0; j<w; j++)
                pixel(j, i) = initializer(j, i);
    }
    // ...
};

我的意图是能够像这样实例化Image:

My intention is to be able to instantiate an Image like this:

Image<int> img0(w, h, 0); // image of zeroes
Image<int> imgF(w, h, [](int j, int i){ // checkerboard image
    return (j/10+i/10) % 2;
});

但是，第二个构造函数签名当然会与第一个构造函数签名冲突.为了解决此冲突，我想限制第二个构造函数的可能模板实例化.

But of course the second constructor signature will conflict with the first constructor signature. To resolve this conflict I want to restrict the second constructor's possible template instantiations.

我不想使其变得过于复杂.你能帮助我吗?我的尝试:

I don't want to make it too complicated. Can you help me? My attempt:

template<typename F, typename = std::enable_if_t< // what now? how to check that F is callable (and if simple to check, with appropriate signature)

推荐答案

您正在寻找 std::is_invocable :

You're looking for std::is_invocable:

template<typename F, typename = std::enable_if_t<
    std::is_invocable<F&, int, int>>

F&，因为您将其作为左值调用，然后，它只是参数类型的列表.

F& because you're invoking it as an lvalue, and then after that it's just a list of types of parameters.

以上要求使用C ++ 17.它可以在C ++ 14上实现，但是在您的情况下，我们也可以采用更简单的方法，然后执行以下操作:

The above requires C++17. It is implementable on C++14, but in your case we can also take a much simpler approach and just do:

template <typename F, typename = decltype(std::declval<F&>()(1, 1))>

F&的原因与上述相同，因此表达式的其余部分更加熟悉.由于我们使用int进行调用，因此我们不在乎INVOKE允许的其他操作(例如，指向成员的指针).

F& for the same reason as above, and the rest of the expression is more familiar. Since we're calling with ints we don't care about the other things that INVOKE allows for (e.g. pointers to members).

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