SFINAE复制构造函数声明 [英] SFINAE Duplicate constructor declaration

问题描述

我想为类创建构造函数，以便编译器在需要时轻松创建类的新实例。

I want to create constructors for a class in a way that the compiler trivially create new instances of it when needed.

这里是一个示例。

class C {
    public:
    C(int) {}; // int constructor
};

然后我声明一个函数：

void F(C _c) {};

我可以用一个int调用它，并由编译器处理C的构造：

I can call this one with an int and have the construction of C handled by the compiler:

F(0); // works

我想做的是实现相同的事情，但是使用lambda作为参数，一些例子：

What I want to do is to achieve the same thing, but with lambdas as parameters, a few examples:

F([]() {});                     //A
F([](int) {});                  //B
F([](int)->int { return 0; });  //C

有了SFINAE，从另一个问题中我学到了以下内容：自动构造函数无法与< functional>对象

With SFINAE and from what I've learned from another question: Auto-constructor not working with <functional> objects

我设法找到一种创建仅匹配特定lambda签名的构造函数的方法，它的工作方式如下：

I managed to sort out a way to create a constructor that matches only a specific lambda signature, it would work out like this:

template<typename F, typename = decltype(function<void(void)>(declval<F&>()))> C(F&& f) {}; //For lambda's like A
template<typename F, typename = decltype(function<void(int)>(declval<F&>()))> C(F&& f) {};  //For lamdba's like B
template<typename F, typename = decltype(function<int(int)>(declval<F&>()))> C(F&& f) {};   //For lambda's like C

现在的问题是，如果我在以下位置添加这三个定义一次，我收到一条错误消息，指出我正在尝试重新定义C的构造函数。这是正确的，因为是的，该构造函数被定义为C（F&& f）三次，但是，我应该如何知道

Now the problem that I have is that if I add these three definitions at once, I get an error stating that I'm trying to redefine the constructor of C. This is correct because, yeah, the constructor's being defined as C(F&& f) three times, however, how should I let know the compiler to use a different signature for each different case?

另一个答案提示我看一下enable_if和is_convertible，但没有设法解决此问题。

The other answer hinted me to look at enable_if and is_convertible but haven't managed to set up a workaround for this issue. Any help is greatly appreciated.

使用：Apple LLVM 6.0版（lang-600.0.57）（基于LLVM 3.5svn）

Using: Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)

推荐答案

您当前的问题是您定义了3次

Your current problem is that you define 3 times

template <typename F, typename> C(F&&);

但具有不同的默认参数（做SFINAE）。

but with different default argument (to do SFINAE).

您可以更改为

// Use to have different type easily
template <std::size_t> struct dummy {}

template<typename F, decltype(function<void(void)>(declval<F&>()), dummy<0>())* = nullptr> C(F&& f);
template<typename F, decltype(function<void(int)>(declval<F&>()), dummy<1>())* = nullptr> C(F&& f);
template<typename F, decltype(function<int(int)>(declval<F&>()), dummy<2>())* = nullptr> C(F&& f);

因此，您有3种不同的签名：

So you have 3 different signatures:

template<typename F, dummy<0>*> C(F&& f);
template<typename F, dummy<1>*> C(F&& f);
template<typename F, dummy<2>*> C(F&& f);

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