c++ - 为什么这个重载函数的调用不明确？

Question

为什么这个构造函数调用不明确？

#include <functional>

class A {
    std::function<int(void)> f_;
    std::function<float(void)> g_;
public:
    A(std::function<int(void)> f) { f_ = f; }
    A(std::function<float(void)> g) { g_ = g; }
    ~A() {}
};

int main()
{
    A a([](){ return (int)1; });
    return 0;
}

注意类型转换。

有没有办法告诉编译器使用哪个构造函数重载？

Answer 1

这是标准中的一个缺陷。参见 DR 2132 :

Consider the following:

#include <functional>

void f(std::function<void()>) {}
void f(std::function<void(int)>) {}

int main() {
  f([]{});
  f([](int){});
}

The calls to f in main are ambiguous. Apparently because the conversion sequences to std::function from the lambdas are identical. The standard specifies that the function object given to std::function "shall be Callable (20.8.11.2) for argument types ArgTypes and return type R." It doesn't say that if this is not the case, the constructor isn't part of the overload set.

尝试使用函数指针作为参数:

A(int f()) { f_ = f; }
A(float g()) { g_ = g; }