c++ - 比较 std::functions 是否相等？

Question

我如何比较两个 C++11 std::function 与 operator==，如果两者都说则返回 true 函数引用同一个函数指针？

Answer 1

operator== for std::function 将 std::function 与空指针进行比较，据我所知标准没有提供任何详细信息。

尽管如此，此提升常见问题解答条目，Why can't I compare boost::function objects with operator== or operator!=?提供了一个基本原理，据我所知应该适用于 std::function以及。引用常见问题解答:

Comparison between boost::function objects cannot be implemented "well", and therefore will not be implemented. [...]

然后它概述了与 Preet 类似的请求解决方案，并继续说:

The problem occurs when the type of the function objects stored by both f and g doesn't have an operator==[...]

并解释了为什么这必须在赋值运算符或构造函数中处理，然后继续说:

All of these problems translate into failures in the boost::function constructors or assignment operator, even if the user never invokes operator==. We can't do that to users.

更新

在 Accessing the target of a tr1::function object 中找到标准原理，这是相当古老的，但与提升常见问题解答一致，并说:

operator== is unimplementable for tr1::function within the C++ language, because we do not have a reliable way to detect if a given type T is Equality Comparable without user assistance.