哪些算法要求函子是纯函数？
一般来说，该标准要求函子是纯函数，因为允许算法将其函子复制到其核心内容。然而，有一些算法（例如find_if）没有合理的实现来进行任何形式的函子复制。
我们可以依靠这个吗？
Generally the standard requires functors to be pure functions because algorithms are allowed to copy their functors to their heart's content. However, there are some algorithms (e.g. find_if) for which no sane implementation would be doing any form of functor copying.
Can we rely on this?
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我认为您想问的是哪些函子由于在任意时间被复制而需要无状态。
我想不出任何需要使用自由函数的算法，但大多数/全部肯定需要对象本身不保存状态。您可以做的是让对象拥有对算法调用外部保存的状态对象的引用。然后，函子的任何副本都可以适当地修改该状态对象。
I think what you're trying to ask is which functors need to be stateless due to being copied at arbitrary times.
I can't think of any algorithms that require free functions to be used, but most/all certainly require the object itself to not hold state. What you can do is have the object have a reference to a state object that's held outside the algorithm call. Any copy of the functor can then modify that state object appropriately.
“纯函数”是什么意思？
如果你的意思是“函数指针”，那么不是；每个标准算法都可以采用函数对象和函数指针。是的，它们必须是可复制的，但是可复制的函子仍然是函子。
如果你的意思是“可复制”，那么是的。标准库要求赋予算法的类型是可复制的。如果您的仿函数不可复制，您可以使用
boost::reference_wrapper将不可复制的对象包装在充当引用的可复制容器中。请注意，此模板已作为std::reference_wrapper添加到 C++0x。What do you mean by "pure function?"
If you mean "function pointer", then no; every standard algorithm can take function objects as well as function pointers. Yes, they must be copyable, but a functor that is copyable is still a functor.
If you mean "copyable", then yes. The standard library requires that the type given to algorithms be copyable. If your functor isn't copyable, you can use a
boost::reference_wrapper<T>to wrap your non-copyable object in a copyable container that acts as a reference. Note that this template was added to C++0x asstd::reference_wrapper<T>.