仿函数(functor)，就是使一个类的使用看上去像一个函数。其实现就是类中实现一个operator()，这个类就有了类似函数的行为，就是一个仿函数类了。
In computer programming, a function object[a] is a construct allowing an object to be invoked or called as if it were an ordinary function, usually with the same syntax (a function parameter that can also be a function). Function objects are often called functors.
A typical use of a function object is in writing callback functions. A callback in procedural languages, such as C, may be performed by using function pointers.[2] However it can be difficult or awkward to pass a state into or out of the callback function. This restriction also inhibits more dynamic behavior of the function. A function object solves those problems since the function is really a fa?ade for a full object, carrying its own state.
