Function objects

What is Function objects?

cppreference中的如下章节介绍了function object:

1) cppreference Function #Function objects

2) cppreference C++ named requirements: FunctionObject

function object的概念可以顾名思义地来进行理解，我们把它拆解开来，逐个理解:

function: 说明它需要支持the function call expression

object: 说明它需要是object，那么这就排除了function lvalue、reference to function

Function object也是first class citizen

C++中object是first class citizen，因此function object也是first class citizen；

cppreference Function #Function objects

Besides function lvalues, the function call expression supports

1) pointers to functions

2) any value of class type that overloads the function-call operator or is convertible to function pointer (including lambda-expressions) (since C++11).

NOTE: lambda expression本质上是一个unnamed function，因此它convertible to function pointer。

上述 "class type that overloads the function-call operator"是functor，在./Functor中进行了描述。

Together, these types are known as *FunctionObject*s, and they are used ubiquitously(无所不在) through the C++ standard library, see for example, usages of BinaryPredicate and Compare.

The standard library also provides a number of pre-defined function object templates as well as the methods to compose new ones (including std::less, std::mem_fn, std::bind, and std::function (since C++11)).

NOTE: 关于standard library提供的内容，参见C++\Language-reference\Functions\Function-as-first-class-citizen\Function-objects\std-function-object。

cppreference C++ named requirements: FunctionObject

Requirements

Expression	Requirements
f(args)	performs a function call

Notes

Functions and references to functions are not function object types, but can be used where function object types are expected due to function-to-pointer implicit conversion.

NOTE: 章节传函数名即可。

Application

Function object是我们首先function as first class citizen的重要方式。