“状态”在 C++ 中是什么意思？

Question

When I was looking for explanations of C++ functor, I saw the following statement, "There are a couple of nice things about functors. One is that unlike regular functions, they can contain state."

Could anybody explain to me what "state" means in C++? Thank you very much.

Answer 1

What does “state” mean ...

The word has multiple meanings and contextual subtleties.

Here is a general definition for the word from a dictionary :

a condition or way of being that exists at a particular time

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... in C++?

There is no C++ specific meaning for the word as far as I know. It is not something specified by the language. The meaning is same as in programming or computer science in general.

Here is a computer science specific definition :

In information technology and computer science, a system is described as stateful if it is designed to remember preceding events or user interactions; the remembered information is called the state of the system.

The state of a C++ program consists primarily of the representation of objects.

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"There are a couple of nice things about functors. One is that unlike regular functions, they can contain state."

While this is "true enough" in practice, it is a simplification. Technically regular functions can "contain" global state. But that is probably ignored by the author of that quote since global state is problematic and something that should be avoided.

Answer 2

Assuming you are actually asking what it means in this context :

It means that a functor (ie an instance of a class implementing the () operator) can store and access information that is related to its particular instance.

A regular function can only access whatever parameters are passed into it, plus global variables etc.

Example with a functor however:

#include <iostream>

struct Counter {
    int operator()() { return ++count; }

  private:
    int count = 0;
};

Counter count1;
Counter count2;

std::cout << count1() << std::endl; // 1
std::cout << count1() << std::endl; // 2
std::cout << count1() << std::endl; // 3

std::cout << count2() << std::endl; // 1
std::cout << count2() << std::endl; // 2
std::cout << count2() << std::endl; // 3

std::cout << count1() << std::endl; // 4
std::cout << count2() << std::endl; // 4

Here, the actual count is encapsulated in the functor instance - it is the functor's state .