一个const成员函数和一个非const成员函数有什么区别？ [英] What's the difference between a const member function and a non-const member function?

问题描述

我非常困惑的const版本和非const版本成员函数如下：

I am very confused about the const version and non-const version member function like below:

value_type& top() { return this.item }
const value_type& top() const { return this.item }

这两个函数有什么区别？

What is the difference between these two functions? In what situation would they be used?

推荐答案

简而言之，它们用于在程序中添加const correctness。

In short, they're used to add 'const correctness' to your program.

value_type& top() { return this.item }

这用于提供对 item 。它被使用，因此您可以修改容器中的元素。

This is used to provide mutable access to item. It is used so you can modify the element in the container.

例如：

c.top().set_property(5);  // OK - sets a property of 'item'
cout << c.top().get_property();  // OK - gets a property of 'item'

这种模式的一个常见示例是返回可变访问到 vector :: operator [int index] 的元素。

One common example for this pattern is returning mutable access to an element with vector::operator[int index].

std::vector<int> v(5);
v[0] = 1;  // Returns operator[] returns int&.

另一方面：

const value_type& top() const { return this.item }

const 访问项。它比以前的版本限制更多 - 但它有一个优势 - 你可以调用它的const对象。

This is used to provide const access to item. It's more restrictive than the previous version - but it has one advantage - you can call it on a const object.

void Foo(const Container &c) {
   c.top();  // Since 'c' is const, you cannot modify it... so the const top is called.
   c.top().set_property(5);  // compile error can't modify const 'item'.
   c.top().get_property();   // OK, const access on 'item'. 
}

按照向量示例：

const std::vector<int> v(5, 2);
v[0] = 5;  // compile error, can't mutate a const vector.
std::cout << v[1];  // OK, const access to the vector.

这篇关于一个const成员函数和一个非const成员函数有什么区别？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！