编程参考 - 在C++类成员函数声明中使用const关键字

在 C++ 中，可以在类成员函数声明中使用 const 关键字来表示函数不修改对象的状态。这就是所谓的 "const 成员函数"。将成员函数声明为 const 时，意味着该函数承诺不更改对象的任何成员变量（标记为可变的变量除外）。

In C++, the const keyword can be used in class member function declarations to indicate that the function does not modify the state of the object. This is known as a "const member function." When you declare a member function as const, it means that the function promises not to change any member variables of the object (except those marked as mutable).

Syntax

要将成员函数声明为 const，需要在函数的参数列表后加上 const 关键字：

To declare a member function as const, you place the const keyword after the function's parameter list:

class MyClass {

public:

    void myFunction() const;

};

Benefits of Using const Member Functions

1. 确保对象的完整性： 通过将函数声明为 const，可以确保调用该函数不会修改对象的状态。这有助于保持对象的完整性，尤其是在处理不可变数据时。

1. Ensures Object Integrity: By declaring a function as const, you ensure that calling this function will not modify the state of the object. This helps in maintaining object integrity, especially when working with immutable data.

2. 允许函数重载： 您可以根据成员函数的常量性对其进行重载。这意味着你可以有两个版本的函数，一个修改对象，另一个不修改对象。

2. Allows Function Overloading: You can overload member functions based on their const-ness. This means you can have two versions of a function, one that modifies the object and one that doesn't.

3. 常量正确性： 它促进了常量正确性的实践，使你的代码更健壮、更易于理解。常量正确性是确保尽可能使用常量的一门学科，它能带来更安全、更可预测的代码。

3. Const Correctness: It promotes the practice of const correctness, making your code more robust and easier to understand. Const correctness is the discipline of ensuring that const is used wherever possible, leading to safer and more predictable code.

Const and Member Variables

const 成员函数仍然可以修改被声明为可变的成员变量。即使在常量成员函数中，mutable 关键字也允许修改成员变量。

A const member function can still modify member variables that are declared as mutable. The mutable keyword allows a member variable to be modified even in a const member function.

class MyClass {

private:

    int value;

    mutable int accessCount; // This can be modified even in a const function

public:

    MyClass(int val) : value(val), accessCount(0) {}

    void display() const {

        ++accessCount; // Allowed because accessCount is mutable

        std::cout << "Value: " << value << ", Access Count: " << accessCount << std::endl;

    }

};

在本例中，accessCount 是一个可变成员变量，因此即使在声明为常量的显示函数中也可以修改。

In this example, accessCount is a mutable member variable, so it can be modified even in the display function, which is declared as const.

在成员函数声明中使用 const 是一种良好的做法，可以防止对对象进行意外修改，从而提高代码的可靠性和可维护性。

Using const in member function declarations is a good practice that enhances the reliability and maintainability of your code by preventing unintended modifications to objects.