C++ compilation error: non-const objects cannot call const member functions, how to solve it?

C is a widely used programming language, but various problems are often encountered during the coding process. One such problem is the compilation error: "Const member functions cannot be called on non-const objects". When this problem occurs, the compiler will prompt, which means that when the const function is called, the object is treated as "non-const", which may lead to unpredictable behavior. This article explains this problem and how to fix it.

1. Non-const objects and const member functions

In C, you can add the const keyword after the member function to define it as a const member function, which means that it The object's state is not modified. For example:

class MyClass {
public:
    void changeValue() { m_value = 1; }
    int getValue() const { return m_value; }
private:
    int m_value;
};

In the above code, the changeValue() function is not a const function, but the getValue() function is a const function, it does not change the state of the object, and it has been marked as const.

2. Cause of the problem

In C, calls between non-const objects and const member functions can be implemented, but when a non-const object calls a const member function , the compiler has a compilation error. The reason is that const member functions ensure that the object's state is not modified, so only const objects can call them. Therefore, non-const objects cannot call const member functions, otherwise problems will occur.

3. Solution

To solve this problem, there are two common methods:

3.1 Convert non-const objects to const objects

Calling const member functions on non-const objects is not allowed, but the object can be declared as const. In this way, when the object is in the const state, the const member function can be called. For example:

MyClass obj;
const MyClass& constObj = obj;
int value = constObj.getValue();

In the above code, the non-const object obj is converted into a const object constObj, and then the getValue() function is called. This way compiler errors can be avoided.

3.2 Modify the const member function to a non-const function

If the const member function really needs to modify the state of the object, then it needs to be modified to a non-const function. Before changing the code of a member function, make sure you understand the meaning of the code and the consequences of the modification. If the object is only modified inside the function or the modification is unnecessary, it is better to keep the function as a const function rather than modify it as a non-const function.

4. Summary

In C, calls between non-const objects and const member functions are very common. When the compiler prompts an error: "Non-const objects cannot call const member functions", you can use two common solutions: convert the non-const object to a const object or modify the const member function to a non-const function. In order to effectively avoid this problem, when writing code, be sure to carefully consider the meaning and role of each function and choose the correct function type.

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