What is the purpose of methods that do not change the state of an object?

Methods that do not change the state of an object (often called const methods in C++) are designed to ensure safety and predictability of the code. They guarantee that calling such a method will not modify the internal data of the object.

The purposes of such methods:

• Ensuring immutability: allows using the object in contexts where changing state is undesirable or prohibited.
• Improving code readability and maintainability: the programmer immediately sees that the method will not modify the object.
• Allowing calls on const objects: for example, if an object is declared as const, only const methods can be called.

Example in C++:

class MyClass {
    int value;
public:
    int getValue() const { return value; } // does not modify state
    void setValue(int v) { value = v; }   // modifies state
};

const MyClass obj;
int val = obj.getValue(); // allowed
// obj.setValue(10); // compile error