Like member functions and member function arguments, the objects of a class can also be declared as const. an object declared as const cannot be modified and hence, can invoke only const member functions as these functions ensure not to modify the object. 
A const object can be created by prefixing the const keyword to the object declaration. Any attempt to change the data member of const objects results in a compile-time error. 

In C++, const member functions are member functions of a class that are marked with the const keyword. When a member function is marked as const, it means that the function cannot modify the state of the object that it is called on. This is because the this pointer, which points to the object on which the member function is being called, is treated as a pointer to a constant object within the function.

For example, consider the following class: 

class Rectangle {
public:
    Rectangle(int width, int height) : width_(width), height_(height) {}
    int area() const {
        return width_ * height_;
    }
private:
    int width_;
    int height_;
};

The area member function is marked as const, which means that it can be called on const objects of the Rectangle class. The function calculates the area of the rectangle using the width and height member variables, but it doesn't modify them. If the area function were to attempt to modify width_ or height_, the compiler would generate an error.

Const member functions are useful for ensuring that objects are not unintentionally modified by member functions. They can also be used to enable const-correctness in C++, which is a programming paradigm that emphasizes the use of const to prevent unintended modifications to objects.