What is a constant member function c plus plus?

Answer 1

A constant member function is an instance method that can only modify the mutable members of the class instance. This is achieved by implicitly declaring the this pointer constant. The this pointer is a hidden parameter that is implicitly passed to every instance function, and that points to the current instance of the class. Static member functions do not have an implicit this pointer and therefore cannot be declared const.

Consider the following simple class that has both mutable and non-mutable member variables, and both constant and nonconstant member functions:

struct foo

{

void constant()const;

void nonconstant();

private:

int data1;

mutable int data2;

};

void foo::constant()const

{

data1++; // not permitted: the implicit this pointer is declared const

data2++; // ok: data2 is mutable

}

void foo::nonconstant()

{

data1++; // ok: the implicit this pointer is declared non-const

data2++; // ok: data2 is mutable

}

Note that data2 can always be modified since it is declared mutable, but data1 can only be modified by nonconstant member functions. Members should only be declared mutable when they are only used internally by a class (such as when locking a mutex for thread safety), or where a value needs to be calculated and cached the first time it is accessed.