What does the const operator mean when used with a method in C++?

Question

Given a declaration like this:

class A {
public:
    void Foo() const;
};

What does it mean?

Google turns up this:

Member functions should be declared with the const keyword after them if they can operate on a const (this) object. If the function is not declared const, in can not be applied to a const object, and the compiler will give an error message.

But I find that somewhat confusing; can anyone out there put it in better terms?

Thanks.

Answer 1

Consider a variation of your class A.

class A {
public:
    void Foo() const;
    void Moo();

private:
    int m_nState; // Could add mutable keyword if desired
    int GetState() const   { return m_nState; }
    void SetState(int val) { m_nState = val; }
};

const A *A1 = new A();
A *A2 = new A();

A1->Foo(); // OK
A2->Foo(); // OK

A1->Moo(); // Error - Not allowed to call non-const function on const object instance
A2->Moo(); // OK

The const keyword on a function declaration indicates to the compiler that the function is contractually obligated not to modify the state of A. Thus you are unable to call non-const functions within A::Foo nor change the value of member variables.

To illustrate, Foo() may not invoke A::SetState as it is declared non-const, A::GetState however is ok because it is explicitly declared const. The member m_nState may not be changed either unless declared with the keyword mutable.

One example of this usage of const is for 'getter' functions to obtain the value of member variables.

@1800 Information: I forgot about mutable!

The mutable keyword instructs the compiler to accept modifications to the member variable which would otherwise cause a compiler error. It is used when the function needs to modify state but the object is considered logically consistent (constant) regardless of the modification.