Question

I got a book, where there is written something like:

class Foo 
{
public:
    int Bar(int random_arg) const
    {
        // code
    }
};

What does it mean?

Answer 1

A "const function", denoted with the keyword const after a function declaration, makes it a compiler error for this class function to change a data member of the class. However, reading of a class variables is okay inside of the function, but writing inside of this function will generate a compiler error.

Another way of thinking about such "const function" is by viewing a class function as a normal function taking an implicit this pointer. So a method int Foo::Bar(int random_arg) (without the const at the end) results in a function like int Foo_Bar(Foo* this, int random_arg), and a call such as Foo f; f.Bar(4) will internally correspond to something like Foo f; Foo_Bar(&f, 4). Now adding the const at the end (int Foo::Bar(int random_arg) const) can then be understood as a declaration with a const this pointer: int Foo_Bar(const Foo* this, int random_arg). Since the type of this in such case is const, no modifications of data members are possible.

It is possible to loosen the "const function" restriction of not allowing the function to write to any variable of a class. To allow some of the variables to be writable even when the function is marked as a "const function", these class variables are marked with the keyword mutable. Thus, if a class variable is marked as mutable, and a "const function" writes to this variable then the code will compile cleanly and the variable is possible to change. (C++11)

As usual when dealing with the const keyword, changing the location of the const key word in a C++ statement has entirely different meanings. The above usage of const only applies when adding const to the end of the function declaration after the parenthesis.

const is a highly overused qualifier in C++: the syntax and ordering is often not straightforward in combination with pointers. Some readings about const correctness and the const keyword:

Const correctness

The C++ ‘const’ Declaration: Why & How