[CPP] Using const

Source:

• "Understanding the const qualifier" by Carlo Wood
• "C++ Pitfalls" by Scott Wheeler

Declaring a const variable

• General Rule: The 'const' qualifier works on everything on the left of it.
• Convention: If a type T is not a reference or a pointer, a constant variable of type T can be declared by putting 'const' in front of T

Passing reference-to-const v.s. passing pointer-to-const

Taking the pointer to a temporary is not allowed but it is possible to pass a temporary to a function as a reference-to-const:

void foobar(T const&);
foobar( T() ); // Allowed.

Constant/non-constant method and constant/non-constant returned value

class Foo
{
public:

/*
* Modifies m_widget and the user
* may modify the returned widget.
*/
Widget *widget();

/*
* Does not modify m_widget but the
* user may modify the returned widget.
*/
Widget *widget() const;

/*
* Modifies m_widget, but the user
* may not modify the returned widget.
*/
const Widget *cWidget();

/*
* Does not modify m_widget and the user
* may not modify the returned widget.
*/
const Widget *cWidget() const;

private:

Widget *m_widget;

}

• Const method: A method that does not modify the member variables of a class. Calling non-constant methods within a const method is not allowed.