A virtual function header followed by a statement

Question

I am not sure of my interpretation of these instructions:

virtual int getRadioChannel(RadioRef r) const = 0;
virtual int getNumChannels() = 0;

For me, the first one means that a called of the function getRadioChannel returns always 0 and the second one does nothing when the function getNumChannels is called. Am I right ?

Thanks in advance.

http://en.wikipedia.org/wiki/Virtual_function#Abstract_classes_and_pure_virtual_functions — chris, Jan 31 '13 at 09:23
"virtual function header" is already the wrong term. What book are you using to learn C++ and the terms used in that context? — PlasmaHH, Jan 31 '13 at 09:35

score 5 · Answer 1 · edited May 23 '17 at 12:12

5

Am I right ?

No, not at all.

These are declarations of functions that are intended to form, in part, an interface.

Read about pure virtual functions in your C++ book.

edited May 23 '17 at 12:12

Community

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answered Jan 31 '13 at 09:23

Lightness Races in Orbit

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score 2 · Answer 2 · answered Jan 31 '13 at 09:25

These are both declaring pure virtual functions; functions without definition in the class they are declared in. Since there are no definitions of the functions, the class cannot be instantiated; only subclasses of the class, which do have these functions defined, can.

In the first case, it's also const function -- essentially, the you're promising not to modify the object in the body of the function. (Specifically: the this pointer in the function body will be const, and the function is callable on const objects.) The second case is just a "normal" pure virtual function declaration.

You can read more about pure virtual functions here, and const functions here.

score 1 · Accepted Answer · answered Jan 31 '13 at 09:23

1

You're not right, the =0 signifies a pure virtual function.

That makes the class abstract and forces all non-abstract derived classes to implement the methods marked as pure.

answered Jan 31 '13 at 09:23

Luchian Grigore

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score 0 · Answer 4 · edited Jan 31 '13 at 09:31

virtual int getRadioChannel(RadioRef r) const { return 0; }

This is what a function that always returns 0 would look like. As others have pointed out, equating the function to 0, makes it a pure virtual function.

A pure virtual function or pure virtual method is a virtual function that is required to be implemented by a derived class, if that class is not abstract. Classes containing pure virtual methods are termed "abstract"; they cannot be instantiated directly. A subclass of an abstract class can only be instantiated directly if all inherited pure virtual methods have been implemented by that class or a parent class.

A virtual function header followed by a statement

4 Answers4