Dependent Origination

Forward declaration of a class with namespace

Posted on: July 4, 2012

I saw code like this in a header file:

namespace X {

class Y;

}

I thought that was wieldy and changed it to

class X::Y;

Guess what? It wouldn’t compile!

The reason? Here. In short, C++ only allows fully-qualified names refering to existing (previously declared) entities. They cannot be used to declare new entities. Once you have this wieldy forward declaration, you can reference it in fully qualified name subsequently. I guess the tricky point here is the compiler couldn’t figure out whether X is a class or a namespace for a form of X::Y.

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