What does the c++ class access keyword "signals" mean?

Question

I came across a declaration similar to this (names changed per NDA):

class Foo
{
    int  bar;

 public:
    explicit Foo (Fu *parent = NULL);

 private:
    void somefunc (String);

 signals:        // ??? what does this do ???
    void windowClosed();
};

This is compiled successfully by g++ 4.4.7 (from about 2012). Additionally, vim recognizes it as a keyword similar to public and private by highlighting them in brown. (Dis)similiarly, vim uses green to highlight the keywords namespace, class, void, int, double, float, char, unsigned, etc.

The Stackoverflow code formatter does not highlight signals above like it does public and private!

It has proved quite difficult to Google for this (lots of noise), but I haven't found anything mentioning it, not even on SO. I also looked in the enhancements section of the g++ documentation.

This codebase is large (23+ million lines), oldish (~1998), and has a distinct accent. For example, the same class definition has the class access private slots: before two member functions. So it is possible there is some #define obfuscation or trickery going on, but I can't find it using grep. It is possible g++ has been altered, but its --version output does not indicate modification.

Answer 1

It isn't a keyword, if you're using a framework such as Qt it adds up 'keywords' to the language to provide additional functionality that isn't part of the standard.

In Qt for example this code gets preproccessed by the MOC(Meta-Object Compiler), signals is defined in qobjectdefs.h as #define signals public, the moc checks for this macro to add meta-code and provide the actual signal-slot functionality, the generated code is put into a file with a name such as 'moc_myClass.cpp'.