CODEWITHSUNDEEP
c++macrosc-preprocessor
Rodrigo Gurgel
Rodrigo Gurgel

Reputation: 1736

#define with two token after the pattern

Every reference on google only shows easy examples, I have this case on a code:

#define XHANDLER(A,B,H) X_TO_BUS_HANDLER(A,B,H) X_FROM_BUS_HANDLER(A,B,H)

namespace{
   X_TO_BUS_HANDLER( some::SomeClassX,
                     bus::SomeBus,
                     foo::SomeHandler );

Does any one know how this define works? One pattern and two token-lists? References please.

I egrepED the code but only found X_TO_BUS_HANDLER been used.

Upvotes: 4

Views: 373

Answers (3)

Arne Mertz
Arne Mertz

Reputation: 24626

The C/C++ preprocessor will replace the pattern for everything that is written in the same line. In your case it looks as if the two token after that pattern are themselves macros, so they will get expanded as well.

Some example:

#define F(x, y) x f(y yParam);
#define G(x, y) y g(x xParam);
#define FG(x, y) F(x, y) G(x, y);

FG(int, double)

//this is the same as:
int f(double yParam);
double g(int xParam);

In your case I guess the two defines X_FROM_... and X_TO_... create some functions or classes that are handlers for passing an X from or to some bus, respectively. The XHANDLER macro will create handlers for both directions.

Upvotes: 3

Some programmer dude
Some programmer dude

Reputation: 409442

Remember that the preprocessor simply replaces macros with their body. So usage of the macro

XHANDLER(a, b, c)

is simply replaced by the text

X_TO_BUS_HANDLER(a, b, c) X_FROM_BUS_HANDLER(a, b, c)

Upvotes: 5

Luchian Grigore
Luchian Grigore

Reputation: 258648

It works like any other define - whenever the preprocessor encounters XHANDLER, it replaces it with X_TO_BUS_HANDLER(A,B,H) X_FROM_BUS_HANDLER(A,B,H) (and parameters).

In your snippet, the macro isn't used.

But something like

XHANDLER(some::SomeClassX, bus::SomeBus, foo::SomeHandler)

would be equivalent to 

X_TO_BUS_HANDLER(some::SomeClassX, bus::SomeBus, foo::SomeHandler) X_FROM_BUS_HANDLER(some::SomeClassX, bus::SomeBus, foo::SomeHandler)

Upvotes: 6

Related Questions