Have macro 'return' a value

Question

I\'m using a macro and I think it works fine -

#define CStrNullLastNL(str) {char* nl=strrchr(str,\'\\n\'); if(nl){*nl=0;}}

So it works to zero out the last

Answer 1

Macro's don't return values. Macros tell the preprocessor to replace whatever is after the #define with whatever is after the thing after the #define. The result has to be valid C++.

What you're asking for is how to make the following valid:

func( {char* nl=strrchr(str,'\n'); if(nl){*nl=0;}} );

I can't think of a good way to turn that into something valid, other than just making it a real function call. In this case, I'm not sure why a macro would be better than an inline function. That's seems to be what you're really asking for.

Answer 2

Returning a value is what inline functions are for. And quite often, said inline functions are better suited to tasks than macros, which are very dangerous and have no type safetly.

Answer 3

#define CStrNullLastNL(str) ({ \
    char* nl=strrchr(str,'\n');\
    if(nl){*nl=0;} \
    nl; \
})

should work.

Edit: ... in GCC.

Answer 4

If you don't have a strict requirement to use only macro, you can do something like this (real life example):

#define Q_XCB_SEND_EVENT_ALIGNED(T) \
    q_xcb_send_event_aligned<T>()

template<typename T> inline
T q_xcb_send_event_aligned()
{
    union {
        T event;
        char padding[32];
    } event;

    memset(&event, 0, sizeof(event));
    return event.event;
}

And then use it in your code like this:

auto event = Q_XCB_SEND_EVENT_ALIGNED(xcb_unmap_notify_event_t);

Answer 5

For a macro to "return a value", the macro itself has to be an expression. Your macro is a statement block, which cannot evaluate to an expression.

You really ought to write an inline function. It will be just as fast and far more maintainable.

Answer 6

Can you use the comma operator? Simplified example:

#define SomeMacro(A) ( DoWork(A), Permute(A) )

Here B=SomeMacro(A) "returns" the result of Permute(A) and assigns it to "B".