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 newline in a string, really its used to chop off the linebreak when it gets left on by fgets.

So, I'm wondering if I can "return" a value from the macro, so it can be called like

func( CStrNullLastNL( cstr ) ) ;

Or will I have to write a function

Answer 1

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

should work.

Edit: ... in GCC.

Answer 2

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 3

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 4

If you really want to do this, get a compiler that supports C++0x style lambdas:

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

Although since CStrNullLastNL is basically a function you should probably rewrite it as a function.

Answer 5

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".

Answer 6

I gave +1 to Mike because he's 100% right, but if you want to implement this as a macro,

char *CStrNullLastNL_nl; // "private" global variable
#define nl ::CStrNullLastNL_nl // "locally" redeclare it
#define CStrNullLastNL( str ) ( \
    ( nl = strrchr( str, '\n') ), /* find newline if any */ \
    nl && ( *nl = 0 ), /* if found, null out */ \
    (char*) nl /* cast to rvalue and "return" */ \
OR  nl? str : NULL /* return input or NULL or whatever you like */
)
#undef nl // done with local usage

Answer 7

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 8

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 9

to return value from macro:

bool my_function(int a) {
    if (a > 100)return true;
    return false;
}
bool val = my_function(200);

#define my_macro(ret_val,a){\
    if(a > 100 ) ret_val = true;\
    ret_val = false;\
}
bool val; my_macro(val, 200);