Simple Macro Function Pointer Trick...

[Chris Thomasson]

I was wondering if the following technique will produce undefined behavior:
_______________
#include <cstdio>

#define CALL_MACRO_FUNCTION(func_ptr)func_ptr()

#define MY_MESSAGE() "Press <ENTER> to exit."

int main(void) {
puts(CALL_MACRO_FUNCTION(MY_MESSAGE));
getchar();
return 0;
}

_______________


I define 'MY_MESSAGE' as a macro function and only pass the name of it to
'CALL_MACRO_FUNCTION' which in turn uses the name to actually invoke it. The
name of a macro function is analogous to a function pointer. This is useful
when you need to have precise control over when a macro will actually
expand.

 

[James Kanze]

I was wondering if the following technique will produce undefined behavior:
_______________
#include <cstdio>

#define CALL_MACRO_FUNCTION(func_ptr)func_ptr()

#define MY_MESSAGE() "Press <ENTER> to exit."

int main(void) {
puts(CALL_MACRO_FUNCTION(MY_MESSAGE));
getchar();
return 0;
}
_______________

Not related to your actual question, but the above shouldn't
compile with a conformant implementation of <cstdio>. puts and
getchar should be in std, and no where else. (I don't think any
of the implementations are conformant in this regard. But

I define 'MY_MESSAGE' as a macro function and only pass the
name of it to 'CALL_MACRO_FUNCTION' which in turn uses the
name to actually invoke it. The name of a macro function is
analogous to a function pointer. This is useful when you need
to have precise control over when a macro will actually
expand.

It should work. When expanding a macro, the preprocessor
rescans the substituted text for further expansions, and it only
recognizes a function style macro if the name is immediately
followed by an opening parentheses. (Thus, for example, in C,
getchar could be a function style macro. But in expressions
like:
int (* pf)() = &getchar ;
or
(getchar)() ;
the macro is not recognized, and the actual function which it
hides is used.)

 

[Chris Thomasson]

I was wondering if the following technique will produce undefined
behavior:
_______________
#include <cstdio> [...]
_______________

Not related to your actual question, but the above shouldn't
compile with a conformant implementation of <cstdio>.

[...]

DOH! Ah crap. Of course your correct. Here is some better code:

________________
#include <cstdio>

#define CALL_MACRO_FUNCTION(func_ptr)func_ptr()

#define MY_MESSAGE() "Press <ENTER> to exit."

int main(void) {
{
using namespace std;
puts(CALL_MACRO_FUNCTION(MY_MESSAGE));
getchar();
}

return 0;
}

________________

It should work.

Yeah. I agree. BTW, thank you for you response.