Mail Archives: djgpp/1999/06/22/06:33:51
A. Jans-Beken writes:
>This works all fine. But now I want to wrap this function...
>
> textprintf(BITMAP *bmp, FONT *f, int x, y, color, char *fmt, ...);
>
>I do not understand how to do this. The C++ primer that I have (old
>edition) is inconclusive. A gues would be:
>
> void textprintf(int x, y, color, char *fmt, ... ) {
> ::textprintf(bob, font, x, y, color, fmt, ... );
> }
>
> But the compiler doesn't like this at all.
Standard C doesn't provide any way to do this. If you think about it
a bit, the reasons should be obvious. This function has been passed
an unknown number of parameters on the stack, and the only way for
it to find out how many and what type of parameters these are, is
for it to parse the fmt string and understand the tokens within it.
Now you want to pass a copy of these arguments on to a new function,
which requires creating a new argument block on the stack, but how
can the compiler know how much data to copy? At this point, it knows
nothing about the details of printf-style format specifiers, which
is the only way to find that information.
If you understand the layout of parameters on the stack, you could
use some inline asm to fake your call to the Allegro routine into
using the exact same parameters as you were called with (you'd need
to remove your stack frame, and then jump directly to the Allegro
function). This is very non-portable, though, and prone to error
if you aren't very careful about how the compiler can optimise
around your asm fragment.
Alternatively, there is a gcc extension that can help with this
situation: see info "c extensions" "constructing calls". I'm not
sure if this will work with a C++ method, though.
Shawn Hargreaves.
- Raw text -