John Smith wrote:
Hi
I'm porting some C++ code to new platforms and have some 1-byte aligned
structures which need a specific size. Since datatypes can vary on different
platforms (which I found out the hard way since longs are not the same size
on win64 and linux x64) I would like to do a check at compile time to make
sure things are correct. This will ease future work.
Is it possible to do something like the following at preprocessing stage:
typedef struct _aaa
{
UINT32 a;
UINT16 b;
}aaa;
#if sizeof(aaa) != (4 + 2)
#error "Wrong settings for this platform"
#endif
The compiler seems to fail no matter which platform I use. I know I can do
this at runtime but it's not useful code so I rather not have it done at
runtime. The only alternative is to enable it as a runtime check when
compiling in debug mode.
Thanks in advance.
-- John
Does your *preprocessor* really evaluate the size of objects? I bet
that's your problem right there. At best, the result of a "sizeof"
isn't known until the compiler runs, and at worst, it isn't known until
run time. Assuming that your struct's size can be evaluated at
compile-time and your compiler is suffiently smart, just put something
like this at the begining of your program (warning: not tested):
if (sizeof(aaa) != (4 + 2))
{
cerr << "Wrong settings for this platform" << endl;
abort();
}
On platforms where the size of the struct really is 6, the compiler will
detect that the condition is always false and optimize it out
completely. On platforms where it isn't, the compiler might even
optimize out the rest of the program.
If you must check this at compile time, I'd suggest putting a check into
your build system. GNU autoconf, for instance, will allow you to
compile and run a trivial program that checks the size of your struct,
and trigger an alert if it's not the size you expected before attempting
to compile your program.
Rennie deGraaf