Re: Why this code must add sizeof()?
On Apr 9, 2:34 pm, "aiooua" <aio...@gmail.com> wrote:
On Apr 9, 2:08 pm, "nevergone" <wyx2006s...@gmail.com> wrote:
so my question is why " (void)sizeof( CompileTimeChecker< (expr) != 0=
( (ERROR_##msg() ) )); \" must add sizeof?
in general, checks for incomplete types are better done within sizeof
because it's runtime overheads are minimum. try to generate the
assembly code (using, g++ -S) with and without the "sizeof" and you'll
see that the sizeof version is better.
however, i have a related, but different problem. i tried the code as
printed in the book[#1] and couldn't get it to work.
---
/* begin code */
template<bool> struct CompileTimeChecker{ CompileTimeChecker(...); };
template<> struct CompileTimeChecker<false> { };
#define STATIC_CHECK(expr, msg) { class ERROR_##msg {};
(void)sizeof(CompileTimeChecker<(expr) != 0>((ERROR_##msg()))); }
I'm not sure about the exact error message you got---it seems a
little strange. But this line is definitly illegal; for
starters, it contains a } for which there is no {. And ##
operators are only legal in macros.
Are you sure you didn't forget a \ at the end of the previous
line? This line would make sense as part of the macro.
int main(){ STATIC_CHECK(1>2,mustFail);}
/* end code */
---
here's what g++ gives me:
---> g++ -Wall -pedantic test.cpp
test.cpp: In function `int main()':
test.cpp:4: warning: ISO C++ forbids applying `sizeof' to a function
type
Which, as I say, is wierd. The only way a compiler could
possibly parse the sizeof here is as part of an expression
statement. (But of course, expression statements are not legal
at namespace scope.)
Note that if you did mean for this line to be part of the macro,
the line number in the error message should tip you off
immediately about the forgotten \. You never get error messages
from continuation lines in a macro. (Just error messages from
where you invoke the macro.)
---
apart from that warning, the compilation goes through.
shouldn't it fail? what am i missing?
Probably just an \.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orient=E9e objet/
Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34