I have seen debug printfs in glibc which internally is defined as (void) 0
, if NDEBUG is defined. Likewise the __noop
for Visual C++ compiler is there too. The former works on both GCC and VC++ compilers, while the latter only on VC++. Now we all know that both the above statements will be treated as no operation and no respective code will be generated; but here's where I've a doubt.
In case of __noop
, MSDN says that it's a intrinsic function provided by the compiler. Coming to (void) 0
~ Why is it interpreted by the compilers as no op? Is it a tricky usage of the C language or does the standard say something about it explicity? Or even that is something to do with the compiler implementation?
(void)0
(+;
) is a valid, but 'does-nothing' C++ expression, that's everything. It doesn't translate to the no-op
instruction of the target architecture, it's just an empty statement as placeholder whenever the language expects a complete statement (for example as target for a jump label, or in the body of an if
clause).
From Chris Lutz's comment:
It should be noted that, when used as a macro (say, #define noop ((void)0)
), the (void)
prevents it from being accidentally used as a value (like in int x = noop;
).
For the above expression the compiler will rightly flag it as an invalid operation. GCC spits error: void value not ignored as it ought to be
and VC++ barks 'void' illegal with all types
.