Aggro wrote:
> I would like to know the answer to this for both C and C++ as I use both
> languages (usually in differen projects).
>
> Assume the code
> if( A && B )
>
> Normally A is executed before B and this is how I have always believed
> it is guaranteed to work. But recently I got second hand information
> that the execution order is not quaranteed. E.g. B could be executed
> before A by some compilers.
>
> Different execution order could cause e.g. application to crash if we
> first test is the pointer valid and then call a function for that
pointer:
> if( p && p->func() )
>
>
> So. Is the execution order guaranteed by the standard in both C and C++
> to be first A and then B or can it change e.g. because of optimization?
> Should I start writing my code in this format to be safe?
>
> if( p )
> if( p->func() )
> ...
Ulrich has answered your question; in C (and probably C++, though I'm
not a C++ expert) both && and || are guaranteed to be executed
left-to-right; and if the result can be generated based on the LHS
alone, the RHS is guaranteed not to be evaluated. Moreover, && and ||
introduce a sequence point between LHS and RHS, so expressions such as
(p && x = *p++) are valid (though not recommended).
Few operators (except ?:, the comma operator, and probably one or two
I've forgotten) have similar semantics. The order of evaluation of the
arguments of most operators is unspecified; in particular, f(x) * 0 and
0 * f(x) may (will?) call f() in either case, even though the result
will always be zero. Similarly A & 0 and 0 & A have no guarantees over
order of evaluation and no sequence points. And finally, x++ * x++ is
undefined behaviour, even though x++ || x++ or x++ && x++ are
well-defined.
Only && and || are "magical" in this way.
Phil
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