Does the C++ standard guarantee that the call
c = std::min(f(x), g(x));
evaluates the functions f and g only once?
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87answers:
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A:
Yes. Since std::min is a function, f(x) and g(x) will be evaluated only once. And returned values won't be copied. See the prototype of the function :
template<typename T>
const T& min ( const T& a, const T& b );
It is a clear difference with preprocessor-genuinely-defined min macro :
#define MIN(A,B) ((A)<(B))?(A):(B)
Benoît
2010-09-08 07:46:11
hmm... Is it possible to pull off a #define version of min AND make sure it calls each function once? Just curious...
Alexander Rafferty
2010-09-08 08:11:07
For completeness' sake: you cannot however assume the order in which the methods will be evaluated, it's unspecified. For example, `gcc` usually evaluates the arguments from right to left.
Matthieu M.
2010-09-08 08:11:32
@Alexander Rafferty: If it were possible it probably would have been done already in existing MIN implementations. Double evaluation of MIN parameters has been a thorn in the programmers side for a few decades.
Constantin
2010-09-08 08:24:51
@Alexander: #define version of min/max, evaluating its params once is possible, but it still would contain the problem of potentially comparing incompatible data types. Linux has GCC-specific implementation of [type-safe define for min/max](http://lxr.free-electrons.com/source/include/linux/kernel.h#L622).
Dummy00001
2010-09-08 10:39:03