Why use “b < a ? a : b” instead of “a < b ? b : a” to implement max template?

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C++ Templates - The Complete Guide, 2nd Edition introduces the max template:

template<typename T> T max (T a, T b) {   // if b < a then yield a else yield b   return  b < a ? a : b; } 

And it explains using “b < a ? a : b” instead of “a < b ? b : a”:

Note that the max() template according to [StepanovNotes] intentionally returns “b < a ? a : b” instead of “a < b ? b : a” to ensure that the function behaves correctly even if the two values are equivalent but not equal.

How to understand "even if the two values are equivalent but not equal."? “a < b ? b : a” seems have the same result for me.


The standard defines std::max(a, b) as follows [alg.min.max] (emphasis is mine):

template<class T> constexpr const T& max(const T& a, const T& b); 

Requires: Type T is LessThanComparable (Table 18).

Returns: The larger value.

Remarks: Returns the first argument when the arguments are equivalent.

Equivalent here means that !(a < b) && !(b < a) is true [alg.sorting#5].

In particular, if a and b are equivalent, both a < b and b < a are false, so the value on the right of : will be returned in the conditional operator, so a has to be on the right, so:

a < b ? b : a 

...seems to be the correct answer. This is the version used by libstdc++ and libc++.

So the information in your quote seems wrong to me...


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