C++'s max() with initializer list returns strings out of order

Question

max({"a", "b", "z", "x"});  returns "x"

while both

max<std::string>({"a", "b", "z", "x"});

and

max({'a', 'b', 'z', 'x'});

return "z" as they should. Why?

Answer 1

"a", "b" etc. are string literals. These decay to const char* when in the initializer list. max just probably gets the one with the highest address, but since it uses operator< to compare them, and they aren't part of the same array, or one-past the same array, the result of the comparison is unspecified (and the invocation of max would result in undefined behaviour, since the requirement is that the type be LessThanComparable).

The other two invocations of max are well defined. chars such as a, b etc. are integral types with a well defined less-than comparison. And std::string has an operator< that implements a lexicographical comparison.

Answer 2

max({"a", "b", "z", "x"}); attempts to compare pointers. This has undefined behaviour (since the results of the individual comparisons are unspecified). It you wanted to compare pointers, you should write:

std::max({"a", "b", "z", "x"}, std::less<const char*>());

By contrast, max({'a', 'b', 'z', 'x'}); compares integral values, and max<std::string> compares strings.