Why std::sort doesn't accept Compare classes declared within a function

Question

I was at work, writing the Comparators in a function (to move later, when I decided where was best), and noticed this peculiarity. I thought about it for a while, and reali

Answer 1

Your code is fine in C++11; there was a restriction about using local types as template arguments in C++03.

Answer 2

In C++03, template arguments could not have internal linkage:

[C++03: 14.6.4.2/1]: For a function call that depends on a template parameter, if the function name is an unqualified-id but not a template-id, the candidate functions are found using the usual lookup rules (3.4.1, 3.4.2) except that:

• For the part of the lookup using unqualified name lookup (3.4.1), only function declarations with external linkage from the template definition context are found.
• For the part of the lookup using associated namespaces (3.4.2), only function declarations with external linkage found in either the template definition context or the template instantiation context are found.

[..]

This was changed (issue #561: "Internal linkage functions in dependent name lookup") in C++11:

[C++11: C.2.6]: 14.6.4.2
Change: Allow dependent calls of functions with internal linkage
Rationale: Overly constrained, simplify overload resolution rules.

resulting in:

[C++11: 14.6.4.2/1]: For a function call that depends on a template parameter, the candidate functions are found using the usual lookup rules (3.4.1, 3.4.2, 3.4.3) except that:

• For the part of the lookup using unqualified name lookup (3.4.1) or qualified name lookup (3.4.3), only function declarations from the template definition context are found.
• For the part of the lookup using associated namespaces (3.4.2), only function declarations found in either the template definition context or the template instantiation context are found.

[..]

(Spot the missing "with external linkage" qualification.)

Since your main()::ComparitorInner& has internal linkage, and the instantiation of std::sort requires this type to be a template parameter (albeit deduced), your code is only valid in C++11.