What can and can't I specialize in the std namespace?

c++, c++11, explicit-specialization, std

Solution

Quoting loosely from the standard:

`numeric_limits` shall not be specialized for non-arithmetic standard types (e.g. `complex<T>`)

"[S]pecializations of `shared_ptr` shall be CopyConstructible, CopyAssignable, and LessThanComparable [and] convertible to `bool`."

"Specializations of `weak_ptr` shall be CopyConstructible and CopyAssignable."

"[T]emplate specializations [of `std::hash`] shall meet the requirements of class template hash."

Anything in `<type_traits>`: "The behavior of a program that adds specializations for any of the class templates defined in this subclause is undefined unless otherwise specified." (only some specializations of `common_type` are explicitly allowed)

Locales have certain required specializations.

All specializations of `istreambuf_iterator` shall have a trivial copy constructor, a constexpr default constructor, and a trivial destructor.

"The effect of instantiating the class template `complex` for any type other than float, double, or long double is unspecified." I take it that means that defining such other specializations is pointless.

"Specializations and instantiations of the `atomic` template shall have a deleted copy constructor, a deleted copy assignment operator, and a constexpr value constructor."

"The class templates `unary_function` and `binary_function` are deprecated. A program shall not declare specializations of these templates."

And of course the overarching clause 17.6.4.2.1, sentence 1 (thanks @sehe and @curiousguy):

The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std unless otherwise specified. A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.

And sentence 2:

The behavior of a C++ program is undefined if it declares

an explicit specialization of any member function of a standard library class template, or

an explicit specialization of any member function template of a standard library class or class template, or

an explicit or partial specialization of any member class template of a standard library class or class template.

A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type and the instantiation meets the standard library requirements for the original template.

Problem

Original source