What can and can't I specialize in the std namespace?
Users are allowed to add explicit specializations to the std
namespace. However, there are a few templates that I am explicitly forbidden from specializing.
What templates can and can't I specialize?
Solution 1:
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 tobool
." -
"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 ofcommon_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
andbinary_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.