I want to provide a hashing functor for a class I am writing so that it
can be a key in an unordered associative container.

Let us call the class MyClass.  Is it acceptable to specialize
std::hash directly (requiring it to be specialised within the std
namespace as std::hash<MyClass>), in order that the default hash type
for unordered associative containers will work with MyClass without
further intervention, or is that regarded as unlawful so that I would
have to explicitly pass a named functor type existing outside std
namespace when instantiating any relevant associative container?

Chris


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On 2011-03-25 17:02, Chris Vine wrote:
> Let us call the class MyClass.  Is it acceptable to specialize
> std::hash directly (requiring it to be specialised within the std
> namespace as std::hash<MyClass>), in order that the default hash
> type for unordered associative containers will work with MyClass
> without further intervention, [...]

Specializing std::hash for MyClass is allowed in that case and common
practice. Citing from the C++0x N3000 draft, 17.6.3.2.1.1 (but the
standards should contain a similar section):

"[...] 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 of external linkage and the specialization meets the
standard library requirements for the original template and is not
explicitly prohibited."

Cheers

Thomas Luzat


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