Specialization of 'template<class _Tp> struct std::less' in different namespace
This is still the way to do it. Unfortunately you cannot declare or define functions within a namespace like you would do with a class: you need to actually wrap them in a namespace block.
If you need to specialize a standard algorithm, you can do so in the std namespace. It is the only thing that you are allowed to do inside that namespace according to the standard.
[lib.reserved.names]/1
It is undefined for a C++ program to add declarations or definitions to namespace std or namespaces within namespace std unless otherwise specified. A program may add template specializations for any standard library template to namespace std. Such a specialization (complete or partial) of a standard library template results in undefined behavior unless the declaration depends on a user-defined name of external linkage and unless the specialization meets the standard library requirements for the original template
Now, the question is whether you actually want to specialize std::less. Note that std::less will call the comparison operator defined for your type, so you can provide that operation instead of specializing the template.
The problem with specializing std::less for your particular type is that it will cause confusion if you provide a different operation than the one performed by operator< for your type. If they perform the same operation, just leave the default std::less definition without specialization.
If you do not want to provide the comparison operator, but still want to use the type in associative containers or with algorithms that require a comparator, you can provide an external comparison functor by other name that will not confuse other readers (and yourself somewhere in the future).
Why are you even doing this?
std::less exists for two purposes only:
- to give a name to operator <, allowing it to be passed as a functor
- to explicitly allow comparing two pointers that aren't in the same array (which is technically illegal if done with raw pointers)
There's no reason for a user to overload it - either overload operator< or use a custom comparator function.
There are std algorithms that can be sensibly specialized - std::swap is a good example - and to do so you do need to declare the specialization inside namespace std.