部分模板特化：std::allocator_traits?

Question

Is it possible to specialize the std::allocator_traits, template like this?

namespace falloc_ {

  template<class Tp> class FAllocator ;

}

// partial spec for all falloc_::FAllocator<U>, std::allocator_traits

template<typename Tp> 
struct std::allocator_traits<falloc_::FAllocator<Tp> > {

 using allocator_type = falloc_::FAllocator<Tp> ;
 using value_type = typename allocator_type::value_type ;

 // ...
 // All the components I need here, I will include all the code if You need them.
 // ...

} ;


namespace falloc_ {

  template<class Tp> class FAllocator {

    public:
       using value_type = Tp ;
  
  } ;

}

The complete code compiles and runs in the std++20 allocator class and allocator, feature class (std::allocator_traits) specifications. In that case, my question is actually about practical correctness, whether according to people who know more about the standard, whether similar feature class overloading is forbidden, or simply assume that since the (complete) code is compileable and executable and works according to the intended plan - can I consider that I can use it?

Additionally, I tested the allocator with the code:

  std::vector<int, falloc_::FAllocator<int> > mv ;

  for (size_t i = 0 ; i < 5 ; i++) {
    mv.push_back(i) ;
  }

  for (int i = 4 ; i > -1 ; i--) {
    std::cout << "mv[" << i << "]: " << mv[i] << '\n' ;
  }

Memory was allocated and freed correctly, I also checked with, Valgrind. Using Gdb, I checked and all functions were called from

std::allocator_traits<falloc_::FAllocator<U> > // U aka int for std::vector<int, falloc_::FAllocator<int> > 

Thank You in advance for any insights, suggestions and advice.

Answer 1

It is permitted to specialize the std::allocator_traits template. See [namespace.std]/2 :

Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace std provided that (a) the added declaration depends on at least one program-defined type and (b) the specialization meets the standard library requirements for the original template.

It is not explicitly prohibited to specialize std::allocator_traits , therefore you may do so for the type FAllocator that you have defined. This rule applies to both full specializations and partial specializations.

Do bear in mind that your specialization must indeed meet all requirements of the original template. The standard already specifies the definition of each member type and the behaviour of each member function to such an extent that if you were to write your own specialization, it would basically be identical to the original, which defeats the purpose of doing so. I suppose you could do something like add logging statements to std::allocator_traits<...>::construct(...) without violating the original requirements, but it's easier to just put those logging statements in the allocator itself. If you insist on putting them in your allocator_traits specialization, you also have to be careful to not run afoul of [dcl.constexpr]/7 .

I cannot see any situation where specializing std::allocator_traits makes sense.