See also the proposal for language-support for scoped allocations:

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p26...

There are some issues with PMR usage that it hopes to resolve, for example:

   struct Aggregate {
      std::pmr::string data1;
      std::pmr::string data2;
      std::pmr::string data3;
   };

   std::pmr::polymorphic_allocator a1;
   Aggregate ag  = {{"Hello", a1}, {"World", a1}, {"!", a1}};

   std::pmr::vector<Aggregate> va(a1);
   va.emplace_back(std::move(ag));   // Correct allocator retained by moves
   va.emplace_back(ag);              // Error, copied lvalue has wrong allocator
   va.resize(5);                     // Error, new values have wrong allocator
   va.resize(1);                     // OK, remove all objects with bad allocators

The proposed syntax is via a "using" keyword:

   std::pmr::polymorphic_allocator a4;
   ScopeAggregate s1 using a4 {"Hello"};