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c++memory-managementstlallocatorc++17

Why did std::allocator lose member types/functions in C++17?

While looking at std::allocator, I see that members:
value_type, pointer, const_pointer, reference, const_reference, size_type, difference_type, and rebind have all been deprecated.

Allocators will also no longer have the members:
address, max_size, construct, or destroy.

Why did this happen? Did it have something to do with polymophic allocators?

Solution

  • If you look at the relevant isocpp paper you can see that the first set you mention is now thought to be better placed in std::allocator_traits. Since the STL (not even standard library) came out, there's been more of a shift to use traits.

    rebind is also a relic. When the STL first came out, aliases and template-template parameters were not supported. With these language features in existence, rebind seems fairly convoluted. E.g., as you can see in an answer to this question, in The C++ Programming Language, 4th edition, section 34.4.1, p. 998, commenting the 'classical' rebind member in default allocator class :

    template<typename U>
     struct rebind { using other = allocator<U>;};

    Bjarne Stroustupr writes this : "The curious rebind template is an archaic alias. It should have been:

    template<typename U>
using other = allocator<U>;

    However, allocator was defined before such aliases were supported by C++."

    So, altogether, it's the standard library catching up with the language and paradigm shifts.