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c++c++11referencestlallocator

Is it still possible to customize STL vector's "reference" type?

Is it possible to customize the reference of a std::vector. Until C++11 it seemed to be possible through the Allocator template parameter. But not anymore?

According to the documentation, http://en.cppreference.com/w/cpp/container/vector, reference is now always value_type and value_type is always the template parameter T.

It seems to be impossible even using allocator_traits, http://en.cppreference.com/w/cpp/memory/allocator_traits

Is there a workaround for this?

If not, does it means that I have to specialize the entire std::vector and probably reproduce all its functionality if I want to have a special reference type based on the allocator?

If this is so, generally speaking what is the logic for all these constrains? To force the user to use std::vector<T, A> to always manage regular memory (in which, value_type = T, reference = T& and pointer = T*?)

Note: I am aware of the std::vector<bool> controversy. However this a bit more general because in principle I need a custom std::vector<RegularType, special_allocator<RegularType>> mainly to control the return type of operator[](int) to a special proxy object.

Specific implementation: I am looking at GCC 6.3.1's stdlib source and in std_vector.h one can read:

template<typename _Tp, ...>
class vector : ...{
   ...
   public:
   typedef typename _Alloc_traits::reference          reference;
};

which seems to indicate that it is still possible to specify a reference type via the allocator (_traits?).

According to the documentation, allocator or the allocator_traits doesn't need to have a reference type. Plus, I don't know how to customize this type anyway.

Is GCC not following the standard? Or is it simply that indirectly allocator_traits<Allocator>::reference is forced to be allocator_traits<Allocator>::value_type&?

Solution

  • If this is so, generally speaking what is the logic for all these constrains?

    The purpose is to stop promising things C++ cannot deliver.

    Back in the C++98/03 days, it was thought that proxy types, specialized references and the like could really work with standard library containers and algorithms. By the time C++11 rolled around, it had become abundantly clear that... no, they really can't. Or at least, not with the same semantics as an actual language reference.

    With that wisdom in mind, C++11 removed a lot of these no-longer-reasonable customization points. The typedefs were left in, but mainly for backwards-compatibility's sake.

    which seems to indicate that it is still possible to specify a reference type via the allocator (_traits?).

    No, it isn't. This is a bug in libstdc++; reference is required to be value_type& for containers now. allocator_traits has no reference customization point.