问题
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&?
回答1:
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.
来源：https://stackoverflow.com/questions/41580521/is-it-still-possible-to-customize-stl-vectors-reference-type