mismatched std::allocator for some of STL containers

问题

Is it technically valid to use mismatched std::allocator specialization (surely, except its specialization for void) as a template parameter for STL containers (not all of them, but enumerated below plus unordered_(multi)map/set)? Following code compiles fine.

#include <list>
#include <forward_list>
#include <deque>
#include <set>
#include <map>

int main()
{
    struct A { bool operator < (A) const { return true; } };
    struct B {};
    struct C {};
    std::list< A, std::allocator< C > > l;
    std::forward_list< A, std::allocator< C > > fl;
    std::deque< A, std::allocator< C > > d;
    std::set< A, std::less< A >, std::allocator< C > > s;
    std::multiset< A, std::less< A >, std::allocator< C > > ms;
    std::map< A, B, std::less< A >, std::allocator< C > > m;
    std::multimap< A, B, std::less< A >, std::allocator< C > > mm;
}

I believe this is due to allocator being immediately rebound to underlying node type without any relation to its source type.

回答1:

I'd say this is UB (at least in C++11) because specifying an allocator which has a different value_type from the value_type of the container violates the allocator-aware container requirements which means those instances do not conform to the general container requirements. Furthermore, I can't find anything in the C++11 standard that says that the allocator types are to be rebound from the type provided as template parameter.

1. Section [container.requirements.general] tells us:

13) All of the containers defined in this Clause and in (21.4) except array meet the additional requirements of an allocator-aware container, as described in Table 99.

2. The Allocator-aware container requirements says:

Requires: allocator_type::value_type is the same as X::value_type.

Section [default.allocator] specifies

typedef T value_type;

as a member of the allocator template in namespace std.

4. Section [multimap.overview] contains:

template <class Key, class T, class Compare = less<Key>,
    class Allocator = allocator<pair<const Key, T> > >
class multimap {
    [...]
    typedef Allocator allocator_type;
    [...]
 };

(With similar findings for the other containers.)

来源：https://stackoverflow.com/questions/42736112/mismatched-stdallocator-for-some-of-stl-containers

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c++

stl

containers

language-lawyer

allocator