I would like to customize std::vector behavior to not default-construct the element type (e.g. int), as it is expensive to do this for a large vector.
Looking at this, the only way I can see to do this is to specialize std::allocator_traits<MyAllocator>::construct. However, this doesn't seem to be possible, because the specialization must be in the same namespace as the original declaration.
Putting a specialization in namespace std already doesn't seem right. And it's actually worse than that, because the STL implementation I am using actually puts std::allocator_traits in namespace std::__u (and that surely varies across STL implementations), so it seems very wrong to do this.
This is confusing because it seems like std::allocator_traits is designed to allow specialization, but I can't figure out how to actually do it. Is this simply a bad idea? If so, is there some other way to solve the problem (avoiding default construction of elements in STL containers)?
Specializing standard library traits classes is not only allowed, it's the main way to provide such functionality. However, in this particular case, it is unnecessary.
The default std::allocator_traits<T>::construct implementation (where T is your allocator type, not the value-type of the container it is being used with) will call the construct member function of T if T has such a function, and it calls placement-new if T doesn't have an appropriate member. So simply give your allocator a construct member and you should be fine.