Customizing std::allocator_traits::construct

Question

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)?

Answer 1

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.