C++ standard requires allocation functions (malloc() and operator new()) to allocate memory suitably aligned for any standard type. As these functions don’t receive the alignment requirement as an argument, in practice it means that the alignment for all allocations is the same, and is that of a standard type with the largest alignment requirement, which … Read more
The allocator requirements table says that construct(c, args), if provided, must “construct an object of type C at c“. It says absolutely nothing about 1) what arguments are to be passed to C‘s constructor or 2) how these arguments are to be passed. That’s the allocator’s choice, and in fact two allocators in the standard … Read more
When std::vector<T> runs out of capacity it has to allocate a new block. You have correctly covered the reasons. IMO it would make sense to augment the allocator interface. Two of us tried to for C++11 and we were unable to gain support for it: [1] [2] I became convinced that in order to make … Read more
Edit: I removed the inheritance of std::allocator as suggested by GManNickG and made the alignment parameter a compile time thing. I recently wrote this piece of code. It’s not tested as much as I would like it so go on and report errors. 🙂 enum class Alignment : size_t { Normal = sizeof(void*), SSE = … Read more
Choice quote from cppreference: This runtime polymorphism allows objects using polymorphic_allocator to behave as if they used different allocator types at run time despite the identical static allocator type The issue with “regular” allocators is that they change the type of the container. If you want a vector with a specific allocator, you can make … Read more
As I mention here, I’ve seen Intel TBB’s custom STL allocator significantly improve performance of a multithreaded app simply by changing a single std::vector<T> to std::vector<T,tbb::scalable_allocator<T> > (this is a quick and convenient way of switching the allocator to use TBB’s nifty thread-private heaps; see page 7 in this document)