Unable to use custom allocator with allocate_shared/make_shared

Question

In my C++11 program, I use shared_ptr for some objects which are actively created and deleted. It so happened that standard allocator with operat

Answer 1

Your custom allocator does not meet the C++ Allocator requirements.

In particular, it does not support being rebound to allocate objects of a different type. Usually allocators are templates, parameterized on the type they allocate memory for. allocate_shared needs to rebind the allocator so it can allocate a block of memory of the appropriate size and type, it does not want to allocate an array of char objects.

// MyAlloc is a correct allocator, since allocator_traits can be instantiated

This is not a correct assumption. Instantiating allocator_traits does not instantiate all its members.

Also, is it OK to use char as value_type for this particular allocator

That makes your allocator an allocator of char, but allocate_shared needs an allocator of some_internal_type_defined_by_the_library and so it tries to use std::allocator_traits::rebind_alloc to get an allocator for that type, but your allocator does not support the rebind requirement.

If your allocator is a template of the form MyAlloc then allocator_traits can determine how to rebind it to MyAlloc, otherwise the type MyAlloc::rebind::other needs to be valid.

The C++ standard shows the following as an example of an allocator supporting the minimum requirements for a C++ Allocator type:

template 
struct SimpleAllocator {
  typedef Tp value_type;
  SimpleAllocator(ctor args);
  template  SimpleAllocator(const SimpleAllocator& other);
  Tp* allocate(std::size_t n);
  void deallocate(Tp* p, std::size_t n);
};
template 
bool operator==(const SimpleAllocator&, const SimpleAllocator&);
template 
bool operator!=(const SimpleAllocator&, const SimpleAllocator&);