C++17 polymorphic memory recources not working

Question

I was trying to understand the c++17 pmr. So I did this and it is not working as I thought, what could go wrong?

template <typename T>
class Memory : public std::experimental::pmr::memory_resource {
  public:
    Memory() { this->memory = allocate(sizeof(T), alignof(T)); }
    void *getMemory() { return this->memory; }

    ~Memory() { deallocate(this->memory, sizeof(T), alignof(T)); }

  private:
    void *do_allocate(std::size_t bytes, std::size_t alignment)
    {
        memory = ::operator new(bytes);
    }
    void do_deallocate(void *p, std::size_t bytes, std::size_t alignment)
    {
        ::operator delete(memory);
    }
    bool do_is_equal(
           const std::experimental::pmr::memory_resource& other) const noexcept
    {
    }
    void *memory;
};

what can be going wrong with my implementation? This is the client..

Memory<std::string> mem;
std::string * st = (std::string*)mem.getMemory();
st->assign("Pius");
std::cout << *st;

Answer 1

The polymorphic resource allocators allocate memory ; that's all they do. Unlike container Allocators, they don't create objects . That's why they return void* s.

Memory resources are not meant to be used by themselves. That's why std::polymorphic_allocator<T> exists. You can also do the object creation/destruction yourself, using placement- new and manual destructor calls.

Also, your memory_resource implementation makes no sense. do_allocate should return the allocated memory, not store it internally. Your function provokes undefined behavior by returning nothing (which your compiler should have warned about).