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

Like this.. You need it templated, you need the rebind and the types and the allocate and deallocate members. It is also nice to have the operators..

#include 

template
struct Allocator
{
    typedef std::size_t size_type;
    typedef std::ptrdiff_t difference_type;
    typedef T* pointer;
    typedef const T* const_pointer;
    typedef T& reference;
    typedef const T& const_reference;
    typedef T value_type;

    template
    struct rebind {typedef Allocator other;};

    Allocator() throw() {};
    Allocator(const Allocator& other) throw() {};

    template
    Allocator(const Allocator& other) throw() {};

    template
    Allocator& operator = (const Allocator& other) { return *this; }
    Allocator& operator = (const Allocator& other) { return *this; }
    ~Allocator() {}

    pointer allocate(size_type n, const void* hint = 0)
    {
        return static_cast(::operator new(n * sizeof(T)));
    }

    void deallocate(T* ptr, size_type n)
    {
        ::operator delete(ptr);
    }
};

template 
inline bool operator == (const Allocator&, const Allocator&)
{
    return true;
}

template 
inline bool operator != (const Allocator& a, const Allocator& b)
{
    return !(a == b);
}


int main()
{
    std::allocate_shared>(Allocator(), 0);
}

At the very LEAST, an allocator could look like:

template
struct Allocator
{
    typedef T value_type;

    Allocator() noexcept {};

    template
    Allocator(const Allocator& other) throw() {};

    T* allocate(std::size_t n, const void* hint = 0)
    {
        return static_cast(::operator new(n * sizeof(T)));
    }

    void deallocate(T* ptr, size_type n)
    {
        ::operator delete(ptr);
    }
};

template 
inline bool operator == (const Allocator&, const Allocator&)
{
    return true;
}

template 
inline bool operator != (const Allocator& a, const Allocator& b)
{
    return !(a == b);
}

This will also work for allocate_shared.. However, being the type of person I am, I prefer to have all the functions.. Even the ones not required/used by said container/function.