How can I pass the exact same state of an custom allocator to multiple containers?

Question

I'm writing an allocator with a reference to another instance of some class which tracks the number of allocated bytes.

below is a minimal example of what I'm trying to do (adapted from here), just without the whole memory tracking class, instead I made a reference to some int which collects the bytes which got allocated so far. This reference gets assigned inside main and should be passed to the CustomAllocator:

#include <limits>   // numeric_limits
#include <iostream>
#include <typeinfo> // typeid

// container
#include <vector>
#include <list>
#include <forward_list>

template<typename T>
class CustomAllocator {
public:
    // type definitions
    typedef T value_type; /** Element type */
    typedef T* pointer; /** Pointer to element */
    typedef T& reference; /** Reference to element */
    typedef const T* const_pointer; /** Pointer to constant element */
    typedef const T& const_reference; /** Reference to constant element */
    typedef std::size_t size_type; /** Quantities of elements */
    typedef std::ptrdiff_t difference_type; /** Difference between two pointers */

    template<typename U>
    struct rebind {
        typedef CustomAllocator<U> other;
    };

    // return maximum number of elements that can be allocated
    size_type max_size () const throw() {
        return std::numeric_limits<std::size_t>::max() / sizeof(T);
    }

    CustomAllocator(std::size_t& memAllocated) :
            m_totalMemAllocated(memAllocated) {
        std::cout << "construct " << typeid(T).name() << std::endl;
    }

    CustomAllocator(const CustomAllocator& src) :
            m_totalMemAllocated(src.m_totalMemAllocated) {
        std::cout << "copy construct " << typeid(T).name() << std::endl;
    }

    template<class U>
    CustomAllocator(const CustomAllocator<U>& src) :
            m_totalMemAllocated(src.getTotalMemAllocated()) {
    }

    // allocate but don't initialize num elements of type T
    pointer allocate(size_type num, const void* = 0) {
        m_totalMemAllocated += num * sizeof(T);
        // print message and allocate memory with global new
        std::cout << "allocate " << num << " element(s)" << " of size "
                << sizeof(T) << std::endl;
        pointer ret = (pointer) (::operator new(num * sizeof(T)));
        std::cout << " allocated at: " << (void*) ret << std::endl;
        return ret;
    }

    // deallocate storage p of deleted elements
    void deallocate(pointer p, size_type num) {
        m_totalMemAllocated -= num * sizeof(T);
        // print message and deallocate memory with global delete
        std::cout << "deallocate " << num << " element(s)" << " of size "
                << sizeof(T) << " at: " << (void*) p << std::endl;
        ::operator delete((void*) p);
    }




    // initialize elements of allocated storage p with value value
    // no need to call rebind with this variadic template anymore in C++11
    template<typename _U, typename ... _Args>
    void construct(_U* p, _Args&&... args) {
        ::new ((void *) p) _U(std::forward<_Args>(args)...);
    }

    // destroy elements of initialized storage p
    template<typename _U>
    void destroy(_U* p) {
        p->~_U();
    }

    // return address of values
    pointer address (reference value) const {
        return &value;
    }
    const_pointer address (const_reference value) const {
        return &value;
    }

private:
    std::size_t& m_totalMemAllocated;
};

template<typename T, typename U>
bool operator==(const CustomAllocator<T> a, const CustomAllocator<U>& b) {
    return true;
}

template<typename T, typename U>
bool operator!=(const CustomAllocator<T>& a, const CustomAllocator<U>& b) {
    return false;
}

int main() {

    std::size_t memAllocated = 0;

    CustomAllocator<int> allocatorInstance(memAllocated);

    std::vector<int> foo(allocatorInstance);
    foo.push_back(23);
    foo.push_back(12);
    foo.push_back(8);

    std::cout << "---" << std::endl;

    // here the same
    std::list<double> bar(allocatorInstance);
    bar.push_back(3.44);
    bar.push_back(1.18);
    bar.push_back(2.25);

    std::cout << "---" << std::endl;

    // debug output
    for (auto x : foo)
        std::cout << x << " ";
    for (auto x : bar)
        std::cout << x << " ";

    std::cout << "\nalloc_count: " << memAllocated << std::endl;

    std::cout << '\n';
    return 0;
}

My Problem here is that I don't know how to pass the exact same state (in the example m_totalMemAllocated) of an allocator instance to the other two containers (here: foo and bar). Since the standard says that C++11 allocators can have a state.

Update:

thank you for the answers so far :)

I know that you usually pass CustomAllocators as a template argument to the std containers; like this:

std::vector<int, CustomAllocator<int> > foo;
std::list<double, CustomAllocator<double> > bar;

also see: Difference between allocator supplied as template parameter and allocator supplied as constructor argument in C++ containers?

but here I do have a state which I'm not able to pass along and the default constructor would get called which I can't use unless I give the reference some default value (but that's not what I want either).

Putting the

std::size_t memAllocated = 0;

from main into the global scope would mean that all the containers which use the CustomAllocator would end up using the globally defined memAllocated. But I want to extend it so I can have some additional memory or instance memAllocated2 which then again gets assigned to some other allocator instances.

Side-Note:

For a stateful version of allocators for different containers than STD-Containers, see

How to track memory usage using EASTL?

Answer 1

To make sure that the state is shared among all allocators instances, the first idea would to make it a static member. But that alone would not be enough, because different template instanciations are indeed different types and each would have its own copy of the static member. So I can imagine only 2 ways: make the state an auxilliary class containing only static members, or use a singleton pattern:

Static members of auxilliary class:

struct CustomAllocatorState {
    static std::size_t& m_totalMemAllocated;
}
std::size_t& CustomAllocatorState::m_totalMemAllocated = 0; # do not forget definition...

template<typename T>
class CustomAllocator {
public:
    ...
    pointer allocate(size_type num, const void* = 0) {
        CustomAllocatorState::m_totalMemAllocated += num * sizeof(T);
        ...

Singleton pattern (you could use any other C++ singleton pattern, this one is damned simple but does not resist to the static initialization fiasco):

class CustomAllocatorState {
    CustomAllocatorState(): m_val(0) {}
    static CustomAllocatorState state;

public:
    int m_val;
    static CustomAllocatorState& getState() {
        return state;
    }
};
CustomAllocatorState CustomAllocatorState::state;

template<typename T>
class CustomAllocator {
public:
    ...
    CustomAllocator() :
        state(CustomAllocatorState::getState()) {
            std::cout << "construct " << typeid(T).name() << std::endl;
    }
    ...
    pointer allocate(size_type num, const void* = 0) {
        state.m_totalMemAllocated += num * sizeof(T);
        ...
private:
    CustomAllocatorState& state;
};

Static members in an auxilliary class is probably simpler, but if you already use a singleton pattern in your application, it can make sense to use it here too.