How to create a C++ 11 non-default-constructible allocator?

Question

This subject came up in this thread about a change to std::list::sort() for Visual Studio 2015:

`std::list<>::sort()` - why the sudden switch to top-down strategy?

The new version of std::list::sort does not require a default constructible std::list, as it only uses iterators, and doesn't create any local lists, so it doesn't matter if lists can't be default constructed. The prior version uses local lists (note - each instance of a list involves a dynamic allocation of a sentinel node):

typedef list<_Ty, _Alloc> _Myt;
    // ...
   const size_t _MAXBINS = 25;
   _Myt _Templist, _Binlist[_MAXBINS];

I'm trying to create a non-default constructible list, with Visual Studio 2015 version to test how the change to std::list::sort() handles this.

First I tried the Microsoft C++ 11 minimal allocator example. udpate - I had to change one line in order for Jonathan Wakely's answer to work and demonstrate the issue:

template <class T>  
struct Mallocator  
{  
    typedef T value_type;  
//  Mallocator() noexcept {}  // replaced this line from the Microsoft example
    Mallocator(T) noexcept {} // no default constructor

    // A converting copy constructor:  
    template<class U> Mallocator(const Mallocator<U>&) noexcept {}  
    template<class U> bool operator==(const Mallocator<U>&) const noexcept  
    {  
        return true;  
    }  
    template<class U> bool operator!=(const Mallocator<U>&) const noexcept  
    {  
        return false;  
    }  
    T* allocate(const size_t n) const;  
    void deallocate(T* const p, size_t) const noexcept;  
};  

template <class T>  
T* Mallocator<T>::allocate(const size_t n) const  
{
    if (n == 0)  
    {  
        return nullptr;  
    }  
    if (n > static_cast<size_t>(-1) / sizeof(T))  
    {  
        throw std::bad_array_new_length();  
    }  
    void* const pv = malloc(n * sizeof(T));  
    if (!pv) { throw std::bad_alloc(); }  
    return static_cast<T*>(pv);  
}  

template<class T>  
void Mallocator<T>::deallocate(T * const p, size_t) const noexcept  
{  
    free(p);  
}  

update - with Mallocator changed to have no default constructor, this now results in a compile error:

typedef unsigned long long uint64_t;
    std::list <uint64_t, Mallocator<uint64_t>> dll; // doubly linked list

Using the suggested change from Jonathan Wakely works and reproduces the issue where the old std::list::sort gets a compile error due to local lists and shows that the new std::list::sort with no local lists works with no default constructor:

    std::list<uint64_t, Mallocator<uint64_t>> dll(Mallocator<uint64_t>(0));

I also tried this method based on a thread here at SO:

struct Allocator {
    void construct(void* p, const void* container) const {};
    void destruct(void* p, const void* container) const {};
};

void* operator new (size_t size, const Allocator& alloc, const void* container)
{
    void* allocated_memory = std::malloc(size);
    if (!allocated_memory) {
        throw std::bad_alloc();
    }

    alloc.construct(allocated_memory, container);
    return allocated_memory;
}

void operator delete(void* p, const Allocator& alloc, const void* container)
{
    alloc.destruct(p, container);
    std::free(p);
}

In main

typedef unsigned long long uint64_t;
// ...
    Allocator alloc;
    std::list<uint64_t> *dll = new(alloc, NULL)std::list<uint64_t>;
    // ...
    operator delete(dll, alloc, NULL);

but this works for both the old and new versions of std::list::sort, so it's getting a default constructor.

So the question was how do I create a non default constructible allocator?

Thanks to the demo from Igor Tandetni and answer from Jonathan Wakely, I was able to change the Microsoft example allocator above (noted in the comments) to not have a default constructor, and reproduce the issue related to the old std::list::sort.

Answer 1

If you default construct a std::list then it will default-construct its allocator, so this variable definition still requires a default constructor:

std::list <uint64_t, Mallocator<uint64_t>> dll; // doubly linked list

If you want to test allocators without default constructors you need to do it differently e.g.

std::list <uint64_t, Mallocator<uint64_t>> dll(Mallocator<uint64_t>(args));

Or:

Mallocator<uint64_t> alloc(some, args, for, your, allocator);
std::list <uint64_t, Mallocator<uint64_t>> dll(alloc);