Construction-free allocator for C++ standard library containers

no_construct_allocator.cpp

// Originally written by https://github.com/kobykahane

#include <string>
#include <vector>
#include <memory>
#include <utility>

#include <Windows.h>

template <typename T> class no_construct_allocator : public std::allocator<T>
{
public:
    no_construct_allocator() {}
    template <typename U> no_construct_allocator(const no_construct_allocator<U>& other) {}
    template <typename U> struct rebind { typedef no_construct_allocator<U> other; };

    template <typename U, typename... Args> void construct(U* p, Args&&... args)
    {
        // Needed for other stuff that's constructed with this allocator like the container proxy object used by vector iterators in debug.
        ::new ((void*)p) U(std::forward<Args>(args)...);
    }

    void construct(wchar_t* p)
    {
        // Do nothing for default constructed wchar_t.
    }

};

template <typename T> using no_construct_vector = std::vector<T, no_construct_allocator<T>>;

int main()
{
    no_construct_vector<wchar_t> buf(5000);

    GetEnvironmentVariable(L"PATH", buf.data(), static_cast<DWORD>(buf.size()));
    
    return 0;
}