Faulty implementation of shared_ptr(nullptr_t) constructor by Jim Barry

The constructor that initializes a shared_ptr from nullptr is faulty. According to the C++11 Standard (section 20.7.2.2), this constructor should be equivalent to the default constructor:

constexpr shared_ptr(nullptr_t) : shared_ptr() { }

Unfortunately, the implementation in Visual C++ 2012 looks like this:

shared_ptr(nullptr_t)
        {    // construct with nullptr
        _Resetp((_Ty *)0);
        }

This causes the wasteful allocation of an internal control block, which pointlessly guards a null pointer. It also causes a compiler error in the case that "_Ty" has a protected destructor.