What exactly is nullptr?

Question

We now have C++11 with many new features. An interesting and confusing one (at least for me) is the new nullptr.

Well, no need anymore for the nasty mac

Answer 1

When you have a function that can receive pointers to more than one type, calling it with NULL is ambiguous. The way this is worked around now is very hacky by accepting an int and assuming it's NULL.

template 
class ptr {
    T* p_;
    public:
        ptr(T* p) : p_(p) {}

        template 
        ptr(U* u) : p_(dynamic_cast(u)) { }

        // Without this ptr p(NULL) would be ambiguous
        ptr(int null) : p_(NULL)  { assert(null == NULL); }
};

In C++11 you would be able to overload on nullptr_t so that ptr p(42); would be a compile-time error rather than a run-time assert.

ptr(std::nullptr_t) : p_(nullptr)  {  }