tt(Shared_ptr)'s default constructor defines a hi(shared_ptr: default)
tt(shared_ptr) not pointing to a particular block of memory:
        verb(
    shared_ptr<type> identifier;
        )
    The hi(shared_ptr: 0-pointer) pointer controlled by the tt(shared_ptr)
object is initialized to tt(0) (zero). Although the tt(shared_ptr) object
itself is not the pointer, its value em(can) be compared to tt(0). Example:
        verb(
    shared_ptr<int> ip;

    if (!ip)
        cout << "0-pointer with a shared_ptr object\n";
        )
    Alternatively, the member ti(get) can be used (cf. section ref(SHAREDMEM)).
