class My
{
    typedef int My::*SafeBoolType;
    int f;

public:
    operator SafeBoolType() { return &My::f; }
};

int _tmain(int argc, _TCHAR* argv[])
{
    My a;
    bool c = a; // OK
    return 0;
}

class My2
{
    typedef void (My2::*SafeBoolType)();
    void F() {}

public:
    operator SafeBoolType() { return &My2::F; }
};

int _tmain(int argc, _TCHAR* argv[])
{
    My2 a;
    bool c = a; // OK
    return 0;
}

#include <memory>
int main()
{
  std::unique_ptr<int> a;
  bool x = a;
}

U>#include <memory>
U>int main()
U>{
U>  std::unique_ptr<int> a;
U>  bool x = a;
U>}
U>

U>в VS2010 код компилируется

typename add_lvalue_reference<T>::type operator*() const;
pointer operator->() const noexcept;
pointer get() const noexcept;
deleter_type& get_deleter() noexcept;
const deleter_type& get_deleter() const noexcept;
explicit operator bool() const noexcept;

2 A conversion function may be explicit (7.1.2), in which case it is only considered as a user-defined conversion
for direct-initialization (8.5). Otherwise, user-defined conversions are not restricted to use in assignments
and initializations. [ Example:

class Y { };

struct Z {
  explicit operator Y() const;
};

void h(Z z) {
  Y y1(z); // OK: direct-initialization
  Y y2 = z; // ill-formed: copy-initialization
  Y y3 = (Y)z; // OK: cast notation
}

U>помогите поковыряться в его коде

struct Z
{
  explicit operator bool() const { return true; }
};

int main()
{
}

error C2071: 'Z::operator bool' : illegal storage class

unique_ptr
{
  ...
  
   _OPERATOR_BOOL() const
        {    // test for non-null pointer
        return (this->_Myptr != pointer() ? _CONVERTIBLE_TO_TRUE : 0);
        }  
}