Is std::cout << "test"; true?

Is std::cout << "test"; true?

>>5
iostream::operator<< returns an iostream&, yes. However, iostream defines a cast to void*¹. So, this can be used in a conditional part of an if expression:
while(cin >> x)
  cout << x + 1;

Of course, operator void* isn't a very safe construct. But operator bool isn't so great either: it still allows you to say cout == cin and get true back, or pass it to a function expecting a boolean parameter. It's best to do something like this (where T is the name of the class implementing this pattern):

    typedef bool(T::* unspecified_bool_type)();
    operator unspecified_bool_type() const {
        return convert_to_bool() ? &T::convert_to_bool : unspecified_bool_type();
    }

    bool convert_to_bool() const {
        return !!(rand()%2);
    }

I wrapped it like this, using the CRTP²:

template<class Derived>
struct testable {
    typedef bool(Derived::* unspecified_bool_type)();

    operator unspecified_bool_type() const {
        return static_cast<Derived*>(this)->convert_to_bool() ? &Derived::convert_to_bool : unspecified_bool_type();
    }

protected:
    //Not const, thus, causes an error.
    void _error_cannot_compare_testables_defaultly() {}

    /**
     * \remarks Don't allow comparisons.
     * Declared as a friend function - it will be found due to argument dependent lookup,
     * similar to friend name injection.
     * This function is obviously invalid, and can never compile, but the compiler won't
     * try to instantiate it until someone actually tries to compare.
     */
    template<class T>
    friend bool operator == (const Derived& left, const T& right) {
        left._error_cannot_compare_testables_defaultly(); return false;
    }

    template<class T>
    friend bool operator != (const Derived& left, const T& right) {
        left._error_cannot_compare_testables_defaultly(); return false;
    }
};
_________________________________
_{¹http://www.cplusplus.com/reference/iostream/ios/operator_voidpt.html
²See http://en.wikipedia.org/wiki/Curiously_Recurring_Template_Pattern}