How does the “return value” of operator >> of istream class work?

To return a self ref you just return *this You don't actually declare an internal self reference. You can have reference members, but you must initialize them in the constructor initializer list: class Foo { Foo(int& some_int_ref) :my_ref_member(some_int_ref).

To return a self ref you just return *this. You don't actually declare an internal self reference. You can have reference members, but you must initialize them in the constructor initializer list: class Foo { Foo(int& some_int_ref) :my_ref_member(some_int_ref) istream has another overload to determine its boolean value.

Example of a chainable member (like operator >>, but in this case just an ordinary function) and giving a class a boolean value (although the latter is a complex subject worthy of its own subject): #include class X { bool B; public: X() :B(false) { } X& toggle() { B =! B; return *this; } operator void*() { return B? This : 0; } }; int main() { X x; x.toggle().toggle().toggle(); if (x) std::cout ".

Thanks for clear explanation. So if I want to implement a similar technique like istream class, what should I provide in my class? – Chan Dec 26 '10 at 8:37.

Void*. The loop test resolves to while (inf. Operator void*()!

= NULL). How does a reference can be evaluated as true or false? But supporting conversion to bool or something convertible to bool.

How does the internal implementation of istream actually work? It just returns a reference to itself (return *this) so it can support chaining.

Thanks! So I have to provide the void*() operator in order to put that statement inside the while loop? – Chan Dec 26 '10 at 8:36 You needn't call it explicitly (as you've already seen).

It's a conversion operator. C++ calls it automatically here because it finds the object being used in a boolean context. If there was an operator bool it would call that instead, but there isn't, so it calls operator void* which is convertible to bool.It's good to understand this stuff because it can seriously bite you in the ass.

:) – Mud Dec 26 '10 at 8:40 Thanks Mud, I got it ;) – Chan Dec 26 '10 at 8:54.

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