User Defined Conversions in C++

Question

Recently, I was browsing through my copy of the C++ Pocket Reference from O\'Reilly Media, and I was surprised when I came across a brief section and example regard

Answer 1

Is this a particularly obscure feature?

Yes, conversion operators aren't used very often. The places I've seen them are for user-defined types that can degrade to built-in ones. Things like a fixed-precision number class that supports converting to/from atomic number types.

Is this relatively portable?

As far as I know, it is. They've been in the standard forever.

Can user-defined conversions to user defined types be done?

Yes, that's one of the features of constructors. A constructor that takes a single argument effectively creates a conversion operator from the argument type to your class's type. For example, a class like this:

class Foo {
public:
    Foo(int n) {
        // do stuff...
    }
}

Let's you do:

Foo f = 123;

If you've used std::string before, odds are you've used this feature without realizing it. (As an aside, if you want to prevent this behavior, declare any single-argument constructors using explicit.)