Effective C++ Item 15 Provide access to raw resources in resource-managing classes

In last two item, I talk about resource-managing using RAII, now comes to the practical part. Often, we encounter a situation where an API accept raw resource instead of RAII object. For example:

// You have a shared pointer
std::tr1::shared_ptr<Foo> foo(createFoo());

// But the API only accept Foo
void acceptFoo(Foo *foo);

In this situation, you need RAII object return raw resources. And there are commonly two ways to do that

1. Explicitly provide method to return raw resource of RAII object. That's how shared_ptr handles this situation. shared_ptr have get() method to return raw resource it contains. In addition, it overload -> and * method. This way is safe and clear, but still some make might think it not natural, and we have a second way to accomplish it.

2. Implicitly convert RAII object to raw resource.

How to do that? Let's see a example

class Font {
public:
    ...
    operator FontHandle() const {
        return f;
    }

private:
    FontHandle f;
};

We overload operator () to implicitly convert Font to FontHandle and eliminate the use of get(). But this way have a downside.

Font f1(getFont());
// oops, intent to copy a Font object, but wrongly write FontHandler
// yet implicitly convertion hold the candle to the devil
FontHandler f2 = f1;

If f1 is destroyed and FontHandler is released then f2 become dangle pointer.