Item 15 - Provide access to raw resources in resource managing classes.

Item 13 introduces the idea of using smart pointers like auto_ptr or shared_ptr to hold the result of a call to a factory function like createInvestment:

std::shared_ptr<Investment> pInv (createInvestment());

Suppose that a function you'd like to use when working with Investment objects is this:

int daysHeld(const Investment* pi);    // return number of days investment has been held

You'd like to call it like this

int days = daysHeld(pInv);    // error!

but the code won't compile: daysHeld wants a raw Investment* pointer, but you're passing an object of type shared_ptr<Investment>. You need a way to convert an object of the RAII class (in this case shared_ptr) into the raw resource it contains (e.g., the underlying Investment*). There are two general ways to do it: explicit conversion and implicit conversion. shared_ptr and auto_ptr both offer a get member function to perform an explicit conversion, i.e., to return (a copy of) the raw pointer inside the smart pointer object:

int days = daysHeld(pInv.get());        // fine, passes the raw pointer in pInv to daysHeld

Like virtually all smart pointer classes shared_ptr and auto_ptr also overload the pointer dereferencing operators (-> and *), and this allows implicit conversion to the underlying raw pointers:

class Investment {            // root class for a hierarchy of investment types
public:
    bool isTaxFree() const;
    ...
};
Investment* createInvestment();                    // factory function

shared_ptr<Investment> pi1(createInvestment());    // shared_ptr manage a resource
bool taxable1 = !(pi1->isTaxFree());               // access resource via operator->
...
auto_ptr<Investment> pi2(createInvestment()):      // auto_ptr manage a resource
bool taxable2 = !((*pi2).isTaxFree());             // access resource via operator*

Things to Remember

• APIs often require access to raw resource, so each RAII class should offer a way to get to the resource it manages.
• Access may be via explicit conversion or implicit conversion. In general, explicit conversion is safer, but implicit conversion is more convenient for clients.