Manage resources automatically using resource handles and RAII (Resource Acquisition Is Initialization)

Reason

To avoid leaks and the complexity of manual resource management. C++'s language-enforced constructor/destructor symmetry mirrors the symmetry inherent in resource acquire/release function pairs such as fopen/fclose, lock/unlock, and new/delete. Whenever you deal with a resource that needs paired acquire/release function calls, encapsulate that resource in an object that enforces pairing for you -- acquire the resource in its constructor, and release it in its destructor.

Example, bad

Consider:

void send(X* x, string_view destination)
{
    auto port = open_port(destination);
    my_mutex.lock();
    // ...
    send(port, x);
    // ...
    my_mutex.unlock();
    close_port(port);
    delete x;
}

In this code, you have to remember to unlock, close_port, and delete on all paths, and do each exactly once. Further, if any of the code marked ... throws an exception, then x is leaked and my_mutex remains locked.

Example

Consider:

void send(unique_ptr<X> x, string_view destination)  // x owns the X
{
    Port port{destination};            // port owns the PortHandle
    lock_guard<mutex> guard{my_mutex}; // guard owns the lock
    // ...
    send(port, x);
    // ...
} // automatically unlocks my_mutex and deletes the pointer in x

Now all resource cleanup is automatic, performed once on all paths whether or not there is an exception. As a bonus, the function now advertises that it takes over ownership of the pointer.

What is Port? A handy wrapper that encapsulates the resource:

class Port {
    PortHandle port;
public:
    Port(string_view destination) : port{open_port(destination)} { }
    ~Port() { close_port(port); }
    operator PortHandle() { return port; }

    // port handles can't usually be cloned, so disable copying and assignment if necessary
    Port(const Port&) = delete;
    Port& operator=(const Port&) = delete;
};

Note

Where a resource is "ill-behaved" in that it isn't represented as a class with a destructor, wrap it in a class or use finally

See also: RAII