Do not pass a pointer or reference obtained from an aliased smart pointer

Reason

Violating this rule is the number one cause of losing reference counts and finding yourself with a dangling pointer. Functions should prefer to pass raw pointers and references down call chains. At the top of the call tree where you obtain the raw pointer or reference from a smart pointer that keeps the object alive. You need to be sure that the smart pointer cannot inadvertently be reset or reassigned from within the call tree below.

Note

To do this, sometimes you need to take a local copy of a smart pointer, which firmly keeps the object alive for the duration of the function and the call tree.

Example

Consider this code:

// global (static or heap), or aliased local ...
shared_ptr<widget> g_p = ...;

void f(widget& w)
{
    g();
    use(w);  // A
}

void g()
{
    g_p = ...; // oops, if this was the last shared_ptr to that widget, destroys the widget
}

The following should not pass code review:

void my_code()
{
    // BAD: passing pointer or reference obtained from a non-local smart pointer
    //      that could be inadvertently reset somewhere inside f or its callees
    f(*g_p);

    // BAD: same reason, just passing it as a "this" pointer
    g_p->func();
}

The fix is simple -- take a local copy of the pointer to "keep a ref count" for your call tree:

void my_code()
{
    // cheap: 1 increment covers this entire function and all the call trees below us
    auto pin = g_p;

    // GOOD: passing pointer or reference obtained from a local unaliased smart pointer
    f(*pin);

    // GOOD: same reason
    pin->func();
}

Enforcement

(Simple) Warn if a pointer or reference obtained from a smart pointer variable (Unique_pointer or Shared_pointer) that is non-local, or that is local but potentially aliased, is used in a function call. If the smart pointer is a Shared_pointer then suggest taking a local copy of the smart pointer and obtain a pointer or reference from that instead.

ES: Expressions and statements

Expressions and statements are the lowest and most direct way of expressing actions and computation. Declarations in local scopes are statements.

For naming, commenting, and indentation rules, see NL: Naming and layout.

General rules:

Declaration rules:

Expression rules:

Statement rules:

Arithmetic rules:

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