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CS253 Dangling Pointers

Dangling Pointers

• After delete is called on a pointer, its value (address) is, theoretically, indeterminate. Accessing the value invokes undefined behavior. ☠
• In many implementations, the address is simply left unchanged.
• After a pointer is deleted, the pointer is “stale” or “dangling”. Even though you still have a pointer to the data, you’re not allowed to access the data.

double *laurel = new double(12.34);
cout << *laurel << '\n';     // should be 12.34
delete laurel;
cout << *laurel << '\n';     // value is unknown

double *hardy = new double;  // will probably re-use space
hardy[0] = 56.78;
cout << *laurel << '\n';     // most likely 56.78 for LAUREL

12.34
2.71094e-320
56.78

Another way

Don’t return a pointer to something that will soon go away.

// Return a cheery message:
const char *message() {
    char buf[] = "Hello there, folks!\n";
    const char *p = buf;
    return p;
}

int main() {
    cout << message();              // Why doesn’t this work!?
    return 0;
}

␀␀

No Reference Counting

• C++ has no garbage collection, and so has no built-in reference counting.
• This is bad code. Bad!

string *smith = new string("Brush your teeth after every meal!");
string *jones = smith;
delete smith;
cout << *jones << '\n';