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Before you can return a pointer from a function, you must specify that you’re returning one. That’s what I do in the refToElement() header.
string* ptrToElement(vector<string>* const pVec, int i)
By starting the header with string*, I’m saying that the function will return a pointer to a string object (and not a string object itself). To specify that a function returns a pointer to an object of a particular type, put an asterisk after the type name of the return type.
The body of the function ptrToElement() contains only one statement, which returns a pointer to the element at position i in the vector pointed to by pVec.
return &((*pVec)[i]);
The return statement might look a little cryptic, so I’ll step through it. Whenever you come upon a complex expression, evaluate it like the computer does—by starting with the innermost part. I’ll start with (*pVec)[i], which means the element in position i of the vector pointed to by pVec. By applying the address of operator (&) to the expression, it becomes the address of the element in position i of the vector pointed to by pVec.
Tra p
Although returning a pointer can be an efficient way to send information back to a calling function, you have to be careful not to return a pointer that points to an out-of-scope object. For example, the following function returns a pointer that, if used, could crash the program.
string* badPointer()
{
string local = "This string will cease to exist once the function ends."; string* pLocal = &local;
return pLocal;
}
That’s because badPointer() returns a pointer to a string that no longer exists after the function ends. A pointer to a non-existent object is called a dangling pointer. Attempting to dereference a dangling pointer can lead to disastrous results. One way to avoid dangling pointers is to never return a pointer to a local variable.
