C++ functions returning a reference and memory

Question

I have in my project a couple of functions that create objects. For example, if you have a function that makes an object like this:

int& f()
{
    int *i = new int(5);
    return *i;
}

and then when you call that you do

int x = f();

or something like

std::cout << f();

what happens to the int that was created in the function? Is the reference handled like a normal variable or am I doing something terribly wrong? And if so, what is the correct way to make objects?

Community · Accepted Answer · 2017-05-23 12:32:52Z

7

This is terribly wrong, indeed. The moment you forget about your reference is the moment you'll leak a resource. And you're doing just that with int x = f();.

Same with the second case, std::cout << f(); - don't use this at all if you aren't going to delete that dynamically allocated int.

^{int is also passed to std::basic_ostream::operator<< by value, a copy is made. This doesn't actually matter, no normal code would accept a reference and then call delete &ref;.}

Here's a link for you: RAII.

I can't know what you want from your contrived example, but consider returning by value, or using smart pointers in case of polymorphism.

edited May 23, 2017 at 12:32

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answered Jul 31, 2016 at 18:18

LogicStuff

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5 Comments

user2271513 Over a year ago

So if I wanted to have a function that initialises an object then returns a reference to that object so you don't have to initialise two objects how would I go about that?

LogicStuff Over a year ago

Do you mean to avoid a copy? RVO should be able to handle that.

Bob__ Over a year ago

@user2271513 Shouldn't you use a constructor to initialize an object?

user2271513 Over a year ago

In this case the constructor is called in the function and then you do some other stuff to the object and then return it. So I have a class with a member function that returns an object of that class based on the members of the class

LogicStuff Over a year ago

@user2271513 Returning such an object by value is perfectly valid common practice. RVO will kick in. Don't optimize prematurely.

AnatolyS · Accepted Answer · 2016-07-31 19:08:05Z

0

if you want to show that function delegates ownership of the created object with new you have to do it explicitly for example with std::unique_ptr. As you can see your function becomes self documented and you need not look into the body to understand who response for deleting:

std::unique_ptr<int> foo() {
   return std::make_unique<int>(5);
}

edited Jul 31, 2016 at 19:08

answered Jul 31, 2016 at 18:58

AnatolyS

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