CODEWITHSUNDEEP
c++cfunctionreturnreturn-value
ontherocks
ontherocks

Reputation: 1999

Returning by value from a function - Why does it work?

Following is a simple function definition that returns an integer

int myFunc()
{
    int localVar = 99;
    return localVar;
}

and it called in main as 

int main()
{
    int y = myFunc();
    cout << y << endl;
    return 0;
}

This works as it is expected. I want to know why? localVar is a local variable and its value is allocated in stack. It goes out of scope as soon as the function ends. So, localVar would have gone out of scope in the call int y = myFunc(); How/Why is it still able to return the correct value?

Upvotes: 2

Views: 125

Answers (4)

Joseph Mansfield
Joseph Mansfield

Reputation: 110738

There are three things to be aware of here:

  • The object localVar
  • The return value of myFunc
  • The object y

Yes, localVar goes out of scope at the end of myFunc. However, before that happens, its value is being copied into the return value of myFunc (that's what the return statement does). Then, this return value is being copied into the object y. It doesn't matter that localVar is now gone - you have a copy of it.

That's exactly what returning by value does. It copies the result of the expression in the return statement into a return value.

Upvotes: 2

SLaks
SLaks

Reputation: 887877

Your function is returning a copy of the value – that's what "return by value" means.
int y = myFunc(); will copy the bytes of the value from a temporary location used by the function invocation to your local variable.

It would only fail if you return the address of the local variable.

Upvotes: 6

ryyker
ryyker

Reputation: 23226

Because of the return type of your function: int myFunc(). It allows you to pass back an int value. The variable localVar is indeed out of scope in main(), but localVar is not being used there, y is, and when used as y = myFunc(), it accepts the int value returned.

Upvotes: 0

RedX
RedX

Reputation: 15185

localVar must not be generated on the stack. If a register is available it could be used also.

If it was in fact created on the stack then the value is copied to a register anyway on returning.

Upvotes: 0

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