Returning local reference from a function

[eyali]

Hi all
Im new to c++ and Im trying to figure somthing out for a few hours now.

I dont get why the following piece of code works

int & func()
{
int x = 5;
int & a = x;
return a;
}

and when I cout << func() i get 5 in the console window

after all x is a local variable and itt not define after func returns.

The following does not compile

int & func()
{
int x = 5;
return x;
}

I dont understand the difference between the 2 function

please help me :)

Thank u in advance

[Peter87]

The first function doesn't return anything so the program has undefined behaviour and anything could happen.

Are you sure the second function gives you an error and not a warning? The problem with that function is that it returns a reference to a variable that is local in the function. When the function ends x will no longer exist so you end up returning a reference to a variable that does not exist. Trying to use that value gives undefined behaviour.

[eyali]

thank u for ur reply

I typed the first function without any return statement...I fixed it now thank u.
I still dont understand why when returning a reference the variable the function return stores number 5?
after all x is destroyed when the function end. Like ur argument for the second function in my original post...

Thank u in advance

[vlad from moscow]

Let consider at first the second code.

int & func()
 {
 int x = 5;
 return x;
 }

Here a local variable created with the return statement is bounded to non-const reference. Such a program is ill-formed. So the compiler issue a diagnostic message.


Now consider the first example

int & func()
 {
 int x = 5;
 int & a = x;
 return a;
 }
 

In this example a local variable is not created while the return statement is executed. You are returninig a reference. So compiler does not issue an error though this code is also ill-formed.
As for me in this example the compiler also shall issue a diagnostic message. This is determined by the quality of the compiler.

[eyali]

I see

Thank u:-)