Return by reference

Question

Please see the following code snippets. In the second function i am returning a reference. I am declaring a local variable in the function and is returning the address. As the variable is local I believe its life ends as it exits the function. My question is why is it possible to access the value from the caller without any exceptions even though the original variable is deleted?

int& b=funcMulRef(20,3);

int* a= funcMul(20,3);


int* funcMul(int x,int y)
{
 int* MulRes = new int;
      *MulRes = (x*y);

 return MulRes;

}

int& funcMulRef(int x,int y)
{
 int MulRes ;
      MulRes = (x*y);

 return MulRes;

}

Regards, JOhn

Answer 1

The behaviour of the second function is simply undefined; anything can happen, and in many circumstances, it will appear to work, simply because nothing has overwritten where the result used to be stored on the stack.

Answer 2

You are accessing data that is no longer in scope.

The memory probably still has the data in it though so it appears to work properly but is likely to be reused at any time and the value will be overwritten.

The next time you call any function or allocate a local stack variable it's very likely to reuse that memory for the new data and overwrite what you had there before. It's underfined behavour.