[JoeSaunders]

Hey,

I am pretty new to C++ and recently did a test. Today I got the test results back however some of the test answers didn't match up to the result of the question. For instance one of the questions was:

What, if anything, is wrong with the following program?

#include <iostream.h>

int * EncryptNumber(int w)
{
	int *z_ptr = &w;
	w++;
	return z_ptr;
}

void main()
{
	int a;
	cin >> a;
	cout << *(EncryptNumber(a));
}

A ) The pointer 'z_ptr' is returned and the value of the variable it points to is displayed on the screen.
B ) The pointer 'z_ptr' is returned and its memory address is displayed on the screen.
C ) The pointer 'z_ptr' is not declared in an appropriate manner, causing the program to crash.
D ) The pointer 'z_ptr' is returned but the variable it points to goes out of scope, causing the program to crash.

I answered the question with A and it was marked as a wrong answer, however I tested the code by compiling it and it worked.

Does anyone else think the answer should have been anything other than A? If so why? Or do you agree A should be the correct answer?

[jimblumberg]

Quote

Does anyone else think the answer should have been anything other than A?

Yes A is not the correct answer. Remember in your function w is a copy of the data, which goes out of scope when the function exits. While your program may or may not crash, it is incorrect.

Also you should consider finding a new compiler, the one you're using is seriously outdated. The program you supplied will not compile using modern compilers.

Jim

[tlhIn`toq]

.

This post has been edited by tlhIn`toq: 26 June 2013 - 12:54 PM

[jimblumberg]

Well that's one of the problems with undefined behavior, sometimes it'll work but when you really need it to work it'll crash the program. Let's try a little experiment. Try running this program and show the results.

#include <iostream>

using namespace std;

int * EncryptNumber(int w)
{
	int *z_ptr = &w;
	w++;
	return z_ptr;
}

int main()
{
	int a;
	cin >> a;
	cout << a << endl;

	int *temp = EncryptNumber(a);

	cout << *temp << endl;

	cout << *(EncryptNumber(a));

	cout << *temp << endl;
}

Jim

[vividexstance]

Jim is correct, because the argument to the function is passed-by-value, it is just a local copy to the function. So by returning a pointer to the address of that local variable, you have undefined behavior and it's most definitely wrong to return ANY local variables by pointer or by reference.

As soon as the function returns, all of its variables including the function arguments, are popped off the runtime stack, so the pointer you have from the function may still be a valid pointer to somewhere inside your program and therefore it won't crash when used in certain ways.

[jjl]

Like the others have said, the pointer points to memory allocated on the functions stack frame. When the function returns, the stack frame is popped, which doesn't necesarrly mean it's deleted, it means that the space is fair game for any other function calls. Nonetheless, the correct answer is D, not A - however it may or may not crash, it's behavior is undefined.

[JoeSaunders]

Perfect thanks guys, I understand now, I was confused because when I ran the program it worked but that doesn't mean A would be correct in every instance where as D more accurately describes how it should function because w goes out of scope once z_ptr is returned.