function returns pointer to int

Question

My main() crashes below when add(4) is called.

As I understand int* add, it should return a pointer to integer. Then, I should be able in main to say:

int * a = add(3);

to return a pointer to int.

Please explain what I'm doing wrong.

#include <cstdlib>
#include <iostream>

using namespace std;

int* add (int a) {
   int * c, d;
   d = a + 1;
   *c = d;
   cout << "c = " << c << endl; 
   return c;
}

int main(int argc, char *argv[])
{
    int a = 4;

    int * c;

    c = add(4); 

    system("PAUSE");
    return EXIT_SUCCESS;
}

No he's not. Reading comprehension, people. *c = d does not make c point to d! — Tyler McHenry
– Tyler McHenry, Commented Aug 18, 2010 at 14:38
Worse than that, you're writing to, and returning, an uninitialised pointer. — Mike Seymour
– Mike Seymour, Commented Aug 18, 2010 at 14:38
@all - oops I saw that and corrected the comment that when your complaints came. What do now? Should I strike out the modification? How to do that? — rubber boots
– rubber boots, Commented Aug 18, 2010 at 14:40
This is unreal, I figured when I clicked this it would be a scramble between 10 people to get the right answer in, and instead I find myself downvoting a half-dozen wrong answers — Michael Mrozek
– Michael Mrozek, Commented Aug 18, 2010 at 14:42

JaredPar · Accepted Answer · 2010-08-18 14:44:55Z

8

The problem is that you have declared an int* but not given it anything to point to. What you need to do is initialize it with a memory location (error checknig omitted)

int* c = new int();
...
*c = d;  // Now works

Later on though you'll need to make sure to free this memory since it's an allocated resource.

A better solution though is to use references. Pointers have several nasty attributes including unitialized values, NULL, need to free, etc ... Most of which aren't present on references. Here is an example of how to use references in this scenario.

void add (int a, int& c) {
   int d;
   d = a + 1;
   c = d;
   cout << "c = " << c << endl; 
}

int c;
add(4, c);

edited Aug 18, 2010 at 14:44

answered Aug 18, 2010 at 14:37

JaredPar

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2 Comments

Mike Seymour Over a year ago

In C++, using malloc is almost certainly the wrong thing to do.

JaredPar Over a year ago

@Mike, agreed. Only read the problem code and assumed it was a C question. Updated

sharptooth · Accepted Answer · 2010-08-18 14:37:07Z

5

In

 *c = d;

the pointer c is not initialized, so your program runs into undefined behavior. You could do something like the following instead:

void add( int what, int* toWhat )
{
    (*toWhat) += what;
}

and call it like this:

int initialValue = ...;
add( 4, &initialValue );

answered Aug 18, 2010 at 14:37

sharptooth

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Comments

Tyler McHenry · Accepted Answer · 2010-08-18 15:14:38Z

3

You never allocate any memory to the pointer c. Pointers must refer to valid memory, and you must allocate that memory yourself with a call to new, e.g. write

int* c = new int();

within the add function. Now c points to a valid block of memory that is large enough to hold an int. When you are done with that memory, call delete c to deallocate it and release it back to the system.

edited Aug 18, 2010 at 15:14

answered Aug 18, 2010 at 14:37

Tyler McHenry

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2 Comments

Mike Seymour Over a year ago

The question is tagged C++, so using malloc would be a very bad idea.

Tyler McHenry Over a year ago

Well, not a bad idea (malloc is perfectly legal in C++) but not the best idea. Will fix, though.

jbernadas · Accepted Answer · 2010-08-18 14:37:31Z

1

You get an error because c is an uninitialized pointer, so it is undefined behaviour.

answered Aug 18, 2010 at 14:37

jbernadas

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