return pointer to data declared in function

Question

I know this won\'T work because the variable x gets destroyed when the function returns:

int* myFunction()
{
    int x = 4; return &x;
}

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Answer 1

One possibility is passing the function a pointer:

void computeFoo(int *dest) {
    *dest = 4;
}

This is nice because you can use such a function with an automatic variable:

int foo;
computeFoo(&foo);

With this approach you also keep the memory management in the same part of the code, ie. you can’t miss a malloc just because it happens somewhere inside a function:

// Compare this:
int *foo = malloc(…);
computeFoo(foo);
free(foo);

// With the following:
int *foo = computeFoo();
free(foo);

In the second case it’s easier to forget the free as you don’t see the malloc. This is often at least partially solved by convention, eg: “If a function name starts with XY, it means that you own the data it returns.”

An interesting corner case of returning pointer to “function” variable is declaring the variable static:

int* computeFoo() {
    static int foo = 4;
    return &foo;
}

Of course this is evil for normal programming, but it might come handy some day.