Assignment inside function that is passed as pointer?

Question

ClassA* pa = NULL;
ClassA* pb = NULL;
assignObject(ClassA* pa,ClassB* pb)
{
pa = new ClassA;
pb = new ClassB;
}

what will be the value of pa,pb after executing the function

EDIT how to pass as pointer is the return if pa,pb is NULL

Answer 1

NULL, because you're passing the pointers in by value.

Answer 2

ClassA* pa = NULL;
ClassA* pb = NULL;

void assignObject(ClassA* &pa,ClassB* &pb)
{
    pa = new ClassA;
    pb = new ClassB;
}

Alternatively:

ClassA* pa = NULL;
ClassA* pb = NULL;

void assignObject(ClassA** pa,ClassB** pb)
{
    *pa = new ClassA;
    *pb = new ClassB;
}

Answer 3

They will be NULL, since you're passing them by value. If you want to pass it by reference, you'd do this:

ClassA* pa = NULL;
ClassA* pb = NULL;
assignObject(ClassA*& pa, ClassB*& pb)
{
    pa = new ClassA;
    pb = new ClassB;
}

Note, I'm not sure what you're trying to accomplish with the global variables. They're never used in this example, since the local variables (function parameters) hide them.

I think you also need to declare a return value type for your function in order for it to be valid C++.

Answer 4

As pointed out in other answers - both will still be NULL after the call. However, there are two possible solutions to this problem:

1) references

void assignObject(ClassA*& pa, ClassB*& pb)
{
    pa = new ClassA;
    pb = new ClassB;
}

ClassA* pa = NULL;
ClassA* pb = NULL;
assignObject(pa, pb); // both will be assigned as expected.

2) pointers

void assignObject(ClassA** pa, ClassB** pb)
{
    assert(pa != NULL); assert(pb != NULL);
    *pa = new ClassA;
    *pb = new ClassB;
}
ClassA* pa = NULL;
ClassA* pb = NULL;
assignObject(&pa, &pb); // both will be assigned as expected.

Most programmers would probably choose references because then they don't need to assert anything (references can never be NULL).