c++,pointers,return,undefined , Return pointer undefined even though it should be


Return pointer undefined even though it should be

Question:

Tag: c++,pointers,return,undefined

I have something like this. Why is lastItem undefined?

Menu*   CreateItem() 
{
    Menu* menuPointer = new Menu;   
    //initialize values     
    return menuPointer; 
}

void AnotherFunction() 
{
    Menu* lastItem = CreateItem();
}

Thanks a lot!

EDIT:

I just figured it out, but I don't really understand why that wasn't working. The more complete context looks something like this

Menu*   Content::CreateItem() 
{
    Menu* menuPointer = new Menu;   
    //initialize values     
    return menuPointer; 
}

void Content::CreateObject(Menu* curItem, Menu* lastItem)
{
     lastItem = CreateItem();
     curItem = lastItem;
}

void Content::Initialize()
{
    Menu*   curItem = 0;
    Menu*   lastItem = 0;
while (...)
{
    if (/*object is complete*/)
    {
        CreateObject(curItem, lastItem);
    }
    //create object little by little
}

}

I changed

Menu*   curItem = 0;
Menu*   lastItem = 0;

to

Menu*   curItem = new Menu;
Menu*   lastItem = new Menu;

Why was this necessary?


Answer:

Your problem is that passing arguments to functions in C++ defaults to being by value.

A consequence of this is that changes to the value of an argument are local to the function.

So, given your edited code

Menu*   Content::CreateItem() 
{
    Menu* menuPointer = new Menu;   
    //initialize values     
    return menuPointer; 
}

void Content::CreateObject(Menu* curItem, Menu* lastItem)
{
     lastItem = CreateItem();
     curItem = lastItem;
}

The CreateItem() function is fine. The value of menuPointer is returned to the caller.

However, the change of curItem in CreateObject() is different. The caller, when doing

Content::CreateObject(someItem, someOtherItem);

passes copies of the values of someItem and someOtherItem. The changes within the function changes values of the copies, not the variables someItem and someOtherItem seen by the caller.

One possible solution is to pass a pointer to a pointer.

void Content::CreateObject(Menu **curItem, Menu **lastItem)
{
     *lastItem = CreateItem();
     *curItem = *lastItem;
}

Note the additional asterisks in the above. The reason this works is that, although a change of value of the pointer is not visible to the caller, a change of what the pointer points at (the pointee if you like) is visible to the caller.

Another solution, as you have already noted in your own self-reply, is to pass a reference to the pointers.

void Content::CreateObject(Menu *&curItem, Menu *&lastItem)
{
     lastItem = CreateItem();
     curItem = lastItem;
}

You need to read your textbook better, because ALL basic textbooks on C++ describe this in one way or another.


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