Passing by reference does not change the variable

Question

I am trying to implement a function to change state of the menu, but my reference is lost when it leaves the function:

void gotoLowerlevel(Menu *item)
{
             


        

Answer 1

You're passing the pointer by value.

Operations on the object it points to will be visible to the outside, but the pointer itself is only a copy.

You might want to use a pointer to pointer.

Answer 2

item in gotoLowerLevel is a local variable even if it is a reference to an object elsewhere. To modify cnsle->currentState you need to either:

• pass in cnsle
• pass in a reference to cnsle->currentState (that is change the method signature to Menu ** itemptr and the call parameter to &cnsle->currentState)
• or return the new value from gotoLowerLevel and assign it: cnsle->currentState = gotoLowerLevel(cnsle->currentState)

My preference would be the last option, as this makes it clear when reading the calling code that currentState may be modified.

Others have explained how to pass a reference. Code for my preferred solutions is:

Menu* gotoLowerlevel(Menu *item)
{
    if (item->chld != 0x00) {
        item = item->chld;
    }
    return item;
}

/* .... */
cnsle->currentState = gotoLowerlevel(cnsle->currentState);