Chapter 1

CleanupStack::PushL(x);
x->UseL();
CleanupStack::PopAndDestroy(x);
}
The cleanup stack class, CleanupStack, is defined in e32base.h. With these changes in
place, here's what happens:
� Immediately after we have allocated the CX and stored its pointer in x, we also push a
copy of this pointer to the cleanup stack.
� We then call UseL().
� If this doesn't fail, our code pops the pointer from the cleanup stack and deletes the
object. We could have used two lines of code for this (CleanupStack::Pop(),
followed by delete x), but this is such a common pattern that the cleanup stack
provides a single function to do both.
� If UseL() does fail, then as part of leave processing, all objects on the cleanup stack
are popped and destroyed anyway.
So our code works whether UseL() leaves or not. Change memorymagic to include this
code, rebuild, set the heap to fail on the second allocation, invoke Use 3, and then exit the
application. Once again, all will be well.
Of course, we could have done this without the aid of the cleanup stack by using code like
this:
case EMagicCmdUse3:
{
CX* x = new(ELeave) CX;
TRAPD(error, x->UseL()):
if(error)
{
delete x:
User::Leave(error);
}
delete x;
}
However, this is much less elegant. The cleanup stack works particularly well for a long
sequence of operations, such as
case EMagicCmdUse3:
{
CX* x = new(ELeave) CX;
CleanupStack::PushL(x);
x->UseL():
x->UseL();
x->UseL();
CleanupStack::PopAndDestroy(x);
}