Deleting an object

[boogerlad]

code:

int x = 1; vector<int> * herp = new vector<int>; vector<int> derp; delete x; delete herp; delete derp;

The program runs and compiles, but I get a warning
warning: the address of 'derp' will never be NULL

What does that warning mean? It just doesn't make any sense to me. If I can delete x, then why can't I delete derp without the warning?

[Tony]

it doesn't make sense to delete items on the stack. You can't delete x either.

Quote:

tony$ cat test.cc
#include <new>
int main(){
int x = 1;
delete x;
return 0;
}

tony$ g++ test.cc
test.cc: In function ?int main()?:
test.cc:4: error: type ?int? argument given to ?delete?, expected pointer

[boogerlad]

sorry, I meant this:

code:

delete &derp;     delete &x;

deletion of x doesn't result in a warning, but derp does.

[Tony]

No. You cannot delete objects from within the stack, even if you try. If you have to ask why, then you need to review how stack memory works.

Edit: it might not give you a compile-time exception (static analysis is hard), but it will break at run-time.

[md]

Why you get that specific warning I'm not sure but really doing things like that *will* cause trouble.

[boogerlad]

Fair enough. That might explain those random crashes. Sorry for the noob questions, I just started learning c++. I just came from java. Deleting the herp object is okay, because I used the new keyword correct? Anything that uses the new key word must be deleted manually?

[md]

boogerlad @ 2011-12-13, 12:21 am wrote:

Fair enough. That might explain those random crashes. Sorry for the noob questions, I just started learning c++. I just came from java. Deleting the herp object is okay, because I used the new keyword correct? Anything that uses the new key word must be deleted manually?

In a word, yes. Herp can be deleted because it was allocated from the heap (hence why you have a pointer to it). Derp and x are both allocated on the stack. In Java *every* object is allocated from the heap, only references and basic numeric types go on the stack; hence why you *always* use new to create new objects. With C++ you can create objects by allocating memory from the heap with new *or* you can create objects on the stack by simply declaring a variable. Things on the stack get automatically destroyed when the stack frame closes (not always the end of a function), things allocated from the heap must be deleted manually.

In your original post what you were doing was attempting to delete stack-allocated objects from the heap - which clearly makes no sense. In fact, it's an undefined operation IIRC and might do any number of things depending on the specific compiler you're using.