understanding the logic behind the new and delete operators in c++

Question

Im trying to understand the delete operator in c++.

I could understand the logic behind using pointers and new operator, yet I understood that "The delete operator eliminates a dynamic variable and returns the memory that the dynamic variable occupied to the freestone." p517, Problem solving with C++ 9th edition.

which, I think doesn't align with the third cout statement. and I expected that the third cout statement to be sth similar to the first one.

int main() {
    int  *p1;
    cout << "p1 address: " << &p1 << " and points to the address "<< p1 << " which has the value: " << *p1<< endl;

    p1 = new int;
    cout << "p1 address: " << &p1 << " and points to the address "<< p1 << " which has the value: " << *p1<< endl;

    delete p1;
    cout << "p1 address: " << &p1 << " and points to the address "<< p1 << " which has the value: " << *p1<< endl;

    cout << endl;
    return 0;
}

I would appreciate any explanation :))

it doesn't change the value of the pointer. What did you expect? btw using the pointer after delete invokes ub — Incomputable, Jul 21 '16 at 09:34
All of those outputs are UB: first prints and then dereferences an uninitialised pointer, second reads an uninitialised variable, third dereferences deleted pointer. — BeyelerStudios, Jul 21 '16 at 09:35
I expected the memory address which it points to, to have different value. like as if the pointer is not initialized ! — Zingo, Jul 21 '16 at 09:35
I didn't know about Undefined Behavior :/ I think its the time to read about it — Zingo, Jul 21 '16 at 09:40
now when I am thinking of it.. what exactly happens when &p1 is printed out - what address is it? does anybody know it? — nayana, Jul 21 '16 at 09:45
@BeyelerStudios ah I forgot that the pointer itself needs to be stored somewhere .. thanks — nayana, Jul 21 '16 at 09:48

score 1 · Accepted Answer · edited May 23 '17 at 11:44

delete doesn't have to change the memory your pointer points to. What it actually does is implementation specific.

What delete needs to do is to deconstruct any object at the given address and return the associated memory to the allocation pool. Some debugger might overwrite the value of a variable when freed, but for trivial types no special deconstruction needs to take place - the memory can be returned to the pool as-is. The pointer isn't changed either: after delete p we call p a dangling pointer that holds an address to freed memory. All access through that pointer is undefined behaviour.

Since handling raw pointers and especially dangling pointers is error prone, it's good to know the C++ smartpointers, e.g. unique_ptr:

std::unique_ptr<int> p;        // initialised to nullptr
p = std::make_unique<int>(13); // new int with value 13
p = std::make_unique<int>(37); // deleted previous int, assigned new int with value 37
// optional (when p goes out of scope its pointee is deleted automatically)
p.reset();                     // deleted the int and reset p to nullptr

understanding the logic behind the new and delete operators in c++

1 Answers1