Calling delete on variable allocated on the stack

unistudent_54883 0
Viewed 59983

Ignoring programming style and design, is it "safe" to call delete on a variable allocated on the stack?

For example:

   int nAmount;
   delete &nAmount;

or

class sample
{
public:
    sample();
    ~sample() { delete &nAmount;}
    int nAmount;
}
delete-operator heap-memory stack c++
12 Answers

No, it is not safe to call delete on a stack-allocated variable. You should only call delete on things created by new.

  • For each malloc or calloc, there should be exactly one free.
  • For each new there should be exactly one delete.
  • For each new[] there should be exactly one delete[].
  • For each stack allocation, there should be no explicit freeing or deletion. The destructor is called automatically, where applicable.

In general, you cannot mix and match any of these, e.g. no free-ing or delete[]-ing a new object. Doing so results in undefined behavior.

user_250500

Well, let's try it:

jeremy@jeremy-desktop:~$ echo 'main() { int a; delete &a; }' > test.cpp
jeremy@jeremy-desktop:~$ g++ -o test test.cpp
jeremy@jeremy-desktop:~$ ./test
Segmentation fault

So apparently it is not safe at all.

Jeremy_Ruten_8130

Keep in mind that when you allocate a block of memory using new (or malloc for that matter), the actual block of memory allocated will be larger than what you asked for. The memory block will also contain some bookkeeping information so that when you free the block, it can easily be put back into the free pool and possibly be coalesced with adjacent free blocks.

When you try to free any memory that you didn't receive from new, that bookkeeping information wont be there but the system will act like it is and the results are going to be unpredictable (usually bad).

Ferruccio_40860

No, Memory allocated using new should be deleted using delete operator and that allocated using malloc should be deleted using free. And no need to deallocate the variable which are allocated on stack.

Vinay_286410

here the memory is allocated using stack so no need to delete it exernally but if you have allcoted dynamically

like int *a=new int()

then you have to do delete a and not delete &a(a itself is a pointer), because the memory is allocated from free store.

Admin1

Motivation: I have two objects, A and B. I know that A has to be instantiated before B, maybe because B needs information calculated by A. Yet, I want to destruct A before B. Maybe I am writing an integration test, and I want server A to shut-down first. How do I accomplish that?

A a{};
B b{a.port()};

// delete A, how?

Solution: Don't allocate A on the stack. Instead, use std::make_unique and keep a stack-allocated smart pointer to a heap-allocated instance of A. That way is the least messy option, IMO.

auto a = std::make_unique<A>();
B b{a->port()};

// ...

a.reset()

Alternatively, I considered moving the destruction logic out of A's destructor and calling that method explicitly myself. The destructor would then call it only if it has not been called previously.

user_10477880
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