Should RAII cause memory leaks?

Question

#define _CRTDBG_MAP_ALLOC
#include <stdlib.h>
#include <crtdbg.h>

struct A
{
    A(char* p)
        : p(p)
    {}

    ~A()
    {   
        delete this->p;
    }

    char* p;
};

int main()
{
    A a(new char);
    _CrtDumpMemoryLeaks();
}

After running in debugging mode, the output window of Visual Studio 2012 shows:

Detected memory leaks!
Dumping objects ->
{142} normal block at 0x007395A8, 1 bytes long.
 Data: < > CD 
Object dump complete.

What's the cause?

miguel.martin · Accepted Answer · 2012-12-22T07:53:20.060

7

Perhaps it's dumping the memory leaks before the destructor is actually called? Try:

int main()
{
     {
         A a(new char);
     }
     _CrtDumpMemoryLeaks();
}

I suggest using the standard's (or boost's) smart pointer classes, such as unique_ptr or shared_ptr, instead of dealing with new/delete directly with raw pointers.

EDIT: Removed the suggestion to set the pointer to NULL, since delete deals with that.

edited Dec 22 '12 at 07:53

answered Dec 22 '12 at 07:47

miguel.martin

1,626
1
12
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Setting a pointer to NULL in a destructor is kind of pointless. – Mark Ransom Dec 22 '12 at 07:48
smart pointers are not for C++ 11 only, they are also available for C++ 03 – user1849534 Dec 22 '12 at 07:48
@MarkRansom Yes I realise that, but as I said, if he wants to reset what `A::p` points to, he should be checking if `p` is `NULL` before deleting and setting it to `NULL` after reseting the pointer. i.e. `if(p) delete p;` – miguel.martin Dec 22 '12 at 07:50
That's also pointless, since `delete` does the correct thing with a NULL pointer i.e. nothing. It's guaranteed by the standard. – Mark Ransom Dec 22 '12 at 07:50
@MarkRansom Wow, I actually didn't know that. Thanks. – miguel.martin Dec 22 '12 at 07:51

score 5 · Answer 2 · answered Dec 22 '12 at 07:47

5

You're dumping the memory before the destructor has had a chance to run, i.e. before the end of the block. Try this and see the difference:

int main()
{
    {
        A a(new char);
    }
    _CrtDumpMemoryLeaks();
}

answered Dec 22 '12 at 07:47

Mark Ransom

299,747
42
398
622

great, looks like I copied you now :P – miguel.martin Dec 22 '12 at 07:48
1

@miguel.martin, it would be hard to copy in 4 seconds. Great minds think alike! – Mark Ransom Dec 22 '12 at 07:49

Should RAII cause memory leaks?

2 Answers2