Question

I guess I don’t fully understand how destructors work in C++. Here is the sample program I wrote to recreate the issue:

#include <iostream>
#include <memory>
#include <vector>

using namespace std;

struct Odp
{
    int id;

    Odp(int id)
    {
        this->id = id;
    }

    ~Odp()
    {
        cout << "Destructing Odp " << id << endl;
    }
};

typedef vector<shared_ptr<Odp>> OdpVec;

bool findOdpWithID(int id, shared_ptr<Odp> shpoutOdp, OdpVec& vec)
{
    shpoutOdp.reset();

    for (OdpVec::iterator iter = vec.begin(); iter < vec.end(); iter++)
    {
        Odp& odp = *(iter->get());
        if (odp.id == id)
        {
            shpoutOdp.reset(iter->get());
            return true;
        }
    }

    return false;
}

int main()
{
    OdpVec vec;

    vec.push_back(shared_ptr<Odp>(new Odp(0)));
    vec.push_back(shared_ptr<Odp>(new Odp(1)));
    vec.push_back(shared_ptr<Odp>(new Odp(2)));

    shared_ptr<Odp> shOdp;
    bool found = findOdpWithID(0, shOdp, vec);
    found = findOdpWithID(1, shOdp, vec);
}

Just before main() concludes, the output of this program is:

Destructing Odp 0
Destructing Odp 1

Why does this happen? I’m retaining a reference to each of the Odp instances within the vector. Does it have something to do with passing a shared_ptr by reference?

UPDATE I thought that shared_ptr::reset decremented the ref count, based on MSDN:

The operators all decrement the
reference count for the resource
currently owned by *this

but perhaps I’m misunderstanding it?

UPDATE 2: Looks like this version of findOdpWithID() doesn’t cause the destructor to be called:

bool findOdpWithID(int id, shared_ptr<Odp> shpoutOdp, OdpVec& vec)
{
    for (OdpVec::iterator iter = vec.begin(); iter < vec.end(); iter++)
    {
        Odp& odp = *(iter->get());
        if (odp.id == id)
        {
            shpoutOdp = *iter;
            return true;
        }
    }

    return false;
}

Answer 1

This line right here is probably what is tripping you up.

shpoutOdp.reset(iter->get());

What you’re doing here is getting (through get()) the naked pointer from the smart pointer, which won’t have any reference tracking information on it, then telling shpoutOdp to reset itself to point at the naked pointer. When shpoutOdp gets destructed, it’s not aware that there is another shared_ptr that points to the same thing, and shpoutOdp proceeds to destroy the thing it’s pointed to.

You should just do

shpoutOdp = *iter;

which will maintain the reference count properly. As an aside, reset() does decrement the reference counter (and only destroys if the count hits 0).