Naked pointer to unique_ptr

Question

I have a structure with a vector and I need to store a pointer to one of its items. With naked pointers, I'd do like this:

#include <iostream>
#include <vector>
#include <memory>
#include <unistd.h>

struct A {
    A () {
        v = {1, 2, 3, 4};
        uv = &v.front();
    }

    std::vector<int> v;
    int* uv;
};

int main()
{
    A a;
    std::cout << *a.uv << std::endl;
    return 0;
}

Now, I can get rid of the naked pointer with make_shared:

struct A {
    A () {
        v = {1, 2, 3, 4};
        uv = std::make_shared<int>(v.front());
    }

    std::vector<int>     v;
    std::shared_ptr<int> uv;
};

Since C++11 doesn't implement make_unique yet, how can I do the same thing with a unique_ptr? I tried assigning the front element by making uv a unique_ptr and calling uv.reset(&v.front());, but I get the following error:

malloc: *** error for object 0x7f856bc0bcf0: pointer being freed was not allocated

Any help? Thanks

Answer 1

The philosophy behind std::unique_ptr is that of ownership, i.e. it's an object that owns a region of memory, tying the lifetime of the latter to its own.

An std::vector follows the same path: it claims ownership of the elements it contains. Thus, when its destructor runs, it triggers the destructors of its elements (if they are user-defined types).

In your case, v owns v.front(), but you also gave this responsibility to uv. Now, both of them will try to destroy the same object.

It is better to use a raw pointer here, or (better) an iterator, since it's used as an observer only.