How can I create a shared_ptr to a std::vector?

Question

I need to create a shared_ptr to a std::vector, what is the correct syntax?

std::vector<uint8_t> mVector;
shared_ptr<std::vector<uint8_t>> mSharedPtr = &mVector;

The code above does not compile.

Thanks.

Answer 1

What you are trying to do is to let a smart pointer manage a stack object. This doesn't work, as the stack object is going to kill itself when it goes out of scope. The smart pointer can't prevent it from doing this.

std::shared_ptr<std::vector<uint8_t> > sp;
{
   std::vector<uint8_t> mVector;
   sp=std::shared_ptr<std::vector<uint8_t> >(&mVector);
}
sp->empty();   // dangling reference, as mVector is already destroyed

---

Three alternatives:

(1) Initialize the vector and let it manage by the shared_ptr:

auto mSharedPtr = std::make_shared<std::vector<uint8_t> >(/* vector constructor arguments*/);

(2) Manage a copy of the vector (by invoking the vector copy constructor):

std::vector<uint8_t> mVector;
auto mSharedPtr = std::make_shared<std::vector<uint8_t> >(mVector);

(3) Move the vector (by invoking the vector move constructor):

std::vector<uint8_t> mVector;
auto mSharedPtr = std::make_shared<std::vector<uint8_t> >(std::move(mVector));
//don't use mVector anymore.

---

Answer 2

First, what you're doing is very wrong, if you give a pointer to a shared_ptr make sure it's dynamically allocated with new, not on the stack. Otherwise you may just as well use a pointer instead of a shared_ptr.

Your code should be:

std::vector<uint8_t> mVector;
/* Copy the vector in a shared pointer */
std::shared_ptr<std::vector<uint8_t> > mSharedPtr ( new std::vector<uint8_t>(mVector) );

or:

std::shared_ptr<std::vector<uint8_t> > mSharedPtr ( new std::vector<uint8_t>() );

As for why it doesn't compile, you need to use the constructor, not the = operator.

As pointed out by @remyabel, make_shared is more efficient:

std::vector<uint8_t> vector;
/* Copy the vector in a shared pointer */
auto sharedPtr = std::make_shared<std::vector<uint8_t>> (vector);