Correct way to initialize boost::shared_ptr

Question

I am just getting started to work with boost::shared_ptr so I have searched around and I see that there are several ways of initializing it:

boost::shared_ptr<MyClass> myclass = boost::shared_ptr<MyClass>(new MyClass());
boost::shared_ptr<MyClass> myclass = new MyClass();
boost::shared_ptr<MyClass> myclass = boost::make_shared<MyClass>();

And of assigning it:

boost::shared_ptr<MyClass> someOtherType::getMyClass();

boost::shared_ptr<MyClass> myclass = someOtherTypePointer->getMyClass();
boost::shared_ptr<MyClass> myclass = boost::make_shared<MyClass>(someOtherTypePointer->getMyClass());

Which one would be the preferred way for the init/assign and why?

Thank you.

Answer 1

(1) boost::shared_ptr<MyClass> c(boost::shared_ptr<MyClass>(new MyClass()));
(2) boost::shared_ptr<MyClass> c(new MyClass());
(3) boost::shared_ptr<MyClass> c(boost::make_shared<MyClass>());

The first one is unnecessarily complex.

(2) and (3) seem similar but use make_shared whenever you can (i.e. when you don't need a custom deleter: Are there any downsides with using make_shared to create a shared_ptr).

make_shared:

• is more efficient. It usually allocates memory for MyClass object and for the shared_ptr's control block with a single memory allocation. In contrast, (2) performs at least two memory allocations. So make_shared reduces allocation overhead, memory fragmentation and improves locality (see GotW #89 point 2)
• avoids explicit new (and, at least with C++11, it's more clear: auto c(std::make_shared<MyClass>());).

The main use of the assignment is when you want to copy a previously-existing boost::shared_ptr, to share ownership of the same object.

If you need to take ownership of a raw pointer you should use reset (boost shared_ptr: difference between operator= and reset?).