What's the difference between:
std::shared_ptr<int> p = std::shared_ptr<int>( new int );
and
std::shared_ptr<int> p = std::make_shared< int >();
?
Which one should I prefer and why?
P. S. Pretty sure this must have been answered already, but I can't find a similar question.
Both examples are rather more verbose than necessary:
std::shared_ptr<int> p(new int); // or '=shared_ptr<int>(new int)' if you insist
auto p = std::make_shared<int>(); // or 'std::shared_ptr<int> p' if you insist
What's the difference?
The main difference is that the first requires two memory allocations: one for the managed object (new int), and one for the reference count. make_shared should allocate a single block of memory, and create both in that.
Which one should I prefer and why?
You should usually use make_shared as it's more efficient. As noted in another answer, it also avoids any possibility of a memory leak, since you never have a raw pointer to the managed object.
However, as noted in the comments, it has a potential disadvantage that the memory won't be released when the object is destroyed, if there are still weak pointers preventing the shared count from being deleted.
EDIT 2020/03/06:
Further recommendations come also from the official Microsoft documentation with associated examples. Keep the focus on the Example 1 snippet:
Whenever possible, use the make_shared function to create a shared_ptr when the memory resource is created for the first time. make_shared is exception-safe. It uses the same call to allocate the memory for the control block and the resource, which reduces the construction overhead. If you don't use make_shared, then you have to use an explicit new expression to create the object before you pass it to the shared_ptr constructor. The following example shows various ways to declare and initialize a shared_ptr together with a new object.
weak_ptr has been destroyed - otherwise, how would the weak_ptr know that its parent is gone?From en.cppreference.com
In contrast, the declaration std::shared_ptr<T> p(new T(Args...)) performs at least two memory allocations, which may incur unnecessary overhead.
Moreover, f(shared_ptr<int>(new int(42)), g()) can lead to memory leak if g throws an exception. This problem doesn't exist if make_shared is used.
So I would recommend the make_shared approach if possible.
shared_ptr and make_shared on Mac (they're still in tr1 namespace so I have to define macros to write portable code using shared pointers). I mean, additional macro is required to use make_shared.
g++ 4.7 doesn't have that problem.
Be aware that make_shared limits you to using the default allocation/deallocation functions so if you want to have more control, make_shared is not an option. In other words, something like
std::shared_ptr<uint8_t>(p, [](uint8_t *p){ /*user code */});
is impossible using make_shared. One could use allocate_shared instead, but only the allocator can be specified, not a deleter. Sometimes one need to control allocation and deletion of the wrapped class.
std::make_shared<int>().make_shared. See the following URL for more information: herbsutter.com/2013/05/29/gotw-89-solution-smart-pointers