Differences between std::make_unique and std::unique_ptr with new

154
votes

Does std::make_unique have any efficiency benefits like std::make_shared?

Compared to manually constructing std::unique_ptr:

std::make_unique<int>(1);         // vs
std::unique_ptr<int>(new int(1));
4
c++c++11c++14smart-pointersunique-ptr
Does make_shared have any efficiency over just writing the long hand code? – Ed Heal
@EdHeal It may, because make_shared can allocate both the space for the object and the space for the control block together in a single allocation. The cost of that is that the object cannot be deallocated separately from the control block, so if you use weak_ptr a lot then you may end up using more memory. – bames53
Perhaps this is a good starting point stackoverflow.com/questions/9302296/… – Ed Heal
See this link for detailed explanation: herbsutter.com/gotw/_102 – Krishna Kanth Yenumula

4 Answers

165
votes

The motivation behind make_unique is primarily two-fold:

  • make_unique is safe for creating temporaries, whereas with explicit use of new you have to remember the rule about not using unnamed temporaries.

    foo(make_unique<T>(), make_unique<U>()); // exception safe

foo(unique_ptr<T>(new T()), unique_ptr<U>(new U())); // unsafe*

  • The addition of make_unique finally means we can tell people to 'never' use new rather than the previous rule to "'never' use new except when you make a unique_ptr".

There's also a third reason:

  • make_unique does not require redundant type usage. unique_ptr<T>(new T()) -> make_unique<T>()

None of the reasons involve improving runtime efficiency the way using make_shared does (due to avoiding a second allocation, at the cost of potentially higher peak memory usage).

* It is expected that C++17 will include a rule change that means that this is no longer unsafe. See C++ committee papers P0400R0 and P0145R3.

18
votes

std::make_unique and std::make_shared are there for two reasons:

  1. So that you don't have to explicitly list the template type arguments.
  2. Additional exception safety over using std::unique_ptr or std::shared_ptr constructors. (See the Notes section here.)

It's not really about runtime efficiency. There is the bit about the control block and the T being allocated all at once, but I think that's more a bonus and less a motivation for these functions to exist.

9
votes

A reason why you would have to use std::unique_ptr(new A()) or std::shared_ptr(new A()) directly instead of std::make_*() is being unable to access the constructor of class A outside of current scope.

1
votes

Consider function call

void function(std::unique_ptr<A>(new A()), std::unique_ptr<B>(new B())) { ... }

Suppose that new A() succeeds, but new B() throws an exception: you catch it to resume the normal execution of your program. Unfortunately, the C++ standard does not require that object A gets destroyed and its memory deallocated: memory silently leaks and there's no way to clean it up. By wrapping A and B into std::make_uniques you are sure the leak will not occur:

void function(std::make_unique<A>(), std::make_unique<B>()) { ... }

The point here is that std::make_unique<A> and std::make_unique<B> are now temporary objects, and cleanup of temporary objects is correctly specified in the C++ standard: their destructors will be triggered and the memory freed. So if you can, always prefer to allocate objects using std::make_unique and std::make_shared.