push_back or emplace_back with std::make_unique

Question

Based on the answers in these questions here, I know that it is certainly preferred to use c++14's std::make_unique than to emplace_back(new X) directly.

That said, is it preferred to call

my_vector.push_back(std::make_unique<Foo>("constructor", "args"));

or

my_vector.emplace_back(std::make_unique<Foo>("constructor", "args"));

That is, should I use push_back or emplace_back when adding an std::unique_ptr constructed from std::make_unique?

==== EDIT ====

and why? c: <-- (tiny smile)

Solution

It doesn't make a difference as far as construction of the new object is concerned; you already have a unique_ptr<Foo> prvalue (the result of the call to make_unique) so both push_back and emplace_back will call the unique_ptr move constructor when constructing the element to be appended to the vector.

If your use case involves accessing the newly constructed element after insertion, then emplace_back is more convenient since C++17 because it returns a reference to the element. So instead of

my_vector.push_back(std::make_unique<Foo>("constructor", "args"));
my_vector.back().do_stuff();

you can write

my_vector.emplace_back(std::make_unique<Foo>("constructor", "args")).do_stuff();

Comments

[Cheers and hth. - Alf]

With an actual argument of the vector's item type, push_back and emplace_back necessarily do the same.

[dyp]

Related: htmlpreview.github.io/?https://github.com/HowardHinnant/pape‌​rs/…

[NHDaly]

@Praetorian whoops thanks.

[Yakk - Adam Nevraumont]

If you have a custom deleter on your unique_ptr, then things can change.

[NHDaly]

Yes agreed, that's why I asked. :)

[j b]

Actually, there is a difference... emplace_back() returns a reference to the added element, whereas push_back() returns void. emplace_back(std::make_unique<>()) is therefore useful in contexts where you need to use the object after it has been added.

[Praetorian]

@JamieBullock Well, there was no difference when I answered the question 3 years ago. Updated the answer to include the C++17 change to the return type. Thanks.