Why use std::make_unique in C++17?

As far as I understand, C++14 introduced std::make_unique because, as a result of the parameter evaluation order not being specified, this was unsafe:

f(std::unique_ptr<MyClass>(new MyClass(param)), g()); // Syntax A

(Explanation: if the evaluation first allocates the memory for the raw pointer, then calls g() and an exception is thrown before the std::unique_ptr construction, then the memory is leaked.)

Calling std::make_unique was a way to constrain the call order, thus making things safe:

f(std::make_unique<MyClass>(param), g());             // Syntax B

Since then, C++17 has clarified the evaluation order, making Syntax A safe too, so here's my question: is there still a reason to use std::make_unique over std::unique_ptr's constructor in C++17? Can you give some examples?

As of now, the only reason I can imagine is that it allows to type MyClass only once (assuming you don't need to rely on polymorphism with std::unique_ptr<Base>(new Derived(param))). However, that seems like a pretty weak reason, especially when std::make_unique doesn't allow to specify a deleter while std::unique_ptr's constructor does.

And just to be clear, I'm not advocating in favor of removing std::make_unique from the Standard Library (keeping it makes sense at least for backward compatibility), but rather wondering if there are still situations in which it is strongly preferred to std::unique_ptr

Solution 1:

You're right that the main reason was removed. There are still the don't use new guidelines and that it is less typing reasons (don't have to repeat the type or use the word new). Admittedly those aren't strong arguments but I really like not seeing new in my code.

Also don't forget about consistency. You absolutely should be using make_shared so using make_unique is natural and fits the pattern. It's then trivial to change std::make_unique<MyClass>(param) to std::make_shared<MyClass>(param) (or the reverse) where the syntax A requires much more of a rewrite.

Solution 2:

make_unique distinguishes T from T[] and T[N], unique_ptr(new ...) does not.

You can easily get undefined behaviour by passing a pointer that was new[]ed to a unique_ptr<T>, or by passing a pointer that was newed to a unique_ptr<T[]>.