make_unique and perfect forwarding

Question

Why is there no std::make_unique function template in the standard C++11 library? I find

std::unique_ptr p(new SomeUs         


        

Answer 1

While nothing stops you from writing your own helper, I believe that the main reason for providing make_shared in the library is that it actually creates a different internal type of shared pointer than shared_ptr(new T), which is differently allocated, and there's no way to achieve this without the dedicated helper.

Your make_unique wrapper on the other hand is mere syntactic sugar around a new expression, so while it might look pleasing to the eye, it doesn't bring anything new to the table. Correction: this isn't in fact true: Having a function call to wrap the new expression provides exception safety, for example in the case where you call a function void f(std::unique_ptr &&, std::unique_ptr &&). Having two raw news that are unsequenced with respect to one another means that if one new expression fails with an exception, the other may leak resources. As for why there's no make_unique in the standard: It was just forgotten. (This happens occasionally. There's also no global std::cbegin in the standard even though there should be one.)

Also note that unique_ptr takes a second template parameter which you should somehow allow for; this is different from shared_ptr, which uses type erasure to store custom deleters without making them part of the type.