Why does C++ need std::make_unique over forwarded unique_ptr constructor?

I don’t ask about new operator like here. Please, read question carefully.

I want to know why we need special function make_unique over special constructor of unique_ptr.

unique_ptr could use constructor like this to make make_unique unneccessary:

template<typename T, typename ...TArgs>
unique_ptr::unique_ptr(TArgs&&... args)
        : inner_ptr(new T(std::forward(args)...))


I want to summarize discussion with Some programmer dude, StoryTeller – Unslander Monica and Raymond Chen

So, there are 2 reasons:

  1. std::make_unique pairs well with std::make_shared which was introduced earlier so this was easier to learn than new constructor for unique_ptr.
  2. There is possible ambiguity between constructor of unique_ptr and constructor of inner value type (T) if this type have own constructor which takes a pointer to self type. E.g.
struct Inner{
  Inner() = default;
  Inner(Inner* parent_node): parent(parent_node){}
  Inner* parent = nullptr;

Inner* parent = make_parent();
// It would be not clear for human which constructor must be called here
std::unique_ptr<Inner> child(parent);

Compiler can deduce which constructor should be called here but it is hard for human. So having function std::make_unique is beneficial because it is clear that all constructors of std::unique_ptr create only unique_ptr and never call inner value constructor while std::make_unique would always call constructor of inner value. This makes code much easier to reason about.

Thanks to everyone for discussion!