Because std::unique_ptr is designed with move semantics in mind, copy initialization and copy assignment are disabled. If you want to transfer the contents managed by std::unique_ptr, you must use move semantics.

std::move_if_noexcept will return a movable r-value if the object has a noexcept move constructor, otherwise it will return a copyable l-value. We can use the noexcept specifier in conjunction with std::move_if_noexcept to use move semantics only when a strong exception guarantee exists (and use copy semantics otherwise).

std::move can be used whenever we want to treat an l-value like an r-value for the purpose of invoking move semantics instead of copy semantics.

the goal of the move constructor and move assignment is to move ownership of the resources from one object to another (which is typically much less expensive than making a copy).

Move semantics means the class will transfer ownership of the object rather than making a copy.