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 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.

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