I see in the highly-upvoted answer here (https://stackoverflow.com/a/37885232/3754760) there are two ways to convert a unique_ptr to a shared_ptr, namely by creating the unique_ptr first and then move-ing it to the shared_ptr, as well as assigning the unique_ptr directly to the shared_ptr.
Example:
std::unique_ptr<std::string> unique = std::make_unique<std::string>("test");
std::shared_ptr<std::string> shared = std::move(unique);
or:
std::shared_ptr<std::string> shared = std::make_unique<std::string>("test");
Are the two above equivalent, performance-wise? Secondly, I see a warning like Moving a temporary object prevents copy elision when doing something like this:
std::shared_ptr<std::string> shared = std::move(std::make_unique<std::string>("test"));
As someone pretty new to smart pointers, could someone explain what this warning means and why it occurs in the 3rd example? Thanks!
Are the two above equivalent
Yes
could someone explain what this warning means and why it occurs in the 3rd example?
Copy Elison only works when the result of a function call returning a temp object is directly assigned to a variable. The compiler can optimize away the temp object and let the function initialize the variable directly. But in the 3rd example, there is an intermediate type-cast being performed between the construction of the temp object and the assignment of the temp object to a variable, so the temp object can't be optimized away.