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c++c++11shared-ptrunique-ptr

Does C++11 unique_ptr and shared_ptr able to convert to each other's type?


Does C++11 standard library provide any utility to convert from a std::shared_ptr to std::unique_ptr, or vice versa? Is this safe operation?


Solution

  • std::unique_ptr is the C++11 way to express exclusive ownership, but one of its most attractive features is that it easily and efficiently converts to a std::shared_ptr.

    This is a key part of why std::unique_ptr is so well suited as a factory function return type. Factory functions can’t know whether callers will want to use exclusive ownership semantics for the object they return or whether shared ownership (i.e., std::shared_ptr) would be more appropriate. By returning a std::unique_ptr, factories provide callers with the most efficient smart pointer, but they don’t hinder callers from replacing it with its more flexible sibling.

    std::shared_ptr to std::unique_ptr is not allowed. Once you’ve turned lifetime management of a resource over to a std::shared_ptr, there’s no changing your mind. Even if the reference count is one, you can’t reclaim ownership of the resource in order to, say, have a std::unique_ptr manage it.

    Reference: Effective Modern C++. 42 SPECIFIC WAYS TO IMPROVE YOUR USE OF C++11 AND C++14. Scott Meyers.

    In short, you can easily and efficiently convert a std::unique_ptr to std::shared_ptr but you cannot convert std::shared_ptr to std::unique_ptr.

    For 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");