Is there a weak_ptr equivalent to shared_from_this?

Question

I have a class which I know will always be owned by a std::shared_ptr. However passing shared_ptr or even weak_ptr to functions and methods that don't need ownership or lifetime guarantees creates unnecessary overhead. To get around this I often pass raw pointers to functions. The class itself inherits from std::enable_shared_from_this so if the function needs to take ownership of the pointer it can use a method of the class to get a shared_ptr.

This is all working beautifully. However there are occasions where I don't really want to make a shared_ptr from a raw pointer, what I want instead is a weak_ptr.

From what I understand of the usual implementation of std::shared_ptr it has two atomic variables used as reference counters; one for shared_ptr, one for weak_ptr.

If all I have is a raw pointer to my class and I want a weak_ptr, I must first create a shared_ptr and convert it. Doing so means the reference counters are altered like this:

• Construct shared_ptr, increment shared_ptr counter
• Copy construct weak_ptr, increment weak_ptr counter
• Allow shared_ptr to go out of scope, decrement shared_ptr counter

This seems to go against the idea that "you don't pay for what you don't use". Is there a way for my class to just provide weak_ptr without first creating a shared_ptr?

Answer 1

Proposal P0033 was accepted for C++17 in the October 2015 meeting, which adds weak_from_this to classes deriving from std::enable_shared_from_this.