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There's no way to use that optimization if you need to be in two or more lists at the same time.

Which is also an issue in the author's example, no? So the point would be moot.

Wouldn't std shared pointer handle that case?

Was it available when ZeroMQ was being planned? It's not a very new project.

boost shared_ptr has been around since 2001. ZeroMQ was started in 2007.