Undefined ref to 'function<void ()()>::operator()() const' when called from free function defined in cc file.

[Peter Dimov]

Chris Fairles wrote:

> Yeah. I'm not a huge fan of this global function<> w/ mutex method...

The main drawback I see is that function<>'s (copy) constructor or 
assignment operator are allowed to throw. You could std::move in 
__once_proxy though.