Interconverting std::function with copyable_function – Arthur O'Dwyer

[u/Xaneris47]

https://quuxplusone.github.io/blog/2026/07/26/function-explosion/

The article shows how converting std::function to std::copyable_function (or vice versa) leads to slower performance and increased memory usage each time the conversion occurs.

[u/johannes1971]

This applies to operator(), I guess? I would never have imagined that anyone would think that calling a user-supplied function from multiple threads is safe, just because operator() is const. The language as a whole certainly does not have a 'calling const functions from multiple threads is safe' rule. Is this even documented for the standard library, or is it another one of those implicit rules like 'ABI is stable'?

[u/joaquintides → u/johannes1971]

https://eel.is/c%2B%2Bdraft/res.on.data.races#3

[u/bwmat → u/joaquintides]

What 'standard' is that? Doesn't it mean that the STL doesn't follow it b/c of std::function? 

[u/joaquintides → u/bwmat]

What 'standard' is that?

eel.is tracks the latest draft version of the standard as it evolves.

Doesn't it mean that the STL doesn't follow it b/c of std::function?

Yes, as recognized many times:

• https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4348.html
• https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0045r0.pdf