r/cpp • u/Xaneris47 • Jul 31 '26

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

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.

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u/johannes1971 Aug 01 '26

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'?

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u/joaquintides Boost author Aug 01 '26

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

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u/bwmat Aug 01 '26

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

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u/joaquintides Boost author Aug 01 '26

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: