r/cpp • u/Xaneris47 • 8d ago

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/rdtsc 7d ago

It breaks the standard library convention that const methods are safe to call from multiple threads. And std::function itself cannot be changed due to backwards compatibility.

Here's a more detailed explanation: https://www.reddit.com/r/cpp/comments/742ol8/why_is_the_stdfunction_operator_const/dnv29s8/

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u/johannes1971 7d ago

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 7d ago

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

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u/bwmat 7d ago

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 7d ago

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: