Interconverting Std:Function with Copyable_function
The article discusses the interconversion between std::function and the newly introduced std::copyable_function in C++26, highlighting design differences and performance implications. It explains how repeated conversions can create a linked‑list‑like chain of callables that degrades call‑operator speed. The author notes that, as of July 2026, only libstdc++ implements copyable_function and exhibits the described suboptimal behavior.
- ▪C++26 adds std::copyable_function to address shortcomings of std::function, while retaining some of its design quirks.
- ▪Converting between std::function and copyable_function can cause O(1) constructions but may build a chain of wrappers that slows invocation after many rounds.
- ▪The conversion from move_only_function to copyable_function is prohibited because copyable_function requires copyable callables.
- ▪libstdc++ is currently the only major STL implementation that supports copyable_function, and its implementation follows the less efficient conversion pattern described.
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| Original publisher | Arthur O’Dwyer |
| Canonical URL | https://quuxplusone.github.io/blog/2026/07/26/function-explosion/ |
| Publication time | Mon, 27 Jul 2026 05:39:33 +0000 |
| Retrieval time | 2026-07-27T05:57:39.433Z |
| Last seen | 2026-07-27T05:57:39.433Z |
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Opening excerpt (first ~120 words) tap to expand
Interconverting std::function with copyable_function C++11 introduced std::function as a type-erased callable-object holder. function was badly designed in a few ways: Its rarely used “go fish” API bloats every user; it advertises operator() const even when the controlled object’s operator() is mutating; it handles what should be a precondition violation by throwing an exception, which again bloats every user and means its operator() can never be noexcept. So C++26 fixed these problems by adding std::copyable_function.
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Excerpt limited to ~120 words for fair-use compliance. The full article is at Arthur O’Dwyer.
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