The C++ Standard Library Has Been Walking Itself Back for Fifteen Years

Summary

The C++ standard library has been walking itself back for fifteen years, and the receipts are public Sandor Dargo's post this month on std::copyable_function closes with a quick-reference table. Four callable wrappers, one recommendation each, and at the bottom of the list one entry that should stop any working C++ engineer cold: std::function: Legacy. Avoid in new code. std::function shipped in C++11. The committee spent fifteen years shipping the wrappers that should replace it. The latest, std::copyable_function, lands in C++26. The recommendation written on top of the new arrival is not "use this when you need a copyable callable." It is "do not use the original." This is not unusual. The C++ committee has been writing that sentence about its own features since C++11 was new. Sometimes the sentence is formal (a paper number, a deprecation in the standard, a removal one cycle later). Sometimes the sentence is what every senior engineer tells every junior engineer on day one ("never reach for that, here is what to use instead"). And sometimes the sentence cannot be written into the standard at all, because the broken thing is locked in by ABI compatibility, so it stays in the standard library as the default that every tutorial reaches for and every production codebase quietly replaces. The pattern is so consistent that it deserves its own catalogue, with paper numbers next to every entry, so the next time someone tells you the new C++ feature is the future you can ask them to estimate how long until the next paper deprecates it. This piece is that catalogue, in three tiers. The first tier is the formal walk-backs the committee has written down. The second tier is the "everyone knows to avoid this" walk-backs that the committee has not formalised. The third tier is the most damning, because it is the standard library containers that almost every C++ codebase uses every day and that the committee cannot fix without breaking ABI. We have receipts on the third tier fr...