Interconverting Std:Function with Copyable_function

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 . (Which preserves some of function ’s suboptimal design decisions: copyable_function<bool()> remains implicitly convertible to copyable_function<void()> , contextually convertible to bool , and assignable from nullptr .)

P2548 “ copyable_function ” (Hava 2023) contains this guidance for implementors:

It is recommended that implementors do not perform additional allocations when converting from a copyable_function instantiation to a compatible move_only_function instantiation, but this is left as quality-of-implementation.

Note that converting in the other direction — from move_only_function to copyable_function — is disallowed: copyable_function can hold only copyable callables, and move_only_function is not copyable.

But copyable_function is interconvertible — both directions — with std::function ! Both of these type-erased wrappers are constructible from anything that is copyable and callable, and themselves are copyable and callable. So we can do this:

std::function<int()> f = []{ return 5; }; std::copyable_function<int() const> cf = std::move(f); std::function<int()> g = std::move(cf); You’d hope the move from f into cf would be implemented something like this:

and vice versa for the move back into g :

But if copyable_function doesn’t know about function , it might just treat f the same as any old rvalue of copyable, callable type: it might move a copy of f onto the heap to own.

And vice versa for the move back into g : function might just treat cf the same as any old rvalue of copyable, callable type, and move a copy of cf onto the heap to own.

This move operation is still relatively performant — it does only O(1) work for each construction — but after a sequence of \(n\) such constructions, we’ve produced a sort of “linked list” of callables of length \(n\). Each time we invoke g , g ’s operator() will call the operator() of its controlled copyable_function , which calls the operator() of its controlled function , which calls the operator() of its controlled lambda (the green object in these diagrams).

As of this writing (July 2026), libstdc++ is the only one of the Big Three STL vendors to have implemented C++26 copyable_function ; and as of this writing, they’ve implemented the “bad” behavior diagrammed above.

Consider the following contrived scenario: We have some kind of “handler callback” stored in a std::function . Periodically we consider whether to modify that handler based on user input, e.g.

using Handler = std::function<bool(char)>; Handler update(Handler h, bool negate) { if (negate) { h = [f = std::move(h)](char c) { return !f(c); }; } return h; } ~~~~ Handler h = original_handler; while (~~~~) { ~~~~ h = update(std::move(h), (input == '!')); } Observe that h is moved into update , and moved out again (thanks to implicit move ). This uses the move constructor of std::function , which, like most move constructors, is quick and non-allocating.

But suppose one of our coworkers updates part of this code to C++26, where we might reasonably prefer to use std::copyable_function in place of std::function . [1] If our coworker updates the whole codebase to use copyable_function consistently, there’s no problem. But what if they update only their part, so that their new copyable_function -based code must interoperate with everyone else’s old function -based code? Then we get something like this:

using OldHandler = std::function<bool(char)>; using NewHandler = std::copyable_function<bool(char)>; OldHandler update(OldHandler h, bool negate); // unchanged, for the benefit of pre-C++26 callers ~~~~ NewHandler h = original_handler; while (~~~~) { ~~~~ h = update(std::move(h), (input == '!')); } This triggers that “linked list” scenario, and h ’s call operator gets slower with every round-trip through update — even when negate is false !

I wrote up this little benchmark ( Godbolt ):

#include <chrono> #include <cstdio> #include <functional> size_t now() { static const auto epoch = std::chrono::high_resolution_clock::now(); auto tp = std::chrono::high_resolution_clock::now(); return std::chrono::duration_cast<std::chrono::microseconds>(tp - epoch).count(); } void print_elapsed_time(int i, size_t call_started) { printf("Invocation on the %dth iteration took %zu us\n", i, now() - call_started); } using OldHandler = std::function<void(int, size_t)>; using NewHandler = std::copyable_function<void(int, size_t) const>; OldHandler update(OldHandler h, bool negate) { if (negate) { /* do something here, but for this example we don't care what */ } return h; } int main() { NewHandler handler = print_elapsed_time; for (int i = 0; i < 160'000; ++i) { handler = update(std::move(handler), false); if (i % 10'000 == 0) { handler(i, now()); } } } Run this program on libstdc++ (as of GCC 16) and you’ll see output something like the following:

Invocation on the 0th iteration took 0 us Invocation on the 10000th iteration took 180 us Invocation on the 20000th iteration took 296 us Invocation on the 30000th iteration took 365 us [...] Invocation on the 130000th iteration took 1057 us Invocation on the 140000th iteration took 1108 us Invocation on the 150000th iteration took 1202 us Each time we invoke handler(i, now()) the “linked list of callables” hanging off handler is 10,000 elements longer than before, and takes about 100 microseconds longer to chase those pointers. Of course it also holds onto many kilobytes more memory than needed, too. This is a “logical memory leak”: that memory is technically still accessible and will be freed by h ’s destructor, but as long as h lives we’ll see our RAM usage creep higher and higher over time, without any change in the “logical” state of our program.

As of July 2026, Microsoft STL makes move_only_function and function interoperate without such “logical memory leaks,” and I bet their copyable_function will also avoid them, once they implement that type. libstdc++’s implementation may improve in the future. Meanwhile, libc++ hasn’t implemented either move_only_function or copyable_function yet, so they still have plenty of time to think about this issue.

Beware of a simpler version of this pitfall, by the way, when writing your own type-erased types. Make sure that when you copy a Printable -holding-an- int , you get another Printable -holding-an- int and not a Printable -holding-a- Printable -holding-an- int . Write unit tests to make sure that still holds when constructing from a Printable& ; from a const Printable& ; from a Printable&& ; and from a const Printable&& . Solving this problem gets more painful if, like the STL, you have more than one type-erased type that need to play well together. Try to avoid getting into that situation if you can; but be aware of this pitfall if you must.

Footnote: Should you abandon function for copyable_function in C++26? Some would say yes. Personally I would say that you ought to abandon both, and write your own type-erased callable instead. It takes less than 100 lines!

In completely green-field C++26 code, sure, I’d recommend copyable_function over function (once your vendor implements it). I would not recommend mixing both types in the same brown-field codebase; I’d pick one and stick with it, while working toward a point where you could update the entire codebase at once.