std::copyable_function is a polymorphic function wrapper that owns a callable target. Unlike std::function, it correctly propagates const, reference qualifiers, and noexcept specifications to the call operator. Unlike std::move_only_function, it requires the stored callable to be copyable.

std::function has a const call operator that invokes non-const callables, allowing mutation through a const reference. Its design predates later C++ features like noexcept and ref-qualifiers. std::copyable_function addresses these issues while remaining copyable, serving as the intended replacement for std::function in new code.

#include <functional>
#include <print>

int main() {
    int counter = 0;

    // const-qualified: can only be called on const wrapper
    std::copyable_function<int() const> fn = [=]() { return counter + 1; };
    std::println("Result: {}", fn());

    // noexcept-qualified: callable must not throw
    std::copyable_function<int() noexcept> safe_fn = []() noexcept { return 123; };
    std::println("Safe result: {}", safe_fn());
}