C++23: two additional noexcept functions

If my math is correct, there are 125 changes / fixes / new features in C++23 and we are progressively covering them on this blog. I try to go from topic to topic. There are some topics with many smaller changes, such as constexpr, there are some significant topics where even one topic must be in its own post such as the stacktrace library, and there are also some shorter posts with few and quite small changes on a given topic. Today we are going to discuss noexcept related changes.

If you haven’t read it, I’d recommend reading my article on noexcept and its effects on binary size

We can definitely see two trends in the proposals accepted for the latest standards. They try to make more and more functions:

• constexpr, and
• noexcept

The two main changes presented today fit into this trend. But let’s start with discussing a bit of noexcept policies in the standard.

noexcept, but with conditions

The current policies on whether something in the standard can be noexcept or not is defined by P1656R2. Let me summarize the gist of it here.

As we got used to it, a destructor should never throw and therefore even if you don’t mark it noexcept they implicitly are!

There might be standard library functions marked unconditionally noexcept if the committee fully agrees that the given function cannot throw.

The standard specifies a couple of special member functions and library functions that can be marked conditionally noexcept, based on the underlying types and the types these functions operate on. Nothing else can be marked noexcept, except for library functions that are designed for compatibility with C. Those can be unconditionally noexcept.

Most often, we mark functions either noexcept or not. So we often mark our functions unconditionally noexcept. But noexcept can take a compile-time computable condition, such as std::is_nothrow_move_constructible_v<T> && std::is_nothrow_assignable_v<T&, U>

Here is the list that can be marked conditionally noexcept according to C++20.

• std::swap
• the copy-constructor and -assignment operator
• the move-constructor and -assignment operator

This list is getting modified in the C++23 standard.

It’s also worth noting that an implementation can mark conditionally noexcept a function even if it’s not listed by the standard so.

Add a conditional noexcept specification to std::exchange

One of the primary use cases for std::exchange is implementing the move constructor and move assignment operator. In a certain way, it’s quite similar to std::swap. Yet, while std::swap and move operations can be conditionally noexcept, it was not the case for std::exchange.

Up until C++23.

P2401R0 makes std::exchange conditionally noexcept. The conditions are the same as for move operations: is_nothrow_move_constructible_v<T> && is_nothrow_assignable_v<T&, U>.

Add a conditional noexcept specification to std::apply

With the introduction of zip algorithms in C++23, people in and around the committee started to talk once again more and more about std::apply.

The reason is that std::apply could be effectively used to implement these new algorithms. In fact, in the previously referenced proposal apply appears quite a few times. Sadly, std::apply is not noexcept.

But, if we have a look into the exposition-only implementation of apply, we can see that it uses invoke and get. The latter is noexcept and the former is conditionally noexcept, so there is no reason why std::apply should not be conditionally noexcept.

And that becomes the new reality with C++23:

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template<class F, class Tuple>
  constexpr decltype(auto) apply(F&& f, Tuple&& t) noexcept(see below);
  
// Let I be the pack 0, 1, ..., (tuple_size_v<remove_reference_t<Tuple>>-1). The exception specification is equivalent to: noexcept(invoke(std::forward<F>(f), get<I>(std::forward<Tuple>(t))...)).

Conclusion

In this article, we reviewed how the standard defines its policies towards the noexcept specification and we also see that two standard library functions (std::apply and std::exchange) are becoming noexcept.

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