Re: [boost] Mother of all Variants

On 6/27/19 6:16 PM, Robert Ramey via Boost wrote:

On 6/26/19 11:45 PM, degski via Boost wrote:

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On Thu, 27 Jun 2019 at 09:35, Robert Ramey via Boost wrote:

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  From time to time I think about this.  Is swap guarenteed not to throw?

As from C++11 std::swap is marked noexcept, so yes. As from C++20 it's also constexpr, so now it can definitely cannot throw.

No, std::swap is marked conditionally noexcept if is_nothrow_move_constructible_v<T> && is_nothrow_move_assignable_v<T>. Though usually this condition is true.

Hmmm - my understanding of noexcept was not that it couldn't throw but than any attempt to throw would result in immediate termination rather than an exception.  (I'm confused about noexcept as well).

Both statements are true: - From the noexcept function perspective, if an exception tries to propagate out of the function scope, call std::terminate(). - From the caller's perspective, a noexcept function will never throw.

In anycase I should have phrased the above as:

Is swap guaranteed to succeed?

In general, no. It is only guaranteed to succeed if it is noexcept. See above when it is noexcept. In C++17 there is is_nothrow_swappable trait that allows to test this.