Why is std::move not [[nodiscard]] in C++20?

Question

I\'ve recently read about [[nodiscard]] in C++17, and as far as I understand it\'s a new feature (design by contract?) which forces you to use the return value.

Answer 1

AFAIK P0600R1 is the only proposal for adding [[nodiscard]] to the standard library that was applied to C++20. From that paper:

We suggest a conservative approach:

[...]

It should not be added when:

• [...]
• not using the return value makes no sense but doesn’t hurt and is usually not an error
• [...]

So, [[nodiscard]] should not signal bad code if this

• [...]
• doesn’t hurt and probably no state change was meant that doesn’t happen

So the reason is that the standard library uses a conservative approach and a more aggresive one is not yet proposed.

Answer 2

The MSVC standard library team went ahead and added several thousand instances of [[nodiscard]] since VS 2017 15.6, and have reported wild success with it (both in terms of finding lots of bugs and generating no user complaints). The criteria they described were approximately:

1. Pure observers, e.g. vector::size(), vector::empty, and even std::count_if()
2. Things that acquire raw resources, e.g. allocate()
3. Functions where discarding the return value is extremely likely to lead to incorrect code, e.g. std::remove()

MSVC does mark both std::move() and std::forward() as [[nodiscard]] following these criteria.

While it's not officially annotated as such in the standard, it seems to provide clear user benefit and it's more a question of crafting such a paper to mark all the right things [[nodiscard]] (again, several thousand instances from MSVC) and apply them -- it's not complex work per se, but the volume is large. In the meantime, maybe prod your favorite standard library vendor and ask them to [[nodiscard]] lots of stuff?