Is it implementation defined which algorithms can accept a mutable lambda?

Question

I found here that lambdas are captured by value. This means that if an algorithm internally uses a second algorithm which accepts the lambda by value, any mutable state of the lambda will not be preserved. I will repost my linked questions example here:

remove_if(begin(values), end(values), [i = 0U, it = cbegin(intervals), end = cend(intervals)](const auto&) mutable {
    return it != end && ++i > it->first && (i <= it->second || (++it, true));
})

So at the time of writing my original question remove_if had implementation defined behavior for a mutable lambda. Is there a list of what other functions are implementation defined?

Where did you find that this has implementation defined behaviour? I genuinely don't see what part of this would even be implementation defined. — SirGuy
– SirGuy, Commented Oct 4, 2018 at 14:53
@SirGuy You can see the problem here: stackoverflow.com/questions/41103743/… basically internally to remove_if, gcc is using find_if which is copying by value. Each copy will create a new copy of i, it, and end. Thus they are not consistently updated. — Jonathan Mee
– Jonathan Mee, Commented Oct 4, 2018 at 14:56
@SirGuy It isn't implementation-defined, but for a different reason: There's no requirement for the implementation to define it. — Deduplicator
– Deduplicator, Commented Oct 4, 2018 at 15:01

Dan M. · Accepted Answer · 2018-10-04 14:56:11Z

4

Yes, it is. See [algorithms.requirements]:

http://eel.is/c++draft/algorithms#requirements-9

[Note: Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely. Programmers for whom object identity is important should consider using a wrapper class that points to a noncopied implementation object such as reference_wrapper, or some equivalent solution. — end note]

answered Oct 4, 2018 at 14:56

Dan M.

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4 Comments

Jonathan Mee Over a year ago

So... this is just the quote from the linked answer. I guess we're saying all of them may internally pass by value.

Dan M. Over a year ago

@JonathanMee yup. "Unless otherwise specified".

Jonathan Mee Over a year ago

Is there other specification on any of the algorithms though, that's the question?

Dan M. Over a year ago

@JonathanMee I didn't see any and I assume there is none since it doesn't make much sense (may needlessly restrict implementation). I'd imagine there are the opposite cases, there the predicate must be copied or be pure (i.e. some parallel algos might not deal nciely with shared mutable state). If you really need to avoid a copy then just use the suggested solution.

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