[PATCH] D83305: [ADT] Fix join_impl using the wrong size when calculating total length

[David Blaikie via Phabricator via llvm-commits]

dblaikie added a comment.

In D83305#2474484 <https://reviews.llvm.org/D83305#2474484>, @njames93 wrote:

> In D83305#2474470 <https://reviews.llvm.org/D83305#2474470>, @dblaikie wrote:
>
>> In D83305#2474466 <https://reviews.llvm.org/D83305#2474466>, @njames93 wrote:
>>
>>> In D83305#2474378 <https://reviews.llvm.org/D83305#2474378>, @dblaikie wrote:
>>>
>>>> Any chance of testing this? Could test the 'capacity' after join to check its the right size?
>>>
>>> How do you suppose I do that, maybe expensive checks to get the capacity after reserve and after all items have been added and ensure they are the same, otherwise the string grew during appending.
>>
>> Oh, nah, nothing that'd require expensive checks - you could add an assert that size == Len at the end of the function (would have to have some way of avoiding the case for short results that are below the starting capacity, though... perhaps checking if capacity grew over the reserve call, and only asserting in that case)). But I was thinking more along the lines of a unit test that tests join and checks the capacity isn't bigger than the size for a few hardcoded samples.
>
> Oh that assert is fine, the unit test won't be a good test. As we don't control std string, it's capacity for calls to reserve will be implementation defined

The spec seems to be a bit more precise than that (using cppreference, which isn't authoritative, but more readable): "If new_cap is greater than the current capacity(), new storage is allocated, and capacity() is made equal or greater than new_cap."

So a unit test that joins some large strings (if it wanted to be fancy, it could even generate a string intentionally larger than std::string's default capacity, whatever it happens to be) and if the resulting string has a larger capacity than std::string's default capacity, verify that the capacity is exactly equal to the string length.


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