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[docs] Add a note on iteration of unordered containers to coding standards

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Summary: Beware of non-determinism due to ordering of pointers

Reviewers: dblaikie, dexonsmith

Reviewed By: dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37525

llvm-svn: 312667
This commit is contained in:
Mandeep Singh Grang 2017-09-06 20:19:10 +00:00
parent cae9ddd062
commit 12350cd943
1 changed files with 15 additions and 0 deletions
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docs/CodingStandards.rst
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@ -811,6 +811,21 @@ As a rule of thumb, use ``auto &`` unless you need to copy the result, and use
for (const auto *Ptr : Container) { observe(*Ptr); }
for (auto *Ptr : Container) { Ptr->change(); }
Beware of non-determinism due to ordering of pointers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
In general, there is no relative ordering among pointers. As a result,
when unordered containers like sets and maps are used with pointer keys
the iteration order is undefined. Hence, iterating such containers may
result in non-deterministic code generation. While the generated code
might not necessarily be "wrong code", this non-determinism might result
in unexpected runtime crashes or simply hard to reproduce bugs on the
customer side making it harder to debug and fix.
As a rule of thumb, in case an ordered result is expected, remember to
sort an unordered container before iteration. Or use ordered containers
like vector/MapVector/SetVector if you want to iterate pointer keys.
Style Issues
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