Recent gcc adds a warning, -Wclass-memaccess, that warns if you use memset or
similar on a non-trivial type -- in other words, a type that requires something
be done to initialize instances of it (set fields, call a user-defined
constructor, init a vptr, etc.). The reason is that initialize-by-memset is
fragile: if your class ever gains initialization, memset will either overwrite
that initialization or falsely assure you that no further initialization is
necessary. Sad!
Mozilla code probably doesn't do this much *directly*, but unfortunately
PodZero and PodArrayZero do.
Fortunately, C++11 lets you use class member-initializers (especially
brace-initializers) to cleanly zero things.[0] If that doesn't cut it,
<algorithm>'s std::fill and std::fill_n are type-safe alternatives that
optimizing compilers will eat for breakfast.
Please don't add new uses of Pod{,Array}Zero, and start removing existing uses
where possible. It's generally not too tricky; I fixed all issues of this sort
building the JS engine last week, and the only serious quirks I hit were for
fairly unusual SpiderMonkey needs. I'll probably rename these to
DeprecatedPod{,Array}Zero to particularly discourage their use, until they can
be removed.
See
<http://whereswalden.com/2018/05/21/psa-stop-using-mozillapodzero-and-mozillapodarrayzero/>
for more information.
Thanks!
Jeff
0. The only exception are bit-fields. The best solution for bit-fields is to
dump them in a class, brace-initialize the whole thing to zeroes, then (if
needed) overwrite any customized fields in the enclosing class's constructor.
See <https://hg.mozilla.org/mozilla-central/rev/7c45180cea08> for an example.
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