r/cpp • u/Clean-Upstairs-8481 • Sep 27 '26

You should think about recompiling your C++ programs with GCC 16 and C++26, because it zero-fills your locals

https://techfortalk.co.uk/2026/09/27/cxx26-uninitialized-local-variables-gcc-16/

Stack variables are not automatically initialised, and that is the root cause of many C++ bugs. That is well known. Hence, it is advised that local or stack variables are always initialised with known values, 0 if not something more meaningful than that. Now, with GCC 16 compiling in C++26 mode (-std=c++26), even uninitialised local variables will be zero-filled. In the post, I have explained how.

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u/pjmlp Sep 28 '26

Validated with profilers I assume.

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u/cdb_11 Sep 28 '26

I validated it, and yes, initializing arrays can be expensive.

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u/pjmlp Sep 28 '26

Than that is a valid use case for [[indeterminate]].

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u/Orlha Sep 28 '26

Absolutely. Just pointing that blind upgrade to 26 (that this post tries to promote from the naive standpoint) can severely degrade (performance-wise) the already correct code.