# Understanding common std.Io.Reader/Writer mistakes (0.15.1)

**URL:** <https://ziggit.dev/t/understanding-common-std-io-reader-writer-mistakes-0-15-1/11793>\
**Category:** Explain\
**Created:** [August 31, 2025, 1:37am UTC](https://ziggit.dev/t/understanding-common-std-io-reader-writer-mistakes-0-15-1/11793 "2025-08-31T01:37:47Z")\
**Posts on this page:** 1\
**Showing post:** 14

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**Author:** ![npc1054657282](https://ziggit.dev/user_avatar/ziggit.dev/npc1054657282/32/5790_2.png) [@npc1054657282](https://ziggit.dev/u/npc1054657282)\
**Post date:** [September 3, 2025, 9:34pm UTC](https://ziggit.dev/t/understanding-common-std-io-reader-writer-mistakes-0-15-1/11793/14 "2025-09-03T21:34:23Z")

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> [@\`@fieldParentPtr()\` is innocent; what is dangerous is modifying a local copy of a structure](https://ziggit.dev/t/fieldparentptr-is-innocent-what-is-dangerous-is-modifying-a-local-copy-of-a-structure/11764/6):
>
> I must apologize for my assertion, as I’ve found that const isn’t always reliable. A value that might be modified by an API could very well be const. A typical example I’ve found so far is ArenaAllocator . Not all of its members are pointers—its state member contains an end\_index , which is actually modified by the API ArenaAllocator.allocator().alloc. However, because this modification is hidden by anyopaque , this value, while actually mutable, can be defined as const by the user without rece…

I’m very sorry. After reflection, I confirmed that my assertion that “copying `const` can guarantee correctness” was wrong.

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