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0 pointsverdagon4y ago0 comments

It's because const pointers aren't "deeply" immutable. For example, if we have a const Car*, we can reach into it to grab a (non-const) Engine* through which we can modify things.

If there was a "imm" keyword in C++ which acted "deeply", that would get us pretty far towards our goal here. However, we'd then find ourselves in cases where we need to (for example) cast from an imm Engine* back to a (non-const) Engine*, often for values returned from functions. That's what this new "region borrow checker" concept would solve.

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3 comments · 2 top-level

fivea4y ago· 1 in thread

> For example, if we have a const Car, we can reach into it to grab a (non-const) Engine through which we can modify things.

Isn't that already deep within undefined behavior territory?

That hardly is an adequate example, given that it spells an eggregious fault, the kind that's covered by any intro to C++ course with clear indications that it's nonsense.

flqn4y ago

Getting a non-const reference out of a const object isn't undefined behaviour.

WalterBright4y ago

In D we call "deeply" immutable "transitive immutable".

Interestingly, a transitive immutable data structure can be implicitly converted to transitive const, but not vice versa.

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