Fixing reinterpret_cast UB with start_lifetime_as in C++23

[ariddl]

Channel

C++ Weekly

Topics

1. Quick review of UB from reinterpret_cast
2. Quick review of std::bit_cast and any of its limitations
3. (New in C++23) start_life_timetime_as and start_lifetime_as_array

Length

Longer form (~10 min)

Note

I know there is an existing suggestion here from 2022 on this C++23 feature, but I really wanted to bump it. Pointer aliasing continues to be a very common source of UB from my experience inside and outside the workforce, and I really want more attention on this topic since a true solution is coming in C++23.

[lefticus]

I have been literally waiting years to do this episode - no compiler supports start_lifetime_as yet, so it effectively doesn't exist!

[Dharmesh946]

indeed.
Yet they are lot's of expectation about type punning and lots of wrong info about what is possible or not with C++. start_life_time_as only covers a special case.
I think that they are many episode suggestions about that and there is already an episode about reinterpret_cast. But there is more to that: implicit_lifetime objects, allocation/deallocation in presence of type-punning, std::launder correct usage (and what it cannot do also),...

[LB--]

@Dharmesh946 you might be interested in this video: A Deep Dive Into C++ Object Lifetimes - Jonathan Müller - C++Now 2024

[Dharmesh946]

@lb thanks a lot, I'll have a look at it