# Re: [std-proposals] Integer overflow arithmetic

From: Marcin Jaczewski <marcinjaczewski86_at_[hidden]>
Date: Sat, 17 Feb 2024 12:59:00 +0100
sob., 17 lut 2024 o 12:49 Tiago Freire via Std-Proposals
<std-proposals_at_[hidden]> napisaĆ(a):
>
>
> >I don't really get the following case in your code:
> > if constexpr (sizeof(T) >= sizeof(U)) return false;
>
> Ok, the statement is phrased in the negative, i.e. If I can represent a value in its destination type then the result is false, meaning that casting this type doesn't change the meaning of the value.
> In this branch std::is_signed_v<T> == std::is_signed_v<U> is true, i.e. that they are both signed or both unsigned
> Then this compares the storage size, and T is the type we are converting too, if the type we are converting too has a greater or equal storage size and it has the same "signed'ness" , then I can always convert it without affecting the representation.
>
> Thus explaining
> > Why would T not be able to represent x in the event that T is larger.
> > Doesn't this imply that "would_cast_modify<long long>(0)" is false?
>
> false means you can represent.
>
>
> > In the case where the signedness is left unchanged by the cast, you can probably do:
> > static_cast<From>(static_cast<To>(x)) == x
> > This simply checks whether the roundtrip conversion preserves the value.
>
> Note we are not checking for round-trip convertible. Take the following example:
>
> constexpr int8_t test_case = -1;
> constexpr uint32_t converted_case = static_cast<uint32_t>(test_case);
> constexpr int8_t round_trip_case = static_cast<int8_t>(converted_case);
> static_assert(test_case == round_trip_case);
>
> This is round-trip convertible but it is not one-way convertible as converted_case cannot represent -1.

Maybe we need two conversions like:
```
static_cast<uint32_t>(static_cast<std::make_unsigned_t<decltype(test_case)>>(test_case))
== test_case
```
This will skip the sign extension on one side. And this will break
this equation if one side was negative.

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