On Tuesday, April 29th, 2025 at 3:43 AM, Aurelien Cassagnes via Std-Proposals <std-proposals_at_[hidden]> wrote:

> There is a fairly arbitrary (unless I missed an important argument) limitation on initialization clause for if statements.
>

> - I have that
> if (auto it = vec.begin(); it != vec.end()) {
>

> - I want this
> if (auto it = vec.begin() /*and*/ auto itt = ++vec.begin(); it != vec.end() && it != itt) {
>

> So you end up using one of the workarounds, which are somewhat hacky imo :/


The particular example can use multi-declarator in
the declaration as https://lists.isocpp.org/std-proposals/2025/04/13529.php
showed. If the two initializers can deduce to
different types, then

    if (auto [it, itt] = std::tuple{ v1.begin(), std::next(v1.begin()) };
        it != v1.end() && itt != v1.end()) {

To push this idea further, you can do this in C++26

    template <class It1, class It2, class It3 = decltype(std::next(std::declval<It1>()))> struct first_last_next{
        It1 first;
        It2 last;
        It3 next = std::next(first);
   
        explicit constexpr operator bool() const noexcept
        {

            return first != last and first != next;

        }
    };
   
    if (auto [it, _, itt] = first_last_next{v1.begin(), v1.end()}) {



(* just a contrived snippet to show the possibility;std::next doesn't return an iterator of a different type. *)

--
Zhihao Yuan, ID lichray
The best way to predict the future is to invent it.
_______________________________________________
> I'd like to release that restriction and have for example
> if ({
> auto it = vec.begin();
> auto itt = ++vec.begin();
> }; it != vec.end() && it != itt) {
> 
> which can be desugared (desugarified ?) to e.g.
> {
> auto it = vec.begin();
> auto itt = vec.begin();
> if (it != vec.end()) {
> }
> }
> 
> Colleague pointed me to a GCC expression statements extension which ?maybe? makes `if ({` ambiguous to parse, so I would have to look into that too…
> 
> Is there any current proposal that aims to lift that restriction ?
> Cheers.
> 
> Sent from Gmail Mobile