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Subject: Re: [std-proposals] Labeled 'continue' and 'break'
From: Charles Milette (charlesmilette_at_[hidden])
Date: 2020-03-10 21:25:12


The suggested syntax is a direct copy-paste from Java.

On Tue, Mar 10, 2020 at 8:50 PM connor horman via Std-Proposals
<std-proposals_at_[hidden]> wrote:
>
> I’m pretty sure Java has something similar, though that would have less confusion (as you cannot use goto in Java). I would support the feature in some way though, I find it rather annoying in some cases when I need to exit nested loops.
>
> On Tue, Mar 10, 2020 at 20:35 Jake Arkinstall via Std-Proposals <std-proposals_at_[hidden]> wrote:
>>
>> I think the break label syntax is a little bit confusing to the casual reader, given that it's actually going to jump past the loop described by the label. Continue too, to some extent, because it jumps to the evaluation of the condition, whereas the immediate code after the label is the initialisation.
>>
>> Of course, this confusion is just caused by expecting goto like behaviour, when the implementation would actually be a goto to a label based on the loop label*. I have actually seen goto used for this circumstance in relatively modern C++ code, particularly when the "return from a lambda" approach doesn't suffice (I.e. when you want to break from a number of loops that depend on a condition).
>>
>> I'd actually make the labels optional and have break N, where N is how many loops to break out of. Same goes for continue. I think this is neater, but that's probably some (rare) nostalgia from my PHP days.
>>
>> * such that
>>
>> label1: for(int i = 0; x < 5; x++){
>> if(x % 3 == 1){
>> continue label1;
>> }else if(x % 3 == 2){
>> break label1;
>> }
>> }
>>
>> would be equivalent to
>>
>>
>> {
>> int x = 0;
>> while(x < 5){
>> if(x % 3 == 1){
>> goto label1_continue;
>> }else if(x % 3 == 2){
>> goto label1_break;
>> }
>> label1_continue:
>> x++;
>> }
>> label1_break:
>> }
>>
>>
>> On Wed, 11 Mar 2020, 00:12 Ryan Nicholl via Std-Proposals, <std-proposals_at_[hidden]> wrote:
>>>
>>>
>>> I'd like to suggest adopting labeled break and continue statements,
>>>
>>> e.g.
>>>
>>> label1: for( int x = 0; x < 5; x++)
>>> {
>>> label2: for (int y = 0; y < 5; y++)
>>> {
>>> for (int z = 0; z < 5; z++)
>>> {
>>> std::cout << x << ',' << y << ',' << z << std::endl;
>>> if (z+y == 8) continue label2;
>>> if (z+x == 3) break label2;
>>> if (x + y + z == 13) break label1;
>>> }
>>> }
>>> }
>>>
>>> Other languages have it, and it's convenient with nested loops.
>>>
>>> --
>>> Std-Proposals mailing list
>>> Std-Proposals_at_[hidden]
>>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>
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>
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