If Statement with Declaration
Jerry via Digitalmars-d
digitalmars-d at puremagic.com
Fri Nov 4 08:27:08 PDT 2016
On Friday, 4 November 2016 at 14:10:51 UTC, Steven Schveighoffer
wrote:
> On 11/3/16 6:29 PM, Jerry wrote:
>>
>> So I was thinking of a way of extending if statements that have
>> declarations. The following being as example of the current
>> use of if
>> statements with declarations:
>>
>> if(int* weDontPollute = someFunc())
>> {
>> // use weDontPollute
>> }
>>
>> That's great and all, but it only works by checking if the
>> variable
>> evaluates to true or false. Which is fine for a pointer but
>> otherwise
>> useless for anything else, like integers where zero is usually
>> valid
>> input (index for array). So currently the only way to do
>> something like
>> this in the language, that i've found, is to use a for
>> statement.
>>
>> for(int i = someFunc(); i >= 0;)
>> {
>> // use i
>>
>> break;
>> }
>>
>> Not that ideal to use a for statement. It makes it hard to
>> read and if
>> the break condition isn't there it might very well be an
>> infinite loop.
>> So I was thinking of some sort of syntax like this:
>>
>> if(int i = someFunc(); i >= 0)
>> {
>> // use i
>> }
>> Thoughts on this sort of feature?
>
>
> Hm... what about something like:
>
> struct BoolCond(T, string cond)
> {
> T val;
> bool opCast(B)() if(is(B == bool))
> {
> return mixin("val " ~ cond);
> }
> }
>
> auto boolcond(string cond, T)(T val)
> {
> return BoolCond!(T, cond)(val);
> }
>
> if(auto i = someFunc.boolcond!(">= 0"))
> {
> ... // use i
> }
>
> -Steve
Well that's just a basic case, what if you want more than one
condition. Using "&&" for example, or a condition along with
another value that's in the scope.
void otherFunc(int input)
{
if(auto i = someFunc.boolcond!(">= input")) // --- error
{
... // use i
}
}
Then you need another work around for something like this:
if(int value; auto i = someFunc(&value))
{
// ...
}
More information about the Digitalmars-d
mailing list