If Statement with Declaration

[Steven Schveighoffer via Digitalmars-d]

On 7/19/17 9:30 AM, sontung 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?


I really like the idea. Only thing I don't like is the possibility for 
abuse/confusion/errors:

if(int i = someFunc(); j >= 0) // typo, or weird relationship, or just 
intentional obfuscation?

It reminds me a bit of why we got rid of the comma operator.

This is why I've liked suggestions in the past like:

if((int i = foo()) >= 0)

That is, you want to use 'if' on an expression while saving the 
expression, but the if is only looking at a property of that expression.

Note this makes if(arr) (the correct meaning, that is ;) much more 
palatable:

if((auto x = getArray()).length)

Don't get me wrong, if this syntax is what gets this idea in, I'm fine 
with it.

One possibility is to require usage of the declared variable in the 
condition.

-Steve