Re: If Statement with Declaration

[Jerry via Digitalmars-d]

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))
{
   // ...
}