Does D have an equvalent of: if (auto = expr; expr)

[Basile B.]

On Friday, 7 February 2020 at 08:52:44 UTC, mark wrote:
> Some languages support this kind of thing:
>
> if ((var x = expression) > 50)
>   print(x, " is > 50")
>
> Is there anything similar in D?

Yes assuming that the expression is bool evaluable. This includes

- pointers: `if (auto p = giveMeSomePtr()){}`
- classes references: `if (auto p = giveMeSomeClasses()){}`
- integers `if (auto p = giveMeAnInt()){}`

and using the in operators as you've been answered previously.
The problem is that this support only one variable and that the 
If condition must be either a variable or a relational 
expression. Not both.

To overcome the limitation of a single variable I've made a 
little template:

---
/**
  * Encapsulates several variables in a tuple that's usable as a 
if condition,
  * as a workaround to the single declaration allowed by the 
language.
  *
  * Params:
  *      a = The expressions giving the variables.
  *          The variables must be evaluable to $(D bool).
  *
  * Returns:
  *      A tuple containing the variables.
  */
auto ifVariables(A...)(auto ref A a)
if (A.length)
{
     static struct IfVariables(A...)
     {
         private A tup;
         alias tup this;

         this() @disable;
         this(this) @disable;

         this(ref A a)
         {
             tup = a;
         }

         bool opCast(T : bool)() const
         {
             static foreach (i; 0 .. A.length)
                 if (!tup[i])
                     return false;
             return true;
         }
     }
     return IfVariables!A(a);
}
///
unittest
{
     assert(ifVariables(new Object, true, new Object));
     assert(!ifVariables(new Object, false, new Object));
     // typical usage
     bool isDlangExpressive(){return true;}
     if (auto a = ifVariables(new Object, isDlangExpressive())) {}
     // use the variables
     if (auto a = ifVariables(new Object, new Object))
     {
         assert(a.length == 2);
         assert(a[0] !is a[1]);
     }
}

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