How can I assign a variable in “if” condition in Dlang?

On Sun, May 03, 2020 at 02:53:21PM +0000, Baby Beaker via Digitalmars-d-learn wrote:
> How can I assign a variable in “if” condition in Dlang?

	if (auto obj = someFunction(...)) {
		// use obj here, it's guaranteed to be
		// true/non-null/etc.
	}


T

-- 
EMACS = Extremely Massive And Cumbersome System

On Sunday, 3 May 2020 at 14:53:21 UTC, Baby Beaker wrote:
> How can I assign a variable in “if” condition in Dlang?

depends on exactly what there's also

if ( (a = 10) )

with extra parens for special purposes

On 5/3/20 11:27 AM, Adam D. Ruppe wrote:
> On Sunday, 3 May 2020 at 14:53:21 UTC, Baby Beaker wrote:
>> How can I assign a variable in “if” condition in Dlang?
> 
> depends on exactly what there's also
> 
> if ( (a = 10) )
> 
> with extra parens for special purposes

That doesn't work. You need to do a comparison or a cast.

-Steve

On Sunday, 3 May 2020 at 15:13:48 UTC, H. S. Teoh wrote:
> On Sun, May 03, 2020 at 02:53:21PM +0000, Baby Beaker via Digitalmars-d-learn wrote:
>> How can I assign a variable in “if” condition in Dlang?
>
> 	if (auto obj = someFunction(...)) {
> 		// use obj here, it's guaranteed to be
> 		// true/non-null/etc.
> 	}

What if someFunction's return type is string? I mean this:

```if.d
void eval (string function () fun)
{
   import std.stdio;
   if (auto s = fun ())
      "true".writeln;
   else
      "false".writeln;
}

void main ()
{
   () { string s; return s; }.eval;       // empty string
   () { string s = ""; return s; }.eval;  // empty string
}
```

It surprisingly [1] [2] prints:

   false
   true

In the string case there is at least not the guarantee that the string is non-empty.

[1] Empty string vs null
    https://forum.dlang.org/thread/fehvcobsjuklxynbqqox@forum.dlang.org

[2] The Nullity Of strings and Its Meaning
    https://forum.dlang.org/thread/qcmtjfpkvcoyvrouvccw@forum.dlang.org

On 5/3/20 12:30 PM, kdevel wrote:
> It surprisingly [1] [2] prints:
> 
>     false
>     true
> 
> In the string case there is at least not the guarantee that the string is non-empty.

Then you have to do it in 2 steps. It would be nice to have a way to do this, but there isn't right now.

-Steve