https://forum.dlang.org/post/17nwtnp4are5q$.1ddtvmj4e23iy.dlg@40tude.net

On Tuesday, 10 May 2005 at 01:06:14 UTC, Derek Parnell wrote:
> On Tue, 10 May 2005 00:30:57 +0000 (UTC), Oliver wrote:
>
>> Hello D-ers
>> 
>> The documentation is very short on the keywords in, out and inout.
>> Is is inout sth like a reference ? But then, what is in and what is out?
>
>
> in:
> The argument is preserved, such that when control returns to the caller,
> the argument as passed by the caller is unchanged. This means that the
> called function can do anything it likes to the argument but those changes
> are never returned back to the caller. There is a bit of confusion here
> when it comes to passing class objects and dynamic arrays. In both these
> cases, a reference to the data is passed. Which means that for 'in'
> references, the called function is free to modify the reference data (which
> is what is actually passed) in the full knowledge that any changes will
> *not* be returned to the caller. However, if you make any changes to the
> data being referenced, that modified data is 'returned'. Which means that,
> for example, if you pass a char[] variable, the reference will be preserved
> but the data in the string can be changed.
>
> out:
> The argument is always initialized automatically by the called function
> before its code is executed. Any changes to the argument by the called
> function are returned to the caller. The called function never gets to see
> the value of the argument as it was before the called function gets
> control. The argument must a RAM item and not a literal or temporary value.
>
> inout:
> The argument is passed to the called function without before its code is
> executed. Any changes to the argument by the called function are returned
> to the caller. In other words, the called function can see what value was
> passed to it before changing it.  The argument must a RAM item and not a
> literal or temporary value.
>
>
> Examples:
>
>   char[] a;
>   int    b;
>
>   void func_one(in char[] X, in int Y)
>   {
>       X[0] = 'a';   // Modifies the string contents.
>       X = "zxcvb";  // Modifies the string reference but is not returned.
>
>       Y = 3; // Modifies the data but is not returned.
>   }
>
>
>   a = "qwerty";
>   b = 1;
>   func_one(a,b);
>
>   writefln("%s %d", a,b); // --> awerty 1
>
>   void func_two(out char[] X, out int Y)
>   {
>       X[0] = 'a';   // Modifies the string contents.
>       X = "zxcvb";  // Modifies the string reference.
>       if (b == 1)
>         Y = 3; // never executed because Y is always zero on entry.
>       else
>         Y = 4; // Modifies the data.
>   }
>
>
>   a = "qwerty";
>   b = 1;
>   func_two(a,b);
>
>   writefln("%s %d", a,b); // --> zxcvb 4
>
>   void func_three(inout char[] X, inout int Y)
>   {
>       X[0] = 'a';   // Modifies the string contents.
>       X = "zxcvb";  // Modifies the string reference.
>       if (b == 1)
>         Y = 3; // Modifies the data.
>       else
>         Y = 4; // Modifies the data.
>   }
>
>
>   a = "qwerty";
>   b = 1;
>   func_two(a,b);
>
>   writefln("%s %d", a,b); // --> zxcvb 3

Thanks a lot for your explanation.
I started to learn D language recently and I have trouble with understanding some part of language.
I do your example on Linux and it works very well, especially func_rthee().
when I try to repeat code on Windows 10, I get errors:
   Error: cannot modify `inout` expression `X[0]`
   Error: cannot modify `inout` expression `X`
   Error: cannot modify `inout` expression `Y`
   Error: cannot modify `inout` expression `Y`
Very strange situation for me.
I expect next behavior of the parameters X and Y could get value and could change it on Windows but now I am not sure what is goin on.

PS. I am using DMD64 D Compiler v2.098.1-dirty on Debian 10 Linux and Windows 10.

On Thursday, 20 January 2022 at 13:19:06 UTC, Sergey wrote:
> https://forum.dlang.org/post/17nwtnp4are5q$.1ddtvmj4e23iy.dlg@40tude.net
>
> On Tuesday, 10 May 2005 at 01:06:14 UTC, Derek Parnell wrote:
>> [...]
>
> Thanks a lot for your explanation.
> I started to learn D language recently and I have trouble with understanding some part of language.
> I do your example on Linux and it works very well, especially func_rthee().
> when I try to repeat code on Windows 10, I get errors:
>    Error: cannot modify `inout` expression `X[0]`
>    Error: cannot modify `inout` expression `X`
>    Error: cannot modify `inout` expression `Y`
>    Error: cannot modify `inout` expression `Y`
> Very strange situation for me.
> I expect next behavior of the parameters X and Y could get value and could change it on Windows but now I am not sure what is goin on.
>
> PS. I am using DMD64 D Compiler v2.098.1-dirty on Debian 10 Linux and Windows 10.

The explanation you quoted is from 2005, and `inout` does not mean the same thing in 2022 as it did in 2005.

The current meaning of inout is explained in the D language specification on dlang.org. Here is a link to the relevant section:

https://dlang.org/spec/function.html#inout-functions

On Thursday, 20 January 2022 at 13:28:54 UTC, Paul Backus wrote:
> On Thursday, 20 January 2022 at 13:19:06 UTC, Sergey wrote:
>> [...]
>
> The explanation you quoted is from 2005, and `inout` does not mean the same thing in 2022 as it did in 2005.
>
> The current meaning of inout is explained in the D language specification on dlang.org. Here is a link to the relevant section:
>
> https://dlang.org/spec/function.html#inout-functions

Thanks a lot.