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digitalmars.D.learn - How to check for combinations of versions

reply Blatnik <blatblatnik gmail.com> writes:
Is there any way to check for multiple conditions in a `version` 
statement?

For example, my platform may have `Version_A` and `Version_B`, 
and both versions provide some shiny feature I want to use. Is 
there some nice way to write:

```D
version (Version_A || Version_B) {
   // Use the cool feature.
} else {
   // Do something less cool but portable.
}
```

Currently I resort to something like this, but I'm curious if 
there's a nicer way to do it.

```D
version (Version_A) {
   enum Cool_Feature_Supported = true;
} else version (Version_B) {
   enum Cool_Feature_Supported = true;
} else {
   enum Cool_Feature_Supported = false;
}

...

void do_something_cool() {
   static if (Cool_Feature_Supported) {
     ...
   } else {
     ...
   }
}
```
May 05 2021
next sibling parent reply Paul Backus <snarwin gmail.com> writes:
On Wednesday, 5 May 2021 at 15:03:16 UTC, Blatnik wrote:

 Currently I resort to something like this, but I'm curious if 
 there's a nicer way to do it.

 ```D
 version (Version_A) {
   enum Cool_Feature_Supported = true;
 } else version (Version_B) {
   enum Cool_Feature_Supported = true;
 } else {
   enum Cool_Feature_Supported = false;
 }
 ```


This is the officially-recommended way to do it. D's `version` 
system is deliberately restricted, in order to avoid the 
"`#ifdef` hell" that often plagues C and C++ projects.

However, if you really want something more expressive, and are 
confident in your ability to use the extra power responsibly, it 
is possible to work around these limitations:

     template hasVersion(string identifier) {
         mixin(
             "version(", identifier, ") enum hasVersion = true;",
             "else enum hasVersion = false;"
         );
     }

     // Usage
     static if (hasVersion!"Version_A" || hasVersion!"Version_B") {
         enum Cool_Feature_Supported = true;
     } else {
         enum Cool_Feature_Supported = false;
     }
May 05 2021
parent Blatnik <blatblatnik gmail.com> writes:
On Wednesday, 5 May 2021 at 15:25:31 UTC, Paul Backus wrote:

 However, if you really want something more expressive, and are 
 confident in your ability to use the extra power responsibly, 
 it is possible to work around these limitations:

     template hasVersion(string identifier) {
         mixin(
             "version(", identifier, ") enum hasVersion = true;",
             "else enum hasVersion = false;"
         );
     }


Hmm, I'll keep this in mind in case things start getting start 
getting more out of hand. Thanks! :)

Mixin strings are still kind of like black magic to me, so I 
prefer not to use them unless I really think they would help a 
lot. In this case, I only do this 3 times in the entire codebase, 
so it's not worth it yet.

And Dennis, sadly that wouldn't work for me since `version = XYZ` 
only affects the current module and I'm using this check in 
multiple modules.

Thanks for the help!
May 05 2021
prev sibling parent Dennis <dkorpel gmail.com> writes:
On Wednesday, 5 May 2021 at 15:03:16 UTC, Blatnik wrote:

 Is there any way to check for multiple conditions in a 
 `version` statement?


No, and that's by design to discourage complex version logic.
The recommended approach is:

```D
version (Version_A) version = Cool_Feature_Supported;
version (Version_B) version = Cool_Feature_Supported;

void do_something_cool() {
   version(Cool_Feature_Supported) {
     ...
   } else {
     ...
   }
}

```
May 05 2021