Dlang equivalent of #define/#ifdef : not... version

[Steven Schveighoffer]

On 4/20/21 2:57 PM, ichneumwn wrote:
> Hi,
> 
> Trying to convert a C header to D. The underlying package exists in 
> different versions and may or may not expose certain functionality 
> (modules/extensions). One should check at compile time that the header 
> provides the relevant definitions, through #ifdef, and do a further 
> run-time check to confirm the functionality is really present. It is the 
> compile time check that I am finding tricky to do/emulate.
> 
>     .h : #define i_am_a_feature 1

enum i_am_a_feature = true;

> 
> The C-code uses this define as a guard:
> 
>     .c : #ifdef i_am_a_feature

static if(i_am_a_feature) {
    ...
}


> So my questions:
> - is there a module-crossing equivalent of "version"?

enum isn't exactly "module crossing", but it's defined within the 
module, which means you can look at it from elsewhere.

> - if not, is there some way I could test for the existence of the enum 
> can_i_test_for_this? A SymbolExists!() or ModuleHasSymbol!() or 
> ModuleHasMember!() ?
This is surprisingly tricky. A template will not accept a symbol by 
alias that doesn't exist, and if you pass in a string and test, it will 
test if that template can see the symbol, not your code. You can 
probably do it inline:

static if(__traits(compiles, {alias x = doesThisExist;}));

-Steve