default arguments for const ref parameters in extern C++ functions

[Mathias LANG]

On Monday, 14 September 2020 at 18:58:44 UTC, 60rntogo wrote:
> On Monday, 14 September 2020 at 17:11:59 UTC, k2aj wrote:
>> AFAIK the only way to have default ref arguments is to use a 
>> global variable:
>> ---
>> extern(C++) struct Foo
>> {
>>   int x;
>> }
>> immutable foo1 = Foo(1);
>> extern(C++) void fun(const ref Foo foo = foo1);
>> ---
>
> Thanks. This appears to work, but feels like a hack. I can live 
> with that, but I'm still wondering if there is a more idiomatic 
> way of solving the problem.

D doesn't let rvalues bind to const ref (well, `in` will do that, 
but it's D-only).
What was described as "global variable", is more of a "manifest 
constant lvalue".
A (static) immutable variable with an initializer will always be 
CTFE-d and available at compile time, so in this case it is 
*exactly* what you need.

Also note that a default parameter doesn't affect the mangling / 
ABI, because it works exclusively on the caller side. Thus, you 
could just remove the default argument and your `extern(C++)` 
binding would work just fine. But if you want to provide the same 
API, `[static] immutable` + initializer is the way to go.