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

[k2aj]

On Monday, 14 September 2020 at 12:44:34 UTC, 60rntogo wrote:
> I'm trying to use a C++ library that has a function declared 
> like this:
>
> ---
> struct Foo
> {
>   int x;
> };
>
> void fun(const Foo& foo = Foo(1));
> ---
>
> I have translated this to a D declaration:
>
> ---
> struct Foo
> {
>   int x;
> }
>
> extern(C++) void fun(const ref Foo foo = Foo(1));
> ---
>
> This yields an error: "Foo(1) is not an lvalue and cannot be 
> modified". I suppose this makes sense with how D references 
> work, but I have no control over how the C++ function is 
> declared. What can I do with this?

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);
---