D source:

extern(C++) void thisWorks(const char* test);
extern(C++) void doesNotLink(const char** test);

void main() {
    char* baz1;
    char** baz2;
    thisWorks(baz1);
    doesNotLink(baz2);
}

CPP source:

#include <stdio.h>

void thisWorks(const char* test) {
    printf("hi\n");
}

void DoesNotLink(const char** test) {
    // not reached
}

The error given by the linker:
test.d:(.text._Dmain+0x21): undefined reference to `doesNotLink(char const* const*)'

What's happening? I know there are differences in const behavior between D and C++, but can't figure a way around it. Any sort of casting trick I tried doesn't seem to help.

On Wednesday, 1 June 2016 at 07:09:16 UTC, abad wrote:
> D source:
>
> extern(C++) void thisWorks(const char* test);
> extern(C++) void doesNotLink(const char** test);
>
> void main() {
>     char* baz1;
>     char** baz2;
>     thisWorks(baz1);
>     doesNotLink(baz2);
> }
>
> CPP source:
>
> #include <stdio.h>
>
> void thisWorks(const char* test) {
>     printf("hi\n");
> }
>
> void DoesNotLink(const char** test) {
>     // not reached
> }
>
> The error given by the linker:
> test.d:(.text._Dmain+0x21): undefined reference to `doesNotLink(char const* const*)'
>
> What's happening? I know there are differences in const behavior between D and C++, but can't figure a way around it. Any sort of casting trick I tried doesn't seem to help.

Try this:

extern(C++) void DoesNotLink(const(char)**);

On Wednesday, 1 June 2016 at 07:17:04 UTC, Mike Parker wrote:
> On Wednesday, 1 June 2016 at 07:09:16 UTC, abad wrote:

>>
>> What's happening? I know there are differences in const behavior between D and C++, but can't figure a way around it. Any sort of casting trick I tried doesn't seem to help.
>
> Try this:
>
> extern(C++) void DoesNotLink(const(char)**);

And I don't know if this was just a typo in your post,  but also note that you 'D'oesNotLink in the CPP source you pasted and 'd'oesNotLink in the D code.

On Wednesday, 1 June 2016 at 07:17:04 UTC, Mike Parker wrote:
> On Wednesday, 1 June 2016 at 07:09:16 UTC, abad wrote:
>
> Try this:
>
> extern(C++) void DoesNotLink(const(char)**);

That does work, though I have to explicitly cast it in my caller as well.
Like this:

doesNotLink(cast(const(char)**)baz2);

It's a bit troublesome as my code will include quite a lot of calls like this.

Casting is not necessary with the method call with the single pointer argument, as it seems to get const'ed automatically.

> And I don't know if this was just a typo in your post,  but also note that you 'D'oesNotLink in the CPP source you pasted and 'd'oesNotLink in the D code.

That was just a typo.

On Wednesday, 1 June 2016 at 07:29:56 UTC, abad wrote:
> That does work, though I have to explicitly cast it in my caller as well.
> Like this:
>
> doesNotLink(cast(const(char)**)baz2);
>
> It's a bit troublesome as my code will include quite a lot of calls like this.
>
> Casting is not necessary with the method call with the single pointer argument, as it seems to get const'ed automatically.

Can you declare it as const char*const* one the C++ side?

On Wednesday, 1 June 2016 at 16:16:26 UTC, Kagamin wrote:
> Can you declare it as const char*const* one the C++ side?

Just to state the problem clearly, D's const is transitive, C++ it is not. C linkage doesn't care about const, so you can specify it however you want. In C++ the const is included in the mangling, so it must match the declaration between C++ and D.

Transitive meaning that const will apply to all elements pointed to. This is why you see linker complain about not being able to find char const * const *