Why can't we use strings in C++ methods?

[Emmanuel Danso Nyarko]

On Saturday, 4 November 2023 at 12:21:45 UTC, Johan wrote:
> On Saturday, 4 November 2023 at 12:01:11 UTC, Emmanuel Danso 
> Nyarko wrote:
>> On Saturday, 4 November 2023 at 11:18:02 UTC, Dadoum wrote:
>>>
>>> ```d
>>> extern (C) void hello(string arg) {
>>>     import std.stdio;
>>>     writeln(arg);
>>> }
>>> ```
>>>
>>> Compiles fine with dmd, ldc2 and gdc.
>>>
>>>
>>> ```d
>>> extern (C++) void hello(string arg) {
>>>     import std.stdio;
>>>     writeln(arg);
>>> }
>>> ```
>>>
>>> Doesn't compile.
>>>
>>> DMD: `Internal Compiler Error: type `string` cannot be mapped 
>>> to C++`
>>> GDC and LDC2: `function 'example.hello' cannot have parameter 
>>> of type 'string' because its linkage is 'extern(C++)'`
>>>
>>> And I am wondering why the type can be mapped to a template 
>>> in C but not in C++. (you can see the template used when you 
>>> compile with `-H --HCf=./header.h`
>>
>> So C-strings are just an array of characters that are governed 
>> by simple functions and D strings also defined the same.
>
> This is not true. D string (=slice) variables store the length 
> of the string in addition to the reference to the array of 
> characters.
>
> The reason this "works" with `extern (C)` is because the C 
> mangling of a function name does not include the type of the 
> parameters. Note that C does not have a `string` type, so to 
> call the function from C you will have to write a different 
> function signature in C (you'll see that `char[]` will not 
> work).
>
> It does not work with `extern(C++)` because the C++ mangling of 
> a function _does_ include the type of the parameters, and there 
> is no built-in C++ type that is equivalent to D's `string`.
>
> -Johan

You're right but that's not what he's looking for I think. He 
wants to understand why it doesn't compile at all. One major 
cause of failed compilation is syntax disagreements And my main 
point is that because C++ doesn't know independent 'string' and 
that the string in C++ is a standard template library, the D 
compiler decides to stop any symbol generated interaction with 
string because C++ syntatically doesn't know 'string'.

Maybe the compiler team could provide a better answer to him but 
that's what I think.