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

[Johan]

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