I was wondering why C++ linkage forbids strings as arguments while we can with the C one.

With C linkage, it's translated to a template that's defined in the automatically generated header, but it just doesn't compile in C++.

On Saturday, 4 November 2023 at 03:00:49 UTC, Dadoum wrote:

Can you provide an example of exactly what you want to do?

On Saturday, 4 November 2023 at 10:08:20 UTC, Imperatorn wrote:

extern (C) void hello(string arg) {
    import std.stdio;
    writeln(arg);
}

Compiles fine with dmd, ldc2 and gdc.

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

On Saturday, 4 November 2023 at 11:18:02 UTC, Dadoum wrote:

extern (C) void hello(string arg) {
    import std.stdio;
    writeln(arg);
}

extern (C++) void hello(string arg) {
    import std.stdio;
    writeln(arg);
}

So C-strings are just an array of characters that are governed by simple functions and D strings also defined the same. So you could see that D strings are possibly built on the architecture of C strings. In C++, string is a complex standard template library that generates complex symbols and cannot be mapped as it is based on a standard library implementation.

On Saturday, 4 November 2023 at 11:18:02 UTC, Dadoum wrote:

extern (C) void hello(string arg) {
    import std.stdio;
    writeln(arg);
}

extern (C++) void hello(string arg) {
    import std.stdio;
    writeln(arg);
}

We can just assume what you're doing on the C++-side. Are you using std::string?

You could try as a pointer + length and it might work, but without seeing your complete code it's quite hard to know what you want to do.

On Saturday, 4 November 2023 at 12:01:11 UTC, Emmanuel Danso Nyarko wrote:

extern (C) void hello(string arg) {
    import std.stdio;
    writeln(arg);
}

extern (C++) void hello(string arg) {
    import std.stdio;
    writeln(arg);
}

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

On Saturday, 4 November 2023 at 12:21:45 UTC, Johan wrote:

extern (C) void hello(string arg) {
    import std.stdio;
    writeln(arg);
}

extern (C++) void hello(string arg) {
    import std.stdio;
    writeln(arg);
}

What I don't understand is why it cannot use the template it defines in the header.

Here it is:

// Automatically generated by LDC Compiler

#pragma once

#include <assert.h>
#include <math.h>
#include <stddef.h>
#include <stdint.h>

#ifdef CUSTOM_D_ARRAY_TYPE
#define _d_dynamicArray CUSTOM_D_ARRAY_TYPE
#else
/// Represents a D [] array
template<typename T>
struct _d_dynamicArray final
{
    size_t length;
    T *ptr;

    _d_dynamicArray() : length(0), ptr(NULL) { }

    _d_dynamicArray(size_t length_in, T *ptr_in)
        : length(length_in), ptr(ptr_in) { }

    T& operator[](const size_t idx) {
        assert(idx < length);
        return ptr[idx];
    }

    const T& operator[](const size_t idx) const {
        assert(idx < length);
        return ptr[idx];
    }
};
#endif

extern "C" void hello(_d_dynamicArray< const char > arg);

And the D compiler can generate templates in the mangled name, as this D code is translated to C++:

class Foo(T) {}

extern (C++) void hello2(Foo!char arg2) {

}

// Automatically generated by LDC Compiler

#pragma once

#include <assert.h>
#include <math.h>
#include <stddef.h>
#include <stdint.h>

#ifdef CUSTOM_D_ARRAY_TYPE
#define _d_dynamicArray CUSTOM_D_ARRAY_TYPE
#else
/// Represents a D [] array
template<typename T>
struct _d_dynamicArray final
{
    size_t length;
    T *ptr;

    _d_dynamicArray() : length(0), ptr(NULL) { }

    _d_dynamicArray(size_t length_in, T *ptr_in)
        : length(length_in), ptr(ptr_in) { }

    T& operator[](const size_t idx) {
        assert(idx < length);
        return ptr[idx];
    }

    const T& operator[](const size_t idx) const {
        assert(idx < length);
        return ptr[idx];
    }
};
#endif

template <typename T>
class Foo;

extern void hello2(Foo<char >* arg2);

On Saturday, 4 November 2023 at 12:07:12 UTC, Imperatorn wrote:

I will use whatever D gives me in the C++ header, because in fact I am using Swift on the other side, so using std::string isn't useful. I already made a conversion function from Swift strings to D strings when they are represented with C linkage.

On Saturday, 4 November 2023 at 12:01:11 UTC, Emmanuel Danso Nyarko wrote:

I don't think the strings are being translated in any way when we change the linkage. Strings in D are dynamic arrays of chars, the C binding code is giving us a template representing D's dynamic arrays, and so I thought that we could also use it in C++ for interoperability with D dynamic arrays.

On Saturday, 4 November 2023 at 12:21:45 UTC, Johan wrote:

extern (C) void hello(string arg) {
    import std.stdio;
    writeln(arg);
}

extern (C++) void hello(string arg) {
    import std.stdio;
    writeln(arg);
}

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.