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
What I don't understand is why it cannot use the template it
defines in the header.
Here it is:
```c++
// 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++:
```d
class Foo(T) {}
extern (C++) void hello2(Foo!char arg2) {
}
```
```c++
// 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);
```
