Hello,

When trying to pass a D function to the C callback, the compiler says:

'Error: cannot implicitly convert expression &this.onProcessCb of type extern (C) bool delegate(const(short*) a, ulong b, void* c) to extern (C) bool function(const(short*), ulong, void*'

because my function is member of a class (compiles when the function is out of the class).

Is there any way to say to solve this ?
The wiki isn't very clear about the C callbacks:
https://dlang.org/spec/interfaceToC.html#callbacks

C code:
----
typedef bool (*onProcessCallback)(const short*, size_t, void*);
----


D Code:
-----
class Game
{
	onProcessCallback m_onProcessCb;
	
	this()
	{
		m_onProcessCb = &onProcessCb; // Error here
	}
	
	void onProcess()
	{
		// ...
	}

	extern(C) bool onProcessCb(const short* a, size_t b, void* c)
	{
		onProcess();
		return true;
	}
}

private extern(C)
{
	// Should call onProcess() when executed by the C lib
	alias onProcessCallback = bool function(const short*, size_t, void*);
}
-----

On Saturday, 2 November 2019 at 17:49:09 UTC, Luh wrote:
> Hello,
>
> When trying to pass a D function to the C callback, the compiler says:
>
> 'Error: cannot implicitly convert expression &this.onProcessCb of type extern (C) bool delegate(const(short*) a, ulong b, void* c) to extern (C) bool function(const(short*), ulong, void*'
>
> because my function is member of a class (compiles when the function is out of the class).
>
> Is there any way to say to solve this ?
> The wiki isn't very clear about the C callbacks:
> https://dlang.org/spec/interfaceToC.html#callbacks
>
> C code:
> ----
> typedef bool (*onProcessCallback)(const short*, size_t, void*);
> ----
>
>
> D Code:
> -----
> class Game
> {
> 	onProcessCallback m_onProcessCb;
> 	
> 	this()
> 	{
> 		m_onProcessCb = &onProcessCb; // Error here
> 	}
> 	
> 	void onProcess()
> 	{
> 		// ...
> 	}
>
> 	extern(C) bool onProcessCb(const short* a, size_t b, void* c)
> 	{
> 		onProcess();
> 		return true;
> 	}
> }
>
> private extern(C)
> {
> 	// Should call onProcess() when executed by the C lib
> 	alias onProcessCallback = bool function(const short*, size_t, void*);
> }
> -----

you are missing a static in your member function's signature.
The callback is not providing a this pointer.

On Saturday, 2 November 2019 at 17:49:09 UTC, Luh wrote:
> Hello,
>
> When trying to pass a D function to the C callback, the compiler says:
>
> 'Error: cannot implicitly convert expression &this.onProcessCb of type extern (C) bool delegate(const(short*) a, ulong b, void* c) to extern (C) bool function(const(short*), ulong, void*'
>
> because my function is member of a class (compiles when the function is out of the class).
>
> Is there any way to say to solve this ?
> The wiki isn't very clear about the C callbacks:
> https://dlang.org/spec/interfaceToC.html#callbacks
>
> C code:
> ----
> typedef bool (*onProcessCallback)(const short*, size_t, void*);
> ----
>
>
> D Code:
> -----
> class Game
> {
> 	onProcessCallback m_onProcessCb;
> 	
> 	this()
> 	{
> 		m_onProcessCb = &onProcessCb; // Error here
> 	}
> 	
> 	void onProcess()
> 	{
> 		// ...
> 	}
>
> 	extern(C) bool onProcessCb(const short* a, size_t b, void* c)
> 	{
> 		onProcess();
> 		return true;
> 	}
> }
>
> private extern(C)
> {
> 	// Should call onProcess() when executed by the C lib
> 	alias onProcessCallback = bool function(const short*, size_t, void*);
> }
> -----

This is because onProcessCb is a member of an object. Therefore, it carries also the information about the context, which includes e.g. all members of the class.

Due to this onProcessCb is a delegate, which is something different from a function, cf.
https://dlang.org/spec/function.html#closures

So, there is a type mismatch.

On Saturday, 2 November 2019 at 18:31:28 UTC, Stefan Koch wrote:
> On Saturday, 2 November 2019 at 17:49:09 UTC, Luh wrote:
>> Hello,
>>
>> When trying to pass a D function to the C callback, the compiler says:
>>
>> 'Error: cannot implicitly convert expression &this.onProcessCb of type extern (C) bool delegate(const(short*) a, ulong b, void* c) to extern (C) bool function(const(short*), ulong, void*'
>>
>> because my function is member of a class (compiles when the function is out of the class).
>>
>> Is there any way to say to solve this ?
>> The wiki isn't very clear about the C callbacks:
>> https://dlang.org/spec/interfaceToC.html#callbacks
>>
>> C code:
>> ----
>> typedef bool (*onProcessCallback)(const short*, size_t, void*);
>> ----
>>
>>
>> D Code:
>> -----
>> class Game
>> {
>> 	onProcessCallback m_onProcessCb;
>> 	
>> 	this()
>> 	{
>> 		m_onProcessCb = &onProcessCb; // Error here
>> 	}
>> 	
>> 	void onProcess()
>> 	{
>> 		// ...
>> 	}
>>
>> 	extern(C) bool onProcessCb(const short* a, size_t b, void* c)
>> 	{
>> 		onProcess();
>> 		return true;
>> 	}
>> }
>>
>> private extern(C)
>> {
>> 	// Should call onProcess() when executed by the C lib
>> 	alias onProcessCallback = bool function(const short*, size_t, void*);
>> }
>> -----
>
> you are missing a static in your member function's signature.
> The callback is not providing a this pointer.

My problem is, that the onProcess function should not be static because it uses a variable inside the class.

like:
-----
class Game
{
	private short[] m_a;
	
	void onProcess(short[] a)
	{
		m_a ~= a;
		// ...
	}

	extern(C) bool onProcessCb(const short* a, size_t b, void* c)
	{
		onProcess();
		return true;
	}
}
-----

So I think I just can't. :(

On Saturday, 2 November 2019 at 19:42:54 UTC, Luh wrote:
> So I think I just can't. :(

Is that `void* c` in the callback a context pointer by any chance?
That's a common thing in C callbacks precisely for purposes like this.
You can cast your class to a void* when you register the callback and in the callback function cast it back to a class and call process on that.

I don't know what C library you're working with so can't give you specifics.

On Saturday, 2 November 2019 at 19:55:58 UTC, Dennis wrote:
> On Saturday, 2 November 2019 at 19:42:54 UTC, Luh wrote:
>> So I think I just can't. :(
>
> Is that `void* c` in the callback a context pointer by any chance?
> That's a common thing in C callbacks precisely for purposes like this.
> You can cast your class to a void* when you register the callback and in the callback function cast it back to a class and call process on that.
>
> I don't know what C library you're working with so can't give you specifics.

Yup that's it !
Many thanks !

On Saturday, 2 November 2019 at 20:42:29 UTC, Luh wrote:
> Yup that's it !
> Many thanks !

One word of warning: ensure the C library does not have the only reference to your Game class instance, or the garbage collector might deallocate it since it does not scan threads created by C libraries.

See:
https://dlang.org/spec/garbage.html#gc_foreign_obj