After being downvoted on stackoverflow for no apperant reason, I figured I'd give it a shot here.

How do I pass a delegate to an external C function taking a function pointer?

If you want some extra rep on StackOverflow you can also answer here: http://stackoverflow.com/questions/22845175/pass-delegates-to-external-c-functions-in-d

On Thu, 03 Apr 2014 17:59:30 +0000, Jeroen Bollen wrote:

> After being downvoted on stackoverflow for no apperant reason, I figured I'd give it a shot here.
> 
> How do I pass a delegate to an external C function taking a function pointer?
> 
> If you want some extra rep on StackOverflow you can also answer here: http://stackoverflow.com/questions/22845175/pass-delegates-to-external-
c-functions-in-d

Does the C function use the function pointer (call it), or just pass it around/store it?

On Thursday, 3 April 2014 at 18:05:26 UTC, Justin Whear wrote:
> On Thu, 03 Apr 2014 17:59:30 +0000, Jeroen Bollen wrote:
>
>> After being downvoted on stackoverflow for no apperant reason, I figured
>> I'd give it a shot here.
>> 
>> How do I pass a delegate to an external C function taking a function
>> pointer?
>> 
>> If you want some extra rep on StackOverflow you can also answer here:
>> http://stackoverflow.com/questions/22845175/pass-delegates-to-external-
> c-functions-in-d
>
> Does the C function use the function pointer (call it), or just pass it
> around/store it?

It will get called.

On Thursday, 3 April 2014 at 17:59:33 UTC, Jeroen Bollen wrote:
> How do I pass a delegate to an external C function taking a function pointer?


You can't do it directly in general, unless you can modify the C function, then you can hack around it, but a delegate and a regular function pointer are pretty different animals.

But perhaps you can magic hack it. Observe:

// a C function that needs a plain function
extern(C) void test(void function() f) {
        // pretend this impl is in C
        f();
}

// just create a random delegate
void delegate() foo(int a) {
        return { import std.stdio; writeln(a); };
}

// what we want to work
void main() {
        auto dg = foo(10);
        dg(); // works

        //test(dg); // won't work
        test(bindDelegate(dg)); // we want this
}

// transform delegate into pointer..
import std.traits;
auto bindDelegate(T, string file = __FILE__, size_t line = __LINE__)(T t) if(isDelegate!T) {
        static T dg;

        dg = t;

        extern(C)
        static ReturnType!T func(ParameterTypeTuple!T args) {
                return dg(args);
        }

        return &func;
}




What bindDelegate does is create a special static variable and function for that specific call.  It is as if we wrote a separate function and global to hold it.

The __FILE__, __LINE__ things are a filthy hack to make it instantitate a separate variable+function pair for different lines so the global variable holding the delegate won't be so easily overwritten.

On Thursday, 3 April 2014 at 18:13:31 UTC, Adam D. Ruppe wrote:
> On Thursday, 3 April 2014 at 17:59:33 UTC, Jeroen Bollen wrote:
>> How do I pass a delegate to an external C function taking a function pointer?
>
>
> You can't do it directly in general, unless you can modify the C function, then you can hack around it, but a delegate and a regular function pointer are pretty different animals.
>
> But perhaps you can magic hack it. Observe:
>
> // a C function that needs a plain function
> extern(C) void test(void function() f) {
>         // pretend this impl is in C
>         f();
> }
>
> // just create a random delegate
> void delegate() foo(int a) {
>         return { import std.stdio; writeln(a); };
> }
>
> // what we want to work
> void main() {
>         auto dg = foo(10);
>         dg(); // works
>
>         //test(dg); // won't work
>         test(bindDelegate(dg)); // we want this
> }
>
> // transform delegate into pointer..
> import std.traits;
> auto bindDelegate(T, string file = __FILE__, size_t line = __LINE__)(T t) if(isDelegate!T) {
>         static T dg;
>
>         dg = t;
>
>         extern(C)
>         static ReturnType!T func(ParameterTypeTuple!T args) {
>                 return dg(args);
>         }
>
>         return &func;
> }
>
>
>
>
> What bindDelegate does is create a special static variable and function for that specific call.  It is as if we wrote a separate function and global to hold it.
>
> The __FILE__, __LINE__ things are a filthy hack to make it instantitate a separate variable+function pair for different lines so the global variable holding the delegate won't be so easily overwritten.

Thanks.

On Thursday, 3 April 2014 at 17:59:33 UTC, Jeroen Bollen wrote:
> After being downvoted on stackoverflow for no apperant reason, I figured I'd give it a shot here.
>
> How do I pass a delegate to an external C function taking a function pointer?
>
> If you want some extra rep on StackOverflow you can also answer here: http://stackoverflow.com/questions/22845175/pass-delegates-to-external-c-functions-in-d

OT: you've got another upvote!

On Friday, 4 April 2014 at 05:20:42 UTC, Mengu wrote:
> On Thursday, 3 April 2014 at 17:59:33 UTC, Jeroen Bollen wrote:
>> After being downvoted on stackoverflow for no apperant reason, I figured I'd give it a shot here.
>>
>> How do I pass a delegate to an external C function taking a function pointer?
>>
>> If you want some extra rep on StackOverflow you can also answer here: http://stackoverflow.com/questions/22845175/pass-delegates-to-external-c-functions-in-d
>
> OT: you've got another upvote!

Ever since I posted it here it started climbing back up the ladder. Doesn't really matter now anyways, I got my answer. :P