Pass delegates to external C functions in D

Question

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

Answer 1

Let me cross post what I said on the newsgroup:

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 is implemented 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 instantiate a separate variable+function pair for different lines so the global variable holding the delegate won't be so easily overwritten.