How to use a member function (delegate) as a function pointer

[Denis Koroskin]

On Mon, 01 Nov 2010 05:27:06 +0300, Peter Federighi <pfederighi at yahoo.com>  
wrote:

> Hello.  I'm new to D. It's been a long time since I've coded anything  
> with
> classes.  Please excuse my ignorance.
>
> Here's a very simplified version of what I'm trying to do:
>
> import std.c.linux.linux;
> import std.stdio;
> class FOO
>     {
>     this()
>         {
>         sa.sa_handler = &handler;
>         sigemptyset(&sa.sa_mask);
>         sa.sa_flags = SA_RESTART;
>         sigaction(SIGALRM, &sa, null);
>         }
>
>     ~this()
>         {
>         sa.sa_handler = SIG_DFL;
>         sigemptyset(&sa.sa_mask);
>         sa.sa_flags = SA_RESTART;
>         sigaction(SIGALRM, &sa, null);
>         }
>
>     private:
>     sigaction_t sa;
>
>     void handler(int signal)
>         {
>         writeln("Got an alarm signal.");
>         }
>     }
>
>
> Is there a way to do it without removing handler() from the class?  When  
> I try
> compiling, I get: Error: cannot implicitly convert expression  
> (&this.handler)
> of type void delegate(int signal) to void C function(int).
>
> Thank you,
> - Peter

No, unfortunately not. A delegate is a function pointer PLUS 'this'. The C  
API you are using allows providing function only, without a context. As  
such, you need to store 'this' pointer somewhere else. It is usually done  
by means of of a special userData variable which is additionally passed to  
the callback (looks like we are out of luck this time), or storing it in a  
global variable:

Foo foo;
void externalHandler(int signal)
{
     assert(foo !is null);
     foo.handler(signal);
}

class Foo
{
     this() {
         assert(foo is null);
         foo = this;
         sa.sa_handler = &externalHandler;
         // ...
     }
     ~this() {
         assert(foo is this);
         foo = null;
         // ...
     }
     // ...
}