Template for function or delegate (nothing else)

[Steven Schveighoffer]

On Thu, 10 Feb 2011 08:39:13 -0500, spir <denis.spir at gmail.com> wrote:

> On 02/09/2011 11:05 PM, Steven Schveighoffer wrote:

>> I don't think you want templates. What you want is a tagged union (and  
>> a struct
>> is MUCH better suited for this):
>>
>> // untested!
>>
>> struct Example
>> {
>> private
>> {
>> bool isDelegate;
>> union
>> {
>> void function() fn;
>> void delegate() dg;
>> }
>> }
>>
>> void setCallback(void function() f) { this.fn = f; isDelegate = false;}
>> void setCallback(void delegate() d) { this.dg = d; isDelegate = true;}
>>
>> void opCall()
>> {
>> if(isDelegate)
>> dg();
>> else
>> fn();
>> }
>> }
>
> Waow, very nice solution.
> I really question the function/delegate distinction (mostly artificial,  
> imo) that "invents" issues necessiting workarounds like that. What does  
> it mean, what does it bring?

A function pointer is compatible with a C function pointer.  C does not  
have delegates, so if you want to do callbacks, you need to use function  
pointers.  There is no way to combine them and keep C compatibility.

> Right, there is one pointer less for funcs. This save 4 or 8 bytes, so  
> to say, nothing; who develops apps with arrays of billions of funcs?  
> There are written in source ;-) Even then, if this saving of apointer  
> was of any relevance, then it should be an implementation detail that  
> does not leak into artificial semantic diff, creating issues on the  
> programmer side. What do you think?

What you want is already implemented.  There is a relatively new phobos  
construct that builds a delegate out of a function pointer.  In fact, my  
code could use it and save the tag:


// again, untested!

import std.functional : toDelegate;

struct Example
{
    private void delegate() dg;

    void setCallback(void function() f) { this.dg = toDelegate(f); }
    void setCallback(void delegate() d) { this.dg = d; }

    void opCall() { dg(); }
}

Note that toDelegate doesn't appear on the docs because of a doc  
generation bug...

-Steve