[Issue 5710] cannot use delegates as parameters to non-global template

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=5710



--- Comment #22 from Steven Schveighoffer <schveiguy at yahoo.com> 2013-05-06 18:10:18 PDT ---
(In reply to comment #21)

> The problem with Rainer's idea is that the callee would need to know the layout
> of the stack frame in the context in which the delegate was bound, which in
> general it cannot know.

The generated function MUST know the context of main, or else it can't call
add.  Remember, add is passed as an alias, not a delegate.  Even if add is
passed as a delegate, it's calling the delegate from main's stack-frame (i.e.
two identical calls separated by a re-assignment of the delegate would call two
different functions).

Bear in mind that each call to doStuff with a different alias is a *different*
instantiation, even for calls with the same inner function types because of the
alias parameter.  It is OK for each instantiation to be aware of the stack
offset for 'this'.

And it's 2 extra words, not 1.  You need to store the 'array of context
pointers' on the stack, which you do not need if you know the layout of the
stack frame.  If you think about it, it is actually very little different from
Kenji's idea:

uint doStuff(uint a, uint b, void** this)  // not void* this
{
    // (*this + 0) points the object Foo
    // (*this + 1) points the stack frame of main()

Rainer's:

uint doStuff(uint a, uint b, void* this)
{
    // this points the stack frame of main()
    // *(this + N) points the object Foo

Note the difference is that N is a compile time constant, like Kenji's 1.

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