<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">I thought this was interesting. Development of closures for C (and ObjectiveC).<div><br></div><div><a href="http://lists.cs.uiuc.edu/pipermail/cfe-dev/2008-August/002670.html">http://lists.cs.uiuc.edu/pipermail/cfe-dev/2008-August/002670.html</a></div><div><br></div><div><span class="Apple-style-span" style="font-family: Times; font-size: 16px; "><pre><font class="Apple-style-span" face="Monaco" size="3"><span class="Apple-style-span" style="font-size: 11px;">Until there is more real documentation, this is a basic idea of
Blocks: it is closures for C. It lets you pass around units of
computation that can be executed later. For example:
void call_a_block(void (^blockptr)(int)) {
blockptr(4);
}
void test() {
int X = ...
call_a_block(^(int y){ print(X+y); }); // references stack var snapshot
call_a_block(^(int y){ print(y*y); });
}
In this example, when the first block is formed, it snapshots the
value of X into the block and builds a small structure on the stack.
Passing the block pointer down to call_a_block passes a pointer to
this stack object. Invoking a block (with function call syntax) loads
the relevant info out of the struct and calls it. call_a_block can
obviously be passed different blocks as long as they have the same type.
From a technical perspective, blocks fit into C in a couple places:
1) a new declaration type (the caret) which work very much like a
magic kind of pointer that can only point to function types. 2) block
literals, which capture the computation 3) a new storage class
__block 4) a really tiny runtime library.
The new storage class comes into play when you want to get mutable
access to variables on the stack. Basically you can mark an otherwise-
auto variable with __block (which is currently a macro that expands to
an attribute), for example:
void test() {
int X = ...
__block int Y = ...
^{ X = 4; }; // error, can't modify a const snapshot.
^{ Y = 4; }; // ok!
}
From the implementation standpoint, roughly the address of a __block
object is captured by the block instead of its value.
The is tricky though because blocks are on the stack, and you may want
to refer to some computation (and its __block captured variables)
after the function returns. To do this, we have a simple form of
reference counting to manage the lifetimes of these. For example, in
this case:
void (^P)(int); // global var
void gets_a_block(void (^blockptr)(int)) {
P = blockptr;
}
void called_sometime_later() {
P(4);
}
if gets_a_block is called with a block on the stack, and
called_sometime_later is called after that stack frame is popped,
badness happens (yay for C!). Instead, we use:
void (^P)(int); // global var
void gets_a_block(void (^blockptr)(int)) {
P = _Block_copy(blockptr); // copies to heap if on the stack with
refcount +1, otherwise increments refcount.
}
void called_sometime_later() {
P(4);
_Block_release(P); // decrements refcount.
P = 0;
}
The semantics of this is that it copies the block off the stack *as
well as any __block variables it references*, and the shared __block
variables are themselves freed when all referencing blocks go away.
The really tiny runtime library implements things like _Block_copy and
friends.
Other interesting things are that the blocks themselves do limited/
optional type inference of the result type:
foo(^int(){ return 4; }); // takes nothing, returns int.
foo(^(){ return 4; }); // same thing, inferred to return int.
</span></font>
</pre></span></div></body></html>