Question

Say I have a function that expects a block:

void foo(Foo (^block)(Bar));

And say I have a function with the same signature, except not a block:

Foo myFunction(Bar);

I can do this:

foo(^(Bar bar) { return myFunction(bar); });

But I would rather do this, which would be equivalent if it worked:

foo(&myFunction);

If I try to, XCode says:

No matching function for call to 'foo'

A block is a function pointer together with some context, so on that level it seems reasonable to want to use a plain function pointer as a block with an empty context. Is it possible?

Answer 1

A block is a function pointer together with some context, so on that
level it seems reasonable to want to use a plain function pointer as a
block with an empty context. Is it possible?

The problem, though, is that a block is not a function pointer together with some context.

A block captures executable code during compilation and state during execution. The executable code follows the C ABI in terms of passing arguments but, like method calls, it has some very specific requirements. Translating it to a C function declaration, a block that returns a BOOL and takes a single int argument would look like this:

BOOL blockLikeFunc(void *block, int arg) { ... }

That is, the first argument to the block’s “function” must be a pointer to the block itself.

Thus, no, you can’t just rip out the block’s “function” pointer and cast it to/from a C function.