Question

The following code compiles:

^{}();

And this compiles:

void (^x)();
(x = ^{})();

But this doesn’t:

(void (^x)() = ^{})();

The error I get is Expected ')'. Is this a bug with llvm or something? It’s totally holding me back from pretending Objective-C is JavaScript.

Answer 1

This wouldn’t make sense in a C-like language. To see why, let’s build the statement from the ground up.

First, we’ll use your working declaration for x:

void (^x)();

Now let’s initialize it in the same statement:

void (^x)() = ^{};

So far so good – x has been initialized with the correct block. So let’s invoke x now. But where will the () go? Naturally, we need to place the () immediately after a block-valued expression. However, in C, declarations are statements, not expressions so

(void (^x)() = ^{})();

doesn’t make sense. The only place the () can go is after the ^{}:

void (^x)() = ^{}();

But ^{}() has type void, not type void (^)().

To sum up: you can’t declare a block variable and invoke it at the same time. you’ll have to either declare and initialize the variable, and then call it

void (^x)() = ^{};
x();

or declare it and then assign and call it

void (^x)();
(x = ^{})();

or just separate all three:

void (^x)();
x = ^{};
x();

As a concluding thought, let’s say it was desirable to declare and invoke blocks at the same time. If we decided to allow code like (void (^x)() = ^{})();, then for the sake of consistency, we would have to also allow code such as ++(void x = 4); or (void x = 1) + (void y = 2);. I hope you’ll agree that these just look strange in C.