incompatible blocks type assigning

Question

I have used blocks a few times but never with declaring a typedef first.

I am trying to setup this definitions on a class (lets call it ClassA)

typedef void (^myBlock)();

@property (nonatomic, copy) myBlock onCompletion;

Then I create an instance of the object this class represents and do this:

ClassA *obj = [[ClassA alloc] init];
obj.onCompletion = ^(){
   // my code here
};

it is complaining "incompatible block pointer types assigning"...

can you guys explain?

Does "// my code here" contain a return statement? It would change the block type if you return anything; — Nikolai Ruhe, Feb 19 '14 at 18:48
Try changing your typedef to `typedef void (^myBlock)(void);`. — rmaddy, Feb 19 '14 at 18:48
@rmaddy That's what I meant with "anything", but you're right, it wasn't clear. So: Does your code contain a return statement with a value? — Nikolai Ruhe, Feb 19 '14 at 18:50
Other than return type, both block types are identical and should match. — Nikolai Ruhe, Feb 19 '14 at 18:51
@maddy, I thought that by leaving it blak would be void... please add your comment as an answer, so I can accept... thanks — Duck, Feb 19 '14 at 18:52
@DesperateDeveloper No, leaving it out means the compiler figures out the return type automatically. So if you return something, that will be the type. — Nikolai Ruhe, Feb 19 '14 at 18:54
@NikolaiRuhe We are talking about the block parameters, not the return value. See my first comment under this question. — rmaddy, Feb 19 '14 at 18:57
For me the code you posted works fine (as long as the block does not return anything). — Nikolai Ruhe, Feb 20 '14 at 15:30

score 1 · Answer 1 · answered Feb 19 '14 at 18:54

1

Although you don't have to specify the return type of a block, you must specify void if your block doesn't take any parameters.

answered Feb 19 '14 at 18:54

limon

Here's a very useful thing for everyone .. http://stackoverflow.com/a/20760583/294884 – Fattie May 26 '14 at 15:49

score 1 · Answer 2 · answered Feb 19 '14 at 18:55

1

Check out this link for block reference: block syntax

Just guessing it might be because you left the block parameter type blank.

typedef void (^myBlock)(void); //Untested

answered Feb 19 '14 at 18:55

Eric

Nikolai Ruhe · Answer 3 · 2014-02-19T19:01:33.333

0

The only difference in the declared block type myBlock and the type of the block literal might be the return type.

If you leave the return type out in a block literal the compiler figures it out automatically:

myBlock block1 = ^() { // everything is fine
   return;
};

myBlock block2 = ^() { // does not work, types differ
   return 1;
};

edited Feb 19 '14 at 19:01

answered Feb 19 '14 at 18:55

Nikolai Ruhe

score 0 · Accepted Answer · answered Feb 19 '14 at 18:59

0

At the request of the OP I am posting my original comment as an answer even though there are other answers stating the same thing:

You need to clearly specify that the block takes no parameters. Your typedef should be:

typedef void (^myBlock)(void);

answered Feb 19 '14 at 18:59

rmaddy

I thought that by leaving it blank it would be void, but I was wrong. Thanks. – Duck Feb 19 '14 at 19:01
I'm not sure but I think when you declare a block or function with empty parentheses (instead of an explicit `void`), the compiler assumes a single `int` parameter. – rmaddy Feb 19 '14 at 19:03
@DesperateDeveloper Are you sure this is the full answer to the problem. For me the code you posted works fine, as long as the block does not return a thing. – Nikolai Ruhe Feb 19 '14 at 19:04
1

@rmaddy The C standard says that a function with no explicit parameters takes an unspecified number of parameters. They will just not be checked by the compiler. – Nikolai Ruhe Feb 19 '14 at 19:09
@NikolaiRuhe I knew I fuzzy on that detail. Either way it is different from no parameters. – rmaddy Feb 19 '14 at 19:10
@rmaddy Yes. I'm feeling a bit uncomfortable with this solution because I can't reproduce the problem like this. Only the one I pointed out. – Nikolai Ruhe Feb 19 '14 at 19:12

4 Answers4