Are blocks passed by reference or by value from one call on stack to another?

Question

When I pass block to other method (not to heap with Block_copy or @property(copy)), is it copied or is it passed by reference?

I mean:

- (void)processBlock:(MyBlockType)block param:(int)param {

}

- (void)someMethod {
    int b1 = 10;
    int a1 = 9;
    [self processBlock:^int(int number, id object) {
        NSLog(@"block");
        return 1 + a1;
    } param:b1];
}

It is NSStackBlock - because it captures "a" variable, so it is allocated on stack. When I pass them to the other method is it copied and stored on processBlock's section of stack, or just passed by reference?

like that:

copyOf myBlock
copyOf b1
processBlock
..........
..other variables..
a1
b1
myBlock
someMethod
..........

Or Like that:

*myBlock (jast a pointer)
copyOf b1
processBlock
..........
..other variables..
a1
b1
myBlock
someMethod
..........

See this [link](http://stackoverflow.com/questions/10424167/passing-blocks-in-objective-c?answertab=active#tab-top) — Greg, Dec 07 '13 at 16:01
@Greg But I did not find out the answer for my question. The link describes only cases when block is passed on heap, and case when stack is poped. I would know if the myBlock instance is copied the same way as b1 variable value is copied. Sorry, if I did not understand the answer correctly. Thank you. — BergP, Dec 07 '13 at 18:34

score 5 · Answer 1 · answered Dec 08 '13 at 21:20

First, let's clear up some terminology, because that's at the root of the confusion.

If we get really technical, you cannot "pass a block" -- You can never deal with "a block". You can only manipulate "pointers to blocks". The type something (^)() is a pointer-to-block type. A block literal is an expression of pointer-to-block type. The functions Block_copy and Block_release take pointer-to-block type. In short, everything you do with blocks must be done with pointers to blocks.

In Objective-C, blocks are objects. You will recognize the analogous concept in Objective-C that you can never deal with "an object" directly. You can only deal with pointers to objects.

With that cleared up, everything in Objective-C, like in C, is passed and assigned by value. There is no "pass by reference".

In the code you show, you are passing a pointer to a block, by value. That pointer points to a block object, which, yes, resides on the stack frame. If someone calls copy on this pointer (and it is not certain from your code that this occurs, but assuming that it does), then the copy function will return another pointer to a new block object, which is in the heap.

CRD · Accepted Answer · 2013-12-08T22:54:42.267

4

Blocks are pointer types and the pointer is passed-by-value.

So a block created on the stack and passed to a function/method is not copied into the call frame of the callee.

However that does not mean the block may not be copied to the heap under some circumstances you may not expect when using ARC. Stack blocks are really an (internal) optimisation which due to the way they were introduced were surfaced to the user. You should not rely on blocks being on the stack.

BTW: "Copying a block" is usually just used to refer to whether the block is copied from the stack to the heap, which is not what you are asking, so you might get answers along that line.

edited Dec 08 '13 at 22:54

answered Dec 07 '13 at 18:35

CRD

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3

There is no pass by reference in C or Objective-C. Period. – newacct Dec 08 '13 at 21:21
1

@newacct - You are correct, I was careless and used sloppy language. I should know better and have tidied it up. Mea culpa. – CRD Dec 08 '13 at 22:55

Are blocks passed by reference or by value from one call on stack to another?

2 Answers2