Question

#import "ViewController.h"

@implementation ViewController

int tnt=1;

-(void)myMethod{

    void(^blk)(void) = ^(void){
        tnt = 3+1;
        NSLog(@"tnt=>%d", tnt);
    };

    blk();
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    [self myMethod];
}

The variable tnt is outside but inside block its working perfectly fine. If i declare the tnt variable inside myMethod it start giving the __block required error.

In both case the tnt variable is outside of the block. Apple docs also says variable outside need __block storage type. Why its working when tnt is declared outside of myMethod.

Answer 1

I would guess that is related to scope. From the documentation:

Variables local to the enclosing lexical scope declared with the
__block storage modifier are provided by reference and so are mutable. Any changes are reflected in the enclosing lexical scope, including
any other blocks defined within the same enclosing lexical scope.
These are discussed in more detail in “The __block Storage Type.”

When you put int tnt=1; outside the lexical scope, I think you won’t have to use the __block modifier. And you have an example, here. You can also read the following:

__block variables live in storage that is shared between the lexical scope of the variable and all blocks and block copies declared or
created within the variable’s lexical scope. Thus, the storage will
survive the destruction of the stack frame if any copies of the blocks
declared within the frame survive beyond the end of the frame (for
example, by being enqueued somewhere for later execution). Multiple
blocks in a given lexical scope can simultaneously use a shared
variable.

Since the int tnt=1; is declared outside the lexical scope, it will survive the destruction of the stack.