This chapter is from the book
This chapter is from the book
This chapter is from the book
This chapter is from the book
Variable Capture Redux
Let's revisit variable capture. Each of the blocks here is the same code, but they are "created" at different times – each after the value of val has changed.
typedef void (^BoringBlock) (void);
int val = 23;
BoringBlock block1 = ^{ NSLog (@"%d", val); };
val = 42;
BoringBlock block2 = ^{ NSLog (@"%d", val); };
val = 17;
BoringBlock block3 = ^{ NSLog (@"%d", val); };
block1 points to a block that has captured val when it had a value of 23. block2 points to a block that has captured val when it had a value of 42, and likewise block3's val captured the value of 17. Invoking the blocks prints out the captured values of val:
block1 (); block2 (); block3 ();
prints
23 42 17
Now, make a single change to the code, making val a __block-scoped variable:
typedef void (^BoringBlock) (void);
__block int val = 23;
BoringBlock block1 = ^{ NSLog (@"%d", val); };
val = 42;
BoringBlock block2 = ^{ NSLog (@"%d", val); };
val = 17;
BoringBlock block3 = ^{ NSLog (@"%d", val); };
Invoking the blocks as above will print:
17 17 17
The same value is printed because all blocks are sharing the same storage for val rather than making copies.