What does the block keyword mean

Question

What exactly does the   block keyword in Objective-C mean  I know it allows you to modify variables within blocks  but I d like to know       What exactly does it tell the compiler  Does it do anything else  If that s all it does then why is it needed in the first place  Is it in the docs anywhere   I can t find it

User · Answer

It means that the variable it is a prefix to is available to be used within a block

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From the Block Language Spec   In addition to the new Block type we also introduce a new storage qualifier    block  for local variables   testme  a   block declaration within a block literal   The   block storage qualifier is mutually exclusive to the existing local storage qualifiers auto  register  and static  testme   Variables qualified by   block act as if they were in allocated storage and this storage is automatically recovered after last use of said variable   An implementation may choose an optimization where the storage is initially automatic and only  quot moved quot  to allocated  heap  storage upon a Block copy of a referencing Block   Such variables may be mutated as normal variables are  In the case where a   block variable is a Block one must assume that the   block variable resides in allocated storage and as such is assumed to reference a Block that is also in allocated storage  that it is the result of a Block copy operation    Despite this there is no provision to do a Block copy or a Block release if an implementation provides initial automatic storage for Blocks   This is due to the inherent race condition of potentially several threads trying to update the shared variable and the need for synchronization around disposing of older values and copying new ones   Such synchronization is beyond the scope of this language specification   For details on what a   block variable should compile to  see the Block Implementation Spec  section 2 3

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block is a storage type that is use to make in scope variables mutable  more frankly if you declare a variable with this specifier  its reference will be passed to blocks not a read-only copy for more details see Blocks Programming in iOS

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hope this will help you   let suppose we have a code like          int stackVariable   1        blockName                    stackVariable               it will give an error like  variable is not assignable  because the stack variable inside the block are by default immutable   adding   block storage modifier  ahead of it declaration make it mutable inside the block i e    block int stackVariable 1

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It tells the compiler that any variable marked by it must be treated in a special way when it is used inside a block  Normally  variables and their contents that are also used in blocks are copied  thus any modification done to these variables don t show outside the block  When they are marked with   block  the modifications done inside the block are also visible outside of it   For an example and more info  see The   block Storage Type in Apple s Blocks Programming Topics   The important example is this one   extern NSInteger CounterGlobal  static NSInteger CounterStatic         NSInteger localCounter   42        block char localCharacter       void   aBlock  void      void              CounterGlobal            CounterStatic          CounterGlobal   localCounter     localCounter fixed at block creation         localCharacter    a      sets localCharacter in enclosing scope               localCounter     unseen by the block     localCharacter    b        aBlock       execute the block        localCharacter now  a      In this example  both localCounter and localCharacter are modified before the block is called  However  inside the block  only the modification to localCharacter would be visible  thanks to the   block keyword  Conversely  the block can modify localCharacter and this modification is visible outside of the block

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bbum covers blocks in depth in a blog post and touches on the   block storage type        block is a distinct storage type      Just like static  auto  and volatile    block is a storage type  It   tells the compiler that the variable   s storage is to be managed   differently       However  for   block variables  the block does not retain  It is up to you to retain and release  as needed          As for use cases you will find   block is sometimes used to avoid retain cycles since it does not retain the argument  A common example is using self     Now using myself inside a block will not    retain the value therefore breaking a   possible retain cycle    block id myself   self

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block is a storage qualifier that can be used in two ways    Marks that a variable lives in a storage that is shared between the lexical scope of the original variable and any blocks declared within that scope  And clang will generate a struct to represent this variable  and use this struct by reference not by value   In MRC    block can be used to avoid retain object variables a block captures  Careful that this doesn t work for ARC  In ARC  you should use   weak instead      You can refer to apple doc for detailed information

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Normally when you don t use   block  the block will copy retain  the variable  so even if you modify the variable  the block has access to the old object   NSString  str     hello   void   theBlock       void         NSLog        str      str     how are you   theBlock      prints   hello    In these 2 cases you need   block   1 If you want to modify the variable inside the block and expect it to be visible outside     block NSString  str     hello   void   theBlock       void         str     how are you      theBlock    NSLog        str     prints  how are you    2 If you want to modify the variable after you have declared the block and you expect the block to see the change     block NSString  str     hello   void   theBlock       void         NSLog        str      str     how are you   theBlock      prints  how are you

[objective-c] What does the "__block" keyword mean?

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