What does the "__block" keyword mean?
ios, objective-c, objective-c-blocks
Solution
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.
Problem
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).