问题
I was going through Apple\'s documentation and I saw something like this (void (^)(void)). Can somebody explain what this statement means? ^ is XOR, right? void XOR void doesn\'t makes much sense to me?
There was also something like (void (^)(BOOL finished))
回答1:
These are blocks which add anonymous functions and function objects to Objective-C. See e.g. Introducing Blocks and Grand Central Dispatch :
Block objects (informally, “blocks”) are an extension to C, as well as Objective-C and C++, that make it easy for programmers to define self-contained units of work. Blocks are similar to — but far more powerful than — traditional function pointers. The key differences are:
- Blocks can be defined inline, as “anonymous functions.”
- Blocks capture read-only copies of local variables, similar to “closures” in other languages
Declaring a block variable:
void (^my_block)(void);
Assigning a block object to it:
my_block = ^(void){ printf("hello world\n"); };
Invoking it:
my_block(); // prints “hello world\n”
Accepting a block as an argument:
- (void)doSomething:(void (^)(void))block;
Using that method with an inline block:
[obj doSomeThing:^(void){ printf("block was called"); }];
回答2:
That's a block, an Apple-specific extension to C, similar to function pointers, or function objects in other languages.
(void (^)(void)) looks like a typecast to the type of a block that takes no parameters and returns nothing. Similarly, (void (^)(BOOL finished)) looks like another typecast, to a block with one boolean parameter and returning nothing.
来源：https://stackoverflow.com/questions/3079624/caret-character-between-types-rather-than-variables-surrounded-by-parentheses