Declaring a Block Reference

Block variables hold references to blocks. You declare them using syntax similar to that you use to declare a pointer to a function, except that you use ^ instead of *. The block type fully interoperates with the rest of the C type system. The following are all valid block variable declarations:

void (^blockReturningVoidWithVoidArgument)(void);

int (^blockReturningIntWithIntAndCharArguments)(int, char);

void (^arrayOfTenBlocksReturningVoidWithIntArgument[10])(int);

Blocks also support variadic (...) arguments. A block that takes no arguments must specify void in the argument list.

Blocks are designed to be fully type safe by giving the compiler a full set of metadata to use to validate use of blocks, parameters passed to blocks, and assignment of the return value. You can cast a block reference to a pointer of arbitrary type and vice versa. You cannot, however, dereference a block reference via the pointer dereference operator (*)—thus a block's size cannot be computed at compile time.

You can also create types for blocks—doing so is generally considered to be best practice when you use a block with a given signature in multiple places:

typedef float (^MyBlockType)(float, float);

MyBlockType myFirstBlock = // ... ;

MyBlockType mySecondBlock = // ... ;

Creating a Block

You use the ^ operator to indicate the beginning of a block literal expression. It may be followed by an argument list contained within (). The body of the block is contained within {}. The following example defines a simple block and assigns it to a previously declared variable (oneFrom)—here the block is followed by the normal ; that ends a C statement.

float (^oneFrom)(float);

oneFrom = ^(float aFloat) {

    float result = aFloat - 1.0;

    return result;

};

If you don’t explicitly declare the return value of a block expression, it can be automatically inferred from the contents of the block. If the return type is inferred and the parameter list is void, then you can omit the (void) parameter list as well. If or when multiple return statements are present, they must exactly match (using casting if necessary).

Global Blocks

At a file level, you can use a block as a global literal:

#import <stdio.h>

int GlobalInt = 0;

int (^getGlobalInt)(void) = ^{ return GlobalInt; };