EXC_BAD_ACCESS时使用递归块 [英] EXC_BAD_ACCESS when using recursive block

问题描述

我试图使用块创建递归。它工作了一段时间，但最终它崩溃，并给我一个不好的访问异常。这是我的代码：

I'm trying to create recursion using blocks. It works for a while, but eventually it crashes and gives me a bad access exception. This is my code:

BOOL (^Block)(Square *square, NSMutableArray *processedSquares) = ^(Square *square, NSMutableArray *processedSquares) {
    [processedSquares addObject:square];

    if (square.nuked) {
        return YES; // Found a nuked square, immediately return
    }

    for (Square *adjacentSquare in square.adjacentSquares) {
        if ([processedSquares containsObject:adjacentSquare]) {
            continue; // Prevent infinite recursion
        }

        if (Block(adjacentSquare, processedSquares)) {
            return YES;
        }
    }

    return NO;
};

__block NSMutableArray *processedSquares = [NSMutableArray array];
BOOL foundNukedSquare = Block(square, processedSquares);

说明：我有一个 Square 有一个BOOL nuked 。它还有一个包含其他正方形的NSArray adjacentSquares 。

Explanation: I have a Square class that has a BOOL nuked. It also has an NSArray adjacentSquares containing other Squares.

我想检查一个正方形， 'connected'正方形，是否nuked。

I want to check whether a square, or one of its 'connected' squares, is nuked or not.

数组 processedSquares 是跟踪我检查过的方格以防止无限递归。

The array processedSquares is to keep track of the squares I have checked to prevent infinite recursion.

当我运行这个，它做了很多的这个块的调用（如预期）。但是在某些时候，它在最后一行崩溃，访问异常不好。

When I run this, it's doing a lot of calls of this block (as expected). But at some point, it crashes at the last line with a bad access exception.

我也在控制台中得到这个：

I also get this in the console:

无法访问地址0x1处的内存

无法访问地址0x1处的内存

无法访问地址0x1处的内存

无法访问地址0x1处的内存

警告：在当前线程的堆栈中取消调用 - objc代码会使此操作不安全。

Cannot access memory at address 0x1
Cannot access memory at address 0x1
Cannot access memory at address 0x1
Cannot access memory at address 0x1
warning: Cancelling call - objc code on the current thread's stack makes this unsafe.

我不熟悉块和递归。任何想法？

I'm not that familiar with blocks and recursion. Any ideas?

backtrace：

As requested, the backtrace:

#0  0x00000001 in ??
#1  0x000115fb in -[Square connectedToNukedSquare] at   Square.m:105
#2  0x00010059 in __-[Bot makeMove]_block_invoke_1 at Bot.m:94
#3  0x91f3f024 in _dispatch_call_block_and_release
#4  0x91f31a8c in _dispatch_queue_drain
#5  0x91f314e8 in _dispatch_queue_invoke
#6  0x91f312fe in _dispatch_worker_thread2
#7  0x91f30d81 in _pthread_wqthread
#8  0x91f30bc6 in start_wqthread

推荐答案

您需要一个 __ block 阻止，将声明更改为：

You need a __block on Block, change the declaration to:

__block BOOL (^Block)(Square *square, NSMutableArray *processedSquares);
Block = ^(Square *square, NSMutableArray *processedSquares) {

Block ）在块中被引用，则其当前的值被复制到块中。在您的代码阻止还没有赋予一个值，因为你正在构造块中的赋值...

When a variable (Block) is referenced within a block then its current value is copied into the block. In your code Block hasn't yet been given a value, as you are constructing the block in the assignment...

__ block 前缀通过引用传递变量 - 在您的块进行递归调用时 code>有一个值，对它的引用用于获取该值，并且递归调用是OK。

The __block prefix passes the variable by reference - by the time your block makes its recursive call Block has a value, the reference to it is used to obtain that value, and the recursive call is OK.

我不知道为什么它的工作对你没有 __块 - 对我失败的直截了当。使用修饰符，我可以递归到至少10,000的深度 - 所以堆栈空间不是问题！

I don't know why its worked at all for you without the __block - failed straightaway for me. With the modifier however I can recurse to at least a depth of 10,000 - so stack space isn't a problem!

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