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block memory management

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block内存管理，先看几个例子

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1.
    int a = 0;
    void (^block)(void) = ^ {
        printf("in block a = %d,%p\n",a,&a);
    };
    a = 1;
    block();
    printf("after block a = %d,%p\n",a,&a);

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2.
    __block int a = 0;
    void (^block)(void) = ^ {
        printf("in block a = %d,%p\n",a,&a);
    };
    a = 1;
    block();
    printf("after block a = %d,%p\n",a,&a);

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3.
    __block int a = 0;
    void (^block)(void) = ^ {
        printf("in block a = %d,%p\n",a,&a);
        a = 2;
    };
    a = 1;
    block();
    printf("after block a = %d,%p\n",a,&a);

输出分别是

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1.
in block a = 0,0x600000252ec0
after block a = 1,0x7ffee71c8104
2.
in block a = 1,0x600000429ef8
after block a = 1,0x600000429ef8
3.
in block a = 1,0x60000042b118
after block a = 2,0x60000042b118

输出中不难看出在1中变量 a 在 block 中被 copy 了生成了新的地址。经过 __block修饰符声明的变量在 block 中就是同一个地址。