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c++ - `(x+7)&~7`,把x增大为稍大于x的8的倍数。这怎么理解?

stl源码剖析的第61页有上面的位运算,不过我不了解,求证明。

阿神阿神2819 天前617

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  • PHPz

    PHPz2017-04-17 15:25:27

    7 = 00000111b
    & is bitwise AND
    so the x&7 has the only 3 low bits of x.


    ~7 = 11111000b

    so the x&~7 will set x's 3 low bits to 0, and it is the multiple of 8.


    (x+n-1)&(n-1),把x增大稍大於x的n的倍數,前提是n必須為2^m
    because only 2^m-1 has the type : high bits must be 1, low bits must be 0 , not intersected.

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