C++ 撤消移位而不截断
我对此有点困惑。值不应该在移位后截断吗 有人知道为什么会这样吗C++ 撤消移位而不截断,c++,bitwise-operators,bit-shift,C++,Bitwise Operators,Bit Shift,我对此有点困惑。值不应该在移位后截断吗 有人知道为什么会这样吗 long a, b, c, n; //assign any value to a, set b and c to 0x000...0 n = 128; //any number works; b = a << n; c = b >> n; a == (b >> n); // True a == c; //True; 附言 我一直理解,如果你将缓冲区向任何方向移动,超出缓冲区大小的值就会被截断
long a, b, c, n;
//assign any value to a, set b and c to 0x000...0
n = 128; //any number works;
b = a << n;
c = b >> n;
a == (b >> n); // True
a == c; //True;
附言
我一直理解,如果你将缓冲区向任何方向移动,超出缓冲区大小的值就会被截断,除非你从原始缓冲区中获取,否则它们基本上会丢失。
这对于我在汇编中使用过的每一个平台都是如此,我认为这将扩展到更高级别的语言
<>但是,在C++中,我可以把一个缓冲区int或长的位移到它所能容纳的位上,如果我在相反的方向上移动,结果等于原来的缓冲区。 看起来C编译器有点奇怪,但它绝对合理。 如果移位数大于以位为单位的操作数长度,则编译器将其除以操作数长度,并使用提示作为移位数
// A << N
L = length of A
if L < N
then N = A % L;
因此,您可以设置这样的班次数,使其撤消禁用
N=63可能是您的答案,它不起作用。对于偶数A=1上的任何值,除了A=0之外,您的条件都是FALSE
放置[61#include <cstdlib>
#include <cstdio>
using namespace std;
void DisplayBinary(unsigned int n, int l)
{
for (int i = l - 1 ; i >= 0; i--)
{
printf("%x", (n & (1 << i)) >> i);
}
}
int main(int argc, char** argv)
{
// x = 340
unsigned int x = 0x154;
for (int numberOfShifts = 0; numberOfShifts < 100; numberOfShifts++) {
printf("numberOfShifts = %d\tresult = ", numberOfShifts);
DisplayBinary(x << numberOfShifts, 32);
printf("\n");
}
return 0;
}
查看输出:
numberOfShifts = 0 result = 00000000000000000000000101010100
numberOfShifts = 1 result = 00000000000000000000001010101000
numberOfShifts = 2 result = 00000000000000000000010101010000
numberOfShifts = 3 result = 00000000000000000000101010100000
numberOfShifts = 4 result = 00000000000000000001010101000000
numberOfShifts = 5 result = 00000000000000000010101010000000
numberOfShifts = 6 result = 00000000000000000101010100000000
numberOfShifts = 7 result = 00000000000000001010101000000000
numberOfShifts = 8 result = 00000000000000010101010000000000
numberOfShifts = 9 result = 00000000000000101010100000000000
numberOfShifts = 10 result = 00000000000001010101000000000000
numberOfShifts = 11 result = 00000000000010101010000000000000
numberOfShifts = 12 result = 00000000000101010100000000000000
numberOfShifts = 13 result = 00000000001010101000000000000000
numberOfShifts = 14 result = 00000000010101010000000000000000
numberOfShifts = 15 result = 00000000101010100000000000000000
numberOfShifts = 16 result = 00000001010101000000000000000000
numberOfShifts = 17 result = 00000010101010000000000000000000
numberOfShifts = 18 result = 00000101010100000000000000000000
numberOfShifts = 19 result = 00001010101000000000000000000000
numberOfShifts = 20 result = 00010101010000000000000000000000
numberOfShifts = 21 result = 00101010100000000000000000000000
numberOfShifts = 22 result = 01010101000000000000000000000000
numberOfShifts = 23 result = 10101010000000000000000000000000
numberOfShifts = 24 result = 01010100000000000000000000000000
numberOfShifts = 25 result = 10101000000000000000000000000000
numberOfShifts = 26 result = 01010000000000000000000000000000
numberOfShifts = 27 result = 10100000000000000000000000000000
numberOfShifts = 28 result = 01000000000000000000000000000000
numberOfShifts = 29 result = 10000000000000000000000000000000
numberOfShifts = 30 result = 00000000000000000000000000000000
numberOfShifts = 31 result = 00000000000000000000000000000000
numberOfShifts = 32 result = 00000000000000000000000101010100 <<<===
numberOfShifts = 33 result = 00000000000000000000001010101000
numberOfShifts = 34 result = 00000000000000000000010101010000
numberOfShifts = 35 result = 00000000000000000000101010100000
numberOfShifts = 36 result = 00000000000000000001010101000000
numberOfShifts = 37 result = 00000000000000000010101010000000
numberOfShifts = 38 result = 00000000000000000101010100000000
numberOfShifts = 39 result = 00000000000000001010101000000000
numberOfShifts = 40 result = 00000000000000010101010000000000
numberOfShifts = 41 result = 00000000000000101010100000000000
numberOfShifts = 42 result = 00000000000001010101000000000000
numberOfShifts = 43 result = 00000000000010101010000000000000
numberOfShifts = 44 result = 00000000000101010100000000000000
numberOfShifts = 45 result = 00000000001010101000000000000000
numberOfShifts = 46 result = 00000000010101010000000000000000
numberOfShifts = 47 result = 00000000101010100000000000000000
numberOfShifts = 48 result = 00000001010101000000000000000000
numberOfShifts = 49 result = 00000010101010000000000000000000
numberOfShifts = 50 result = 00000101010100000000000000000000
numberOfShifts = 51 result = 00001010101000000000000000000000
numberOfShifts = 52 result = 00010101010000000000000000000000
numberOfShifts = 53 result = 00101010100000000000000000000000
numberOfShifts = 54 result = 01010101000000000000000000000000
numberOfShifts = 55 result = 10101010000000000000000000000000
numberOfShifts = 56 result = 01010100000000000000000000000000
numberOfShifts = 57 result = 10101000000000000000000000000000
numberOfShifts = 58 result = 01010000000000000000000000000000
numberOfShifts = 59 result = 10100000000000000000000000000000
numberOfShifts = 60 result = 01000000000000000000000000000000
numberOfShifts = 61 result = 10000000000000000000000000000000
numberOfShifts = 62 result = 00000000000000000000000000000000
numberOfShifts = 63 result = 00000000000000000000000000000000
numberOfShifts = 64 result = 00000000000000000000000101010100 <<<===
numberOfShifts = 65 result = 00000000000000000000001010101000
numberOfShifts = 66 result = 00000000000000000000010101010000
numberOfShifts = 67 result = 00000000000000000000101010100000
numberOfShifts = 68 result = 00000000000000000001010101000000
numberOfShifts = 69 result = 00000000000000000010101010000000
numberOfShifts = 70 result = 00000000000000000101010100000000
numberOfShifts = 71 result = 00000000000000001010101000000000
numberOfShifts = 72 result = 00000000000000010101010000000000
numberOfShifts = 73 result = 00000000000000101010100000000000
numberOfShifts = 74 result = 00000000000001010101000000000000
numberOfShifts = 75 result = 00000000000010101010000000000000
numberOfShifts = 76 result = 00000000000101010100000000000000
numberOfShifts = 77 result = 00000000001010101000000000000000
numberOfShifts = 78 result = 00000000010101010000000000000000
numberOfShifts = 79 result = 00000000101010100000000000000000
numberOfShifts = 80 result = 00000001010101000000000000000000
numberOfShifts = 81 result = 00000010101010000000000000000000
numberOfShifts = 82 result = 00000101010100000000000000000000
numberOfShifts = 83 result = 00001010101000000000000000000000
numberOfShifts = 84 result = 00010101010000000000000000000000
numberOfShifts = 85 result = 00101010100000000000000000000000
numberOfShifts = 86 result = 01010101000000000000000000000000
numberOfShifts = 87 result = 10101010000000000000000000000000
numberOfShifts = 88 result = 01010100000000000000000000000000
numberOfShifts = 89 result = 10101000000000000000000000000000
numberOfShifts = 90 result = 01010000000000000000000000000000
numberOfShifts = 91 result = 10100000000000000000000000000000
numberOfShifts = 92 result = 01000000000000000000000000000000
numberOfShifts = 93 result = 10000000000000000000000000000000
numberOfShifts = 94 result = 00000000000000000000000000000000
numberOfShifts = 95 result = 00000000000000000000000000000000
numberOfShifts = 96 result = 00000000000000000000000101010100 <<<===
numberOfShifts = 97 result = 00000000000000000000001010101000
numberOfShifts = 98 result = 00000000000000000000010101010000
numberOfShifts = 99 result = 00000000000000000000101010100000
请使用一个简化的示例进行查看:
long left_shift(unsigned long a) {
return (a << 128);
}
long right_shift(unsigned long a) {
return ((a << 128) >> 128);
}
正如HolyBlackCat所说,这是未定义的行为。gcc通过将移位的结果处理为0来实现它,即使在右移位函数中,我们也可以从逻辑上推断结果。clang在left_shift函数中什么也不做,这也是一个完美的cromulent。发布可以编译的代码。这是未定义的行为。看看编译器生成的汇编代码,你会在其中找到答案。另外请注意,移位有符号类型与移位无符号类型有不同的行为,特别是在右移期间。Idc关于符号,只有二进制,而且行为可以用某些值来定义,但对于大于0的任何其他值,它的行为都是相同的。没错,33到63之间的任何值都会截断这些值。谢谢这取决于a中的值,当它溢出时,看起来C编译器的工作有点奇怪…-,这个答案可能很糟糕,但至少这个答案是正确的。这个答案充其量只是误导。
left_shift(unsigned long):
xor eax, eax
ret
right_shift(unsigned long):
xor eax, eax
ret