>> actually do?
This question already has an answer here:
while( x -->> 0 ) // x runs to 0
This is actually a hybrid of the -- (post-decrement) and >> (bitshift right) operators, better formatted as:
while (x-- >> 0)
For this specific usage, with 0 on the right hand side, x is decremented with each loop iteration due to the postfix -- , and the prior (pre-decrement) value is shifted right 0 bits by >> 0 which does nothing at all when x is non-negative.
When x is 1 then the post-decrement reduces it to 0 and the resultant value of bitshifting that is 0, which will cause the loop to terminate.
More generally, if you try to use >> on a negative value (eg x starts at 0 or a negative value great than INT_MIN , so x-- yields a negative value) the result is implementation defined , which means you have to consult your compiler documentation. You could use your compiler documentation to reason about how it would behave in the loop....
Relevant part of the Standard: 5.8/3:
The value of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type or if E1 has a signed type and a non-negative value, the value of the result is the integral part of the quotient of E1/2^E2 . If E1 has a signed type and a negative value, the resulting value is implementation-defined.
BTW /- for Visual Studio, per http://msdn.microsoft.com/en-us/library/336xbhcz.aspx, the implementation defined behaviour is "No shift operation is performed if additive-expression is 0.". I can't find anything in the GCC manual about this (would have expected it here).
while( x -->> 0 ) // x runs to 0
No, the "goes to operator" is --> with only one > sign. It decreases x by one and then compares the result to zero.
The -- >> 0 "runs to operator" decreases x and then bitshifts the result rightward by zero. Bitshifting by zero does nothing for nonnegative x , otherwise it's implementation-defined (usually does nothing, but could be random). Zero bitshifted by zero is zero, which is interpreted as false , at which point the loop will terminate.
So it "works" but it's a terrible way of expressing a loop.
- 递减，但在递减之前返回变量的值，>>右移右边的操作数，即0（也称为非操作数），然后隐式地将结果与0进行比较。
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