问题
Let's say I want to bit shift i twice to the left and store the value in f.
f = i << 2;
Is that correct? How exactly do I do this in C/C++?
回答1:
Yes.
f = i << 2
Shifts are useful in a number of bit twiddling operations.
This used to be a great way to multiply a number by four. However, these days, optimizing compilers tend to take care of that for you.
Keep in mind that the two leftmost bits are discarded.
回答2:
As an additional note: Even though your question is tagged C++, it is probably worth noting that C and C++ took slightly different paths with regard to shifting negative values. In C++ the result of doing << or >> on a negative value is implementation-defined. In C >> is implementation-defined, while << produces undefined behavior.
回答3:
Yes, i << 2, f = i << 2, or f <<= 2 are all things one might want to do to shift bits.
More shift things to keep in mind:
you have
>>as well. At the bit level,>>works differently for signed and unsigned types.the priority of
<<and>>is below that of+and-, which fools some people, as one might imagine them to be more like*and/.
回答4:
For the sake of completeness to help you with your bit operations you can check out this page: uow TEXTBOOK -> bitops.html
来源：https://stackoverflow.com/questions/1604368/bit-shifting-left