Is there a logic shift operator in C++?

Steve Pope posted: >>Is there a logic shift operator in C++? I tried ">>", it is an >>arithmetic shift operator. As far as I know, it's implementation-defined as to whether it's logical shift or arithmetic shift. > No, there is no built-in logical shift right. You could probably make one yourself by using a combination of things: (1) Macros to determine whether the system is sign-magnitude, one's complement or two's complement. (2) Usage of the IMAX_BITS macro (you can do a Google Groups search for this.) (3) Usage of bitwise operators. It wouldn't be a mammoth task. -- Frederick Gotham

yuyang08@gmail.com writes: > Is there a logic shift operator in C++? I tried ">>", it is an > arithmetic shift operator. It's arithmetical or logical depending on whether or not the integer you are shifting is signed or not. Jens

"Jens Theisen" writes: >> Is there a logic shift operator in C++? I tried ">>", it is an >> arithmetic shift operator. > > It's arithmetical or logical depending on whether or not the integer > you are shifting is signed or not. Actually, there is an established meaning for those two phrases. Hint: The operation performed does not depend on the value of the operand.

osmium wrote: > "Jens Theisen" writes: > >>> Is there a logic shift operator in C++? I tried ">>", it is an >>> arithmetic shift operator. >> It's arithmetical or logical depending on whether or not the integer >> you are shifting is signed or not. > > Actually, there is an established meaning for those two phrases. Yes, and they mean exactly the same thing when applied to unsigned integers. -- Clark S. Cox III clarkcox3@gmail.com

"Clark S. Cox III" writes: > osmium wrote: >> "Jens Theisen" writes: >> >>>> Is there a logic shift operator in C++? I tried ">>", it is an >>>> arithmetic shift operator. >>> It's arithmetical or logical depending on whether or not the integer >>> you are shifting is signed or not. >> >> Actually, there is an established meaning for those two phrases. > > Yes, and they mean exactly the same thing when applied to unsigned > integers. The hardware instruction repertoires that I know of that provide both arithmetic and logical shifts have four shift instructions, not three. Unless you are terribly clever it is going to take longer to force a preliminary step of seeing what the value of the operand is, before doing the shift. YMMV.

osmium wrote: > "Clark S. Cox III" writes: > > > osmium wrote: > > > "Jens Theisen" writes: > > > > > > > > Is there a logic shift operator in C++? I tried ">>", it is an > > > > > arithmetic shift operator. > > > > It's arithmetical or logical depending on whether or not the integer > > > > you are shifting is signed or not. > > > Actually, there is an established meaning for those two phrases. > > > > Yes, and they mean exactly the same thing when applied to unsigned > > integers. > > The hardware instruction repertoires that I know of that provide both > arithmetic and logical shifts have four shift instructions, not three. > Unless you are terribly clever it is going to take longer to force a > preliminary step of seeing what the value of the operand is, before doing > the shift. YMMV. What are you talking about hardware instruction repertoires for? I was simply pointing out that, for unsigned integers, arithmetic and logical shifts are 100% identical; There is no sign bit to sign extend; there is no decision to be made. -- Clark S. Cox III clarkcox3@gmail.com

osmium wrote: > "Jens Theisen" writes: > >>> Is there a logic shift operator in C++? I tried ">>", it is an >>> arithmetic shift operator. >> >> It's arithmetical or logical depending on whether or not the integer >> you are shifting is signed or not. > > Actually, there is an established meaning for those two phrases. > > Hint: The operation performed does not depend on the value of the operand. No, but on the type. Hint: "singned"/"unsigned" doesn't say anything about the value.

osmium wrote: > "Clark S. Cox III" writes: > >> osmium wrote: >>> "Jens Theisen" writes: >>> >>>>> Is there a logic shift operator in C++? I tried ">>", it is an >>>>> arithmetic shift operator. >>>> It's arithmetical or logical depending on whether or not the integer >>>> you are shifting is signed or not. >>> >>> Actually, there is an established meaning for those two phrases. >> >> Yes, and they mean exactly the same thing when applied to unsigned >> integers. > > The hardware instruction repertoires that I know of that provide both > arithmetic and logical shifts have four shift instructions, not three. Really? I don't know many, but those that I know only have three, because there is no difference between a logic and an arithmetic left shift, but there is one for the right shift. Sometimes, however, there are two different names on assembler level for the left shift that just resolve to the same instruction.

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7/11/2012 4:16:44 AM