C ++避免位操作中的完全移位

Question

template<class _Diff,
        class _Urng>
        class _Rng_from_urng
    {   // wrap a URNG as an RNG
    public:
        explicit _Rng_from_urng(_Urng& _Func)
            : _Ref(_Func), _Bits(CHAR_BIT * sizeof(_Udiff)), _Bmask(_Udiff(-1))
        {   // construct from URNG
            for (; (_Urng::max)() - (_Urng::min)() < _Bmask; _Bmask >>= 1)
                --_Bits;
        }

        _Diff operator()(_Diff _Index)
        {   // adapt _Urng closed range to [0, _Index)
            for (;;)
            {   // try a sample random value
                _Udiff _Ret = 0;    // random bits
                _Udiff _Mask = 0;   // 2^N - 1, _Ret is within [0, _Mask]

                while (_Mask < _Udiff(_Index - 1))
                {   // need more random bits
                    _Ret <<= _Bits - 1; // avoid full shift
                    _Ret <<= 1;
                    _Ret |= _Get_bits();
                    _Mask <<= _Bits - 1;    // avoid full shift
                    _Mask <<= 1;
                    _Mask |= _Bmask;
                }

                // _Ret is [0, _Mask], _Index - 1 <= _Mask, return if unbiased
                if (_Ret / _Index < _Mask / _Index
                    || _Mask % _Index == _Udiff(_Index - 1))
                    return (_Ret % _Index);
            }
        }
    };
}

以上代码是从stl粘贴过来的。 "avoid full shift" 是什么意思？我觉得两个位移位可以合并成一个操作

_Ret <<= _Bits - 1; // avoid full shift 
_Ret <<= 1;

我将代码更改为：

_Ret<<=_Bits;

这里有什么不同？

Answer 1

[expr.shift]/1

7.6.7 Shift operators

The shift operators << and >> group left-to-right.

shift-expression:

additive-expression

shift-expression << additive-expression

shift-expression >> additive-expression

The operands shall be of integral or unscoped enumeration type and integral promotions are performed. The type of the result is that of the promoted left operand. The behavior is undefined if the right operand is negative, or greater than or equal to the length in bits of the promoted left operand.

C ++避免位操作中的完全移位

C++ avoid full shift in bit operation

c++

bit

[expr.shift]/1

7.6.7 Shift operators