14.8 Shift operators

王朝other·作者佚名  2006-03-10
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The << and >> operators are used to perform bit shifting operations.

shift-expression:

additive-expression

shift-expression << additive-expression

shift-expression >> additive-expression

For an operation of the form x << count or x >> count, binary operator

overload resolution (?4.2.4) is

applied to select a specific operator implementation. The operands are

converted to the parameter types of

the selected operator, and the type of the result is the return type of the

operator.

Chapter 14 Expressions

163

When declaring an overloaded shift operator, the type of the first operand

must always be the class or struct

containing the operator declaration, and the type of the second operand

must always be int.

The predefined shift operators are listed below.

?Shift left:

int operator <<(int x, int count);

uint operator <<(uint x, int count);

long operator <<(long x, int count);

ulong operator <<(ulong x, int count);

The << operator shifts x left by a number of bits computed as described

below.

The high-order bits outside the range of the result type of x are

discarded, the remaining bits are shifted

left, and the low-order empty bit positions are set to zero.

?Shift right:

int operator >>(int x, int count);

uint operator >>(uint x, int count);

long operator >>(long x, int count);

ulong operator >>(ulong x, int count);

The >> operator shifts x right by a number of bits computed as described

below.

When x is of type int or long, the low-order bits of x are discarded, the

remaining bits are shifted

right, and the high-order empty bit positions are set to zero if x is

non-negative and set to one if x is

negative.

When x is of type uint or ulong, the low-order bits of x are discarded, the

remaining bits are shifted

right, and the high-order empty bit positions are set to zero.

For the predefined operators, the number of bits to shift is computed as

follows:

?When the type of x is int or uint, the shift count is given by the

low-order five bits of count. In other

words, the shift count is computed from count & 0x1F.

?When the type of x is long or ulong, the shift count is given by the

low-order six bits of count. In

other words, the shift count is computed from count & 0x3F.

If the resulting shift count is zero, the shift operators simply return the

value of x.

Shift operations never cause overflows and produce the same results in

checked and unchecked contexts.

When the left operand of the >> operator is of a signed integral type, the

operator performs an arithmetic

shift right wherein the value of the most significant bit (the sign bit) of

the operand is propagated to the

high-order empty bit positions. When the left operand of the >> operator is

of an unsigned integral type, the

operator performs a logical shift right wherein high-order empty bit

positions are always set to zero. To

perform the opposite operation of that inferred from the operand type,

explicit casts can be used. [Example:

For example, if x is a variable of type int, the operation

unchecked((int)((uint)x >> y)) performs a

logical shift right of x. end example]

 
 
 
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