what is the meaning of the following expression in c++

What is the meaning of the following expression in c++?

(variable1 | (variable2 << 8))

What is the meaning of it开发者_运维问答? And what does it represent?

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It concatenates the two variables.
Suppose you have two chars, a and b. a|b<<8 shifts the b 8 bits to the left, | sets every bit that is in a or b.
So in this example the result would be "ab".

'a' is 97, 'b' is 98, so bitwise the following happens:

a:      01100001
b:              01100010
b<<8:   0110001000000000
a|b<<8: 0110001001100001

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| is Bitwise OR
<< is Bitwise left shift operator

   (variable1 | (variable2 << 8))

Left Shifts the variable2(8 bit) by 8 and then ORs the result with variable1(8 bit), resulting output will combine two variables variable1 and variable2 to be represented as one variable(16 bit).

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You might think of it as "concatenating" two variables in a bitwise fashion.

If:

x = 00000000 00001000 (16-bit binary)
y = 00000000 00100010 (16-bit binary)

Then:

    (y << 8) = 00100010 00000000
x | (y << 8) = 00100010 00001000

What it actually means in the context of the code in which you found it is anybody's guess.

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In actual fact, "concatenating" is not accurate if x has any bits set in the most significant byte:

If:

x = 01000000 00001000 (16-bit binary)
y = 00000000 00100010 (16-bit binary)

Then:

    (y << 8) = 00100010 00000000
x | (y << 8) = 01100010 00001000

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If variable1 and variable1 are 8-bit values, then it combines them into a single 16-bit value.

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It would make sense if both variables where bytes. In that case it would combine them into one larger variable, so that first come 8 bits of variable2 and then 8 bits of variable1.

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In your code, the 8 least significant (rightmost) bits of variable1 are appended to the bits of variable2 from the right, with the bits of variable2 being shifted left by 8.

If denote the bits of variable1 as

xxxxxxxxxxxxxxxxxxxxxxxxwxxxxxxx

and the bits of variable2 as

yyyyyyyyzyyyyyyyyyyyyyyyyyyyyyyy

then expression

(variable1 | (variable2 << 8))

would result in

zyyyyyyyyyyyyyyyyyyyyyyywxxxxxxx

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I don't know what you mean by "meaning" - but this is one variable being bitwise OR'ed with another variable which has been left-shifted by 8 bits (which you can think of as being multiplied by 256).

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If both variable1 and variable are less than 256, the statement is the same as variable1 + (variable2*256).

More generally though, | is binary or and and << is left shift.

So if we start with:

variable1 = 321;
variable2 = 123;

The binary values would be:

variable1 =>  0000 0001 0100 0001
variable2 =>  0000 0000 0111 1011

Left shifting variable2 by 8 results in:

              0111 1011 0000 0000

So variable1 | (variable2 << 8) equals

              0111 1011 0100 0001

Which is 32065. This is less than 31519 which is the result of (321 + (123 * 256)) because variable1 and variable2 << 8 have some bits in common.