C++: 什么 (a< <b) mean?< div><div id="text_translate"><p> 我有一个 C++ header 文件，其中包含以下定义：</p><pre> #define CACHE_NUM_WAYS (1<<1) #define CACHE_DATA_SIZE (1<<8)</pre><p> 它在代码的rest中用作integer。</p><p> 这是什么意思？ 它的价值是什么？</p></div></b)>

Question

I have a C++ header file that contains the following definitions:

#define CACHE_NUM_WAYS    (1<<1)
#define CACHE_DATA_SIZE   (1<<8)

It is used as an integer in the rest of the code.

What does it mean? And what is its value?

Answer 1

1 << 1 means:

00000000 00000001 changes to 00000000 00000010

1 << 8 means:

00000000 00000001 changes to 00000001 00000000

It's a bit shift operation. For every 1 on the right, you can think of yourself as multiplying the value on the left by 2. So, 2 << 1 = 4 and 2 << 2 = 8. This is much more efficient than doing 1 * 2.

Also, you can do 4 >> 1 = 2 (and 5 >> 1 = 2 since you round down) as the inverse operation.

Answer 2

Those are bitwise shift operators.

http://msdn.microsoft.com/en-us/library/336xbhcz(v=vs.80).aspx

<< is shifting left so it is actually multiplying by 2 for << 1 and by 2^8 for << 8.

Answer 3

<< is bitwise shift left (there is also >> bitwise shift right) if you have 32 bit integer

1      = 00000000 00000000 00000000 00000001 = 1
1 << 1 = 00000000 00000000 00000000 00000010 = 2
1 << 8 = 00000000 00000000 00000001 00000000 = 256

Answer 4

a<<b for integers means "shift left". The bitwise representation of a is shifted left b bits. This is the same as multiplying by (2 to the power of b ).

So in your example, (1<<1) is 1*(2^1) is 2 , (1<<8) is 1*(2^8) is 256 .

It is worth pointing out that in general, as with other operators in c++, << may be overridden to perform other functions. By default, input/output streams override this operator to let you write concise code to send a bunch of parameters to the stream. So you may see code like this:

cout << something << somethingelse

and << does not mean left shift in this context.

Answer 5

The operator << is a bitwise left-shift operator.

So when you write 1<<17 , the binary representation of 1 is shifted left by 17 bits as:

//before (assume 1 is represented by 32-bit)
1 << 17  
0000 0000 0000 0000 0000 0000 0000 0001 << 17 (before - binary representation)

//after
0000 0000 0000 0010 0000 0000 0000 0000       (after - binary representation)

Answer 6

In CPP, "<<" stands for the left-shift operator and ">>" stands for the right-shift operator.

x<<y is equivalent to multiplying x with 2^y and x>>y is equivalent to dividing x by 2^y.

In theoretical terms, the left shift operator shifts the bits of the first operand to the left and the operand on the right in this case decides the number of bits to shift to the left and similarly, the right shift operator shifts the bits of the first operand to the right and the operand on the right decides the number of bits to shift to the right.

Answer 7

1<<8

here is how i understood it. in the most layman way, understand it as 1 (00000001) in binary number system is now promoted to the 8 places(bits) ahead from where it was earlier, and for every place as we know it moves the value(2 in binary) gets exponentially multiplied so it becomes 1*(2^8)=256.

so 1<<6 will be 1*(2^6)= 64 and so on

Answer 8

a<<b means (a * pow(2,b))-> a * 2^b // left-sift operator <<

a>>b means (a / pow(2,b))-> a / 2^b // right-sift operator >>