右移 unsigned char 填充了一个？

Question

On my system, (unsigned char) -1 is (expectedly for chars of 8 bits) 1111 1111 in binary (255 decimal).

Similarly, (unsigned char) -1 >> 1 is as expected 0111 1111. The left was filled with a zero.

~((unsigned char) -1 >> 1) is 1000 0000 as expected.

Now I want to generate the unsigned char 0010 0000 for example.

I tried ~((unsigned char) -1 >> 1) >> 2 , but this outputs 1110 0000.... what the? Why is the left being filled with ones all of a sudden?

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How can I generate an unsigned char with the nth bit (from the left) enabled?

I would like

n   unsigned char
0   1000 0000
1   0100 0000
2   0010 0000
3   0001 0000
... 
7   0000 0001

At the moment, ~((unsigned char) -1 >> 1) >> n is giving me

n   unsigned char
0   1000 0000
1   1100 0000
2   1110 0000
3   1111 0000
... 
7   1111 1111

This problem exists for uint8_t , uint16_t , but no longer happens at uint32_t and greater.

Answer 1

Due to integer promotions, unsigned char is promoted to int when pretty much any operation is performed on the value.

When you shift right the left operand undergoes integer promotions, and unsigned char would become int . The value of the expression will be that of the left operand after promotions - therefore the result of ((unsigned char) -1 >> 1) has value 127, of type int . If you ~ (int)127 you'll get an int with sign-bit set; shifting that right will have implementation-defined behaviour.

The solution is to add a cast around the outer ~ :

(unsigned char)~((unsigned char) -1 >> 1) >> 2

Or alternatively, & the value with 0xFF :

(~((unsigned char) -1 >> 1) >> 2) & 0xFF

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As for setting the n th bit from left of unsigned char , the best solution is to *left-shift 1U to the position, as shown by Allan Wind .

Answer 2

Another way to set the nth bit (from the left):

1 << (CHAR_BIT - n - 1)

Answer 3

You might take advantage of std::numeric_limits<T>::digits to create a less obfuscated and flexible code:

    unsigned char data=0;
    
    const uint8_t sizeBit=std::numeric_limits<decltype(data)>::digits;
    for(uint8_t n=0;n<sizeBit;n++) {
        data = 1 << (sizeBit - (n+1));
        std::bitset<sizeBit> x(data);
        std::cout << x << '\n';
    }

Prints:

10000000
01000000
00100000
00010000
00001000
00000100
00000010
00000001