What is going on with bitwise operators and integer promotion?

问题

I have a simple program. Notice that I use an unsigned fixed-width integer 1 byte in size.

#include <cstdint>
#include <iostream>
#include <limits>

int main()
{
    uint8_t x = 12;
    std::cout << (x << 1) << '\n';
    std::cout << ~x;

    std::cin.clear();
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    std::cin.get();

    return 0;
}

My output is the following.

24
-13

I tested larger numbers and operator << always gives me positive numbers, while operator ~ always gives me negative numbers. I then used sizeof() and found...

When I use the left shift bitwise operator(<<), I receive an unsigned 4 byte integer.

When I use the bitwise not operator(~), I receive a signed 4 byte integer.

It seems that the bitwise not operator(~) does a signed integral promotion like the arithmetic operators do. However, the left shift operator(<<) seems to promote to an unsigned integral.

I feel obligated to know when the compiler is changing something behind my back. If I'm correct in my analysis, do all the bitwise operators promote to a 4 byte integer? And why are some signed and some unsigned? I'm so confused!

Edit: My assumption of always getting positive or always getting negative values was wrong. But from being wrong, I understand what was really happening thanks to the great answers below.

回答1:

[expr.unary.op]

The operand of ~ shall have integral or unscoped enumeration type; the result is the one’s complement of　its operand. Integral promotions are performed.

[expr.shift]

The shift operators << and >> group left-to-right. [...] The operands shall be of integral or unscoped enumeration type and integral promotions are performed.

What's the integral promotion of uint8_t (which is usually going to be unsigned_char behind the scenes)?

[conv.prom]

A prvalue of an integer type other than bool, char16_t, char32_t, or wchar_t whose integer conversion rank (4.13) is less than the rank of int can be converted to a prvalue of type int if int can represent all the values of the source type; otherwise, the source prvalue can be converted to a prvalue of type unsigned int.

So int, because all of the values of a uint8_t can be represented by int.

What is int(12) << 1 ? int(24).

What is ~int(12) ? int(-13).

回答2:

For performance reasons the C and C++ language consider int to be the "most natural" integer type and instead types that are "smaller" than an int are considered sort of "storage" type.

When you use a storage type in an expression it gets automatically converted to an int or to an unsigned int implicitly. For example:

// Assume a char is 8 bit
unsigned char x = 255;
unsigned char one = 1;

int y = x + one; // result will be 256 (too large for a byte!)
++x;             // x is now 0

what happened is that x and one in the first expression have been implicitly converted to integers, the addition has been computed and the result has been stored back in an integer. In other words the computation has NOT been performed using two unsigned chars.

Likewise if you have a float value in an expression the first thing the compiler will do is promoting it to a double (in other words float is a storage type and double is instead the natural size for floating point numbers). This is the reason for which if you use printf to print floats you don't need to say %lf int the format strings and %f is enough (%lf is needed for scanf however because that function stores a result and a float can be smaller than a double).

C++ complicated the matter quite a bit because when passing parameters to functions you can discriminate between ints and smaller types. Thus it's not ALWAYS true that a conversion is performed in every expression... for example you can have:

void foo(unsigned char x);
void foo(int x);

where

unsigned char x = 255, one = 1;
foo(x);       // Calls foo(unsigned char), no promotion
foo(x + one); // Calls foo(int), promotion of both x and one to int

回答3:

I tested larger numbers and operator << always gives me positive numbers, while operator ~ always gives me negative numbers. I then used sizeof() and found...

Wrong, test it:

uint8_t v = 1;
for (int i=0; i<32; i++) cout << (v<<i) << endl;

gives:

1
2
4
8
16
32
64
128
256
512
1024
2048
4096
8192
16384
32768
65536
131072
262144
524288
1048576
2097152
4194304
8388608
16777216
33554432
67108864
134217728
268435456
536870912
1073741824
-2147483648

uint8_t is an 8-bit long unsigned integer type, which can represent values in the range [0,255], as that range in included in the range of int it is promoted to int (not unsigned int). Promotion to int has precedence over promotion to unsigned.

回答4:

Look into two's complement and how computer stores negative integers.
Try this

#include <cstdint>
#include <iostream>
#include <limits>
int main()
{
uint8_t x = 1;
int shiftby=0;
shiftby=8*sizeof(int)-1;
std::cout << (x << shiftby) << '\n'; // or std::cout << (x << 31) << '\n';

std::cout << ~x;

std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
std::cin.get();
return 0;
}

The output is -2147483648

In general if the first bit of a signed number is 1 it is considered negative. when you take a large number and shift it. If you shift it so that the first bit is 1 it will be negative

** EDIT **
Well I can think of a reason why shift operators would use unsigned int. Consider right shift operation >> if you right shift -12 you will get 122 instead of -6. This is because it adds a zero in the beginning without considering the sign

来源：https://stackoverflow.com/questions/30473958/what-is-going-on-with-bitwise-operators-and-integer-promotion