The bitwise operators
C++ provides 6 bit manipulation operators, often called bitwise operators:
| Operator | Symbol | Form | Operation |
|---|---|---|---|
| left shift | << | x << y | all bits in x shifted left y bits |
| right shift | >> | x >> y | all bits in x shifted right y bits |
| bitwise NOT | ~ | ~x | all bits in x flipped |
| bitwise AND | & | x & y | each bit in x AND each bit in y |
| bitwise OR | | | x | y | each bit in x OR each bit in y |
| bitwise XOR | ^ | x ^ y | each bit in x XOR each bit in y |
Author's note
In the following examples, we will largely be working with 4-bit binary values. This is for the sake of convenience and keeping the examples simple. In actual programs, the number of bits used is based on the size of the object (e.g. a 2 byte object would store 16 bits).
For readability, we’ll also omit the 0b prefix outside of code examples (e.g. instead of 0b0101, we’ll just use 0101).
Bitwise left shift (<<) and bitwise right shift (>>) operators
The bitwise left shift (<<) operator shifts bits to the left. The left operand is the expression to shift the bits of, and the right operator is an integer number of bits to shift left by.
So when we say x << 1, we are saying "shift the bits in the variable x left by 1 place". New bits shifted in from the right side receive the value 0.
0011 << 1 is 0110
0011 << 2 is 1100
0011 << 3 is 1000
Note that in the third case, we shifted a bit off the end of the number! Bits that are shifted off the end of the binary number are lost forever.
The bitwise right shift (>>) operator shifts bits to the right.
1100 >> 1 is 0110
1100 >> 2 is 0011
1100 >> 3 is 0001
Note that in the third case we shifted a bit off the right end of the number, so it is lost.
Here's an example of doing some bit shifting:
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#include <iostream> #include <bitset> int main() { std::bitset<4> x { 0b1100 }; std::cout << x << '\n'; std::cout << (x >> 1) << '\n'; // shift right by 1, yielding 0110 std::cout << (x << 1) << '\n'; // shift left by 1, yielding 1000 return 0; } |
This prints:
1100 0110 1000
Note that the results of applying the bitwise shift operators to a signed integer are compiler dependent prior to C++20.
Warning
Prior to C++20, don't shift a signed integer (and even then, it's probably still better to use unsigned)
What!? Aren't operator<< and operator>> used for input and output?
They sure are.
Programs today typically do not make much use of the bitwise left and right shift operators to shift bits. Rather, you tend to see the bitwise left shift operator used with std::cout to output text. Consider the following program:
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#include <iostream> int main() { unsigned int x { 0b0100 }; x = x << 1; // use operator<< for left shift std::cout << std::bitset<4>(x); // use operator<< for output return 0; } |
This program prints:
1000
In the above program, how does operator<< know to shift bits in one case and output x in another case? The answer is that std::cout has overloaded (provided an alternate definition for) operator<< that does console output rather than bit shifting.
When the compiler sees that the left operand of operator<< is std::cout, it knows that it should call the version of operator<< that std::cout overloaded to do output. If the left operand some other type, then operator<< knows it should do its usual bit-shifting behavior.
The same applies for operator>>.
Note that if you're using operator << for both output and left shift, parenthisization is required:
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#include <iostream> #include <bitset> int main() { std::bitset<4> x{ 0b0110 }; std::cout << x << 1 << '\n'; // print value of x (0110), then 1 std::cout << (x << 1) << '\n'; // print x left shifted by 1 (1100) return 0; } |
This prints:
01101 1100
The first line prints the value of x (0110), and then the literal 1. The second line prints the value of x left-shifted by 1 (1100).
We will talk more about operator overloading in a future section, including discussion of how to overload operators for your own purposes.
Bitwise NOT
The bitwise NOT operator (~) is perhaps the easiest to understand of all the bitwise operators. It simply flips each bit from a 0 to a 1, or vice versa. Note that the result of a bitwise NOT is dependent on what size your data type is.
Flipping 4 bits:
~0100 is 1011
Flipping 8 bits:
~0000 0100 is 1111 1011
In both the 4-bit and 8-bit cases, we start with the same number (binary 0100 is the same as 0000 0100 in the same way that decimal 7 is the same as 07), but we end up with a different result.
We can see this in action in the following program:
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#include <iostream> #include <bitset> int main() { std::cout << std::bitset<4>(~0b0100u) << ' ' << std::bitset<8>(~0b0100u); return 0; } |
This prints:
1011 11111011
Bitwise OR
Bitwise OR (|) works much like its logical OR counterpart. However, instead of applying the OR to the operands to produce a single result, bitwise OR applies to each bit! For example, consider the expression 0b0101 | 0b0110.
To do (any) bitwise operations, it is easiest to line the two operands up like this:
0 1 0 1 OR 0 1 1 0
and then apply the operation to each column of bits.
If you remember, logical OR evaluates to true (1) if either the left, right, or both operands are true (1), and 0 otherwise. Bitwise OR evaluates to 1 if either the left, right, or both bits are 1, and 0 otherwise. Consequently, the expression evaluates like this:
0 1 0 1 OR 0 1 1 0 ------- 0 1 1 1
Our result is 0111 binary.
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#include <iostream> #include <bitset> int main() { std::cout << (std::bitset<4>(0b0101) | std::bitset<4>(0b0110)); return 0; } |
This prints:
0111
We can do the same thing to compound OR expressions, such as 0b0111 | 0b011 | 0b0001. If any of the bits in a column are 1, the result of that column is 1.
0 1 1 1 OR 0 0 1 1 OR 0 0 0 1 -------- 0 1 1 1
Here's code for the above:
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#include <iostream> #include <bitset> int main() { std::cout << (std::bitset<4>(0b0111) | std::bitset<4>(0b0011) | std::bitset<4>(0b0001)); return 0; } |
This prints:
0111
Bitwise AND
Bitwise AND (&) works similarly to the above. Logical AND evaluates to true if both the left and right operand evaluate to true. Bitwise AND evaluates to true (1) if both bits in the column are 1. Consider the expression 0b0101 & 0b0110. Lining each of the bits up and applying an AND operation to each column of bits:
0 1 0 1 AND 0 1 1 0 -------- 0 1 0 0
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#include <iostream> #include <bitset> int main() { std::cout << (std::bitset<4>(0b0101) & std::bitset<4>(0b0110)); return 0; } |
This prints:
0100
Similarly, we can do the same thing to compound AND expressions, such as 0b0001 & 0b0011 & 0b0111. If all of the bits in a column are 1, the result of that column is 1.
0 0 0 1 AND 0 0 1 1 AND 0 1 1 1 -------- 0 0 0 1
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#include <iostream> #include <bitset> int main() { std::cout << (std::bitset<4>(0b0001) & std::bitset<4>(0b0011) & std::bitset<4>(0b0111)); return 0; } |
This prints:
0001
Bitwise XOR
The last operator is the bitwise XOR (^), also known as exclusive or.
When evaluating two operands, XOR evaluates to true (1) if one and only one of its operands is true (1). If neither or both are true, it evaluates to 0. Consider the expression 0b0110 ^ 0b0011:
0 1 1 0 XOR 0 0 1 1 ------- 0 1 0 1
It is also possible to evaluate compound XOR expression column style, such as 0b0001 ^ 0b0011 ^ 0b01111. If there are an even number of 1 bits in a column, the result is 0. If there are an odd number of 1 bits in a column, the result is 1.
0 0 0 1 XOR 0 0 1 1 XOR 0 1 1 1 -------- 0 1 0 1
Bitwise assignment operators
Similar to the arithmetic assignment operators, C++ provides bitwise assignment operators in order to facilitate easy modification of variables.
| Operator | Symbol | Form | Operation |
|---|---|---|---|
| Left shift assignment | <<= | x <<= y | Shift x left by y bits |
| Right shift assignment | >>= | x >>= y | Shift x right by y bits |
| Bitwise OR assignment | |= | x |= y | Assign x | y to x |
| Bitwise AND assignment | &= | x &= y | Assign x & y to x |
| Bitwise XOR assignment | ^= | x ^= y | Assign x ^ y to x |
For example, instead of writing x = x >> 1;, you can write x >>= 1;.
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#include <bitset> #include <iostream> int main() { std::bitset<4> bits { 0b0100 }; bits >>= 1; std::cout << bits; return 0; } |
This program prints:
0010
Summary
Summarizing how to evaluate bitwise operations utilizing the column method:
When evaluating bitwise OR, if any bit in a column is 1, the result for that column is 1.
When evaluating bitwise AND, if all bits in a column are 1, the result for that column is 1.
When evaluating bitwise XOR, if there are an odd number of 1 bits in a column, the result for that column is 1.
In the next lesson, we'll explore how these operators can be used in conjunction with bit masks to facilitate bit manipulation.
Quiz time
Question #1
a) What does 0110 >> 2 evaluate to in binary?
b) What does the following evaluate to in binary: 0011 | 0101?
c) What does the following evaluate to in binary: 0011 & 0101?
d) What does the following evaluate to in binary (0011 | 0101) & 1001?
Question #2
A bitwise rotation is like a bitwise shift, except that any bits shifted off one end are added back to the other end. For example 0b1001 << 1 would be 0b0010, but a left rotate by 1 would result in 0b0011 instead. Implement a function that does a left rotate on a std::bitset<4>. For this one, it's okay to use std::bitset<4>::test() and std::bitset<4>::set().
The following code should execute:
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#include <iostream> #include <bitset> std::bitset<4> rotl(std::bitset<4> bits) { // Your code here } int main() { std::bitset<4> bits1{ 0b0001 }; std::cout << rotl(bits1) << '\n'; std::bitset<4> bits2{ 0b1001 }; std::cout << rotl(bits2) << '\n'; } |
and print the following:
0010 0011
 O.3 -- Bit manipulation with bitwise operators and bit masks |
 Index |
 O.1 -- Bit flags and bit manipulation via std::bitset |
Sorry if it seems like I'm being nitpicky here.
On the first 'Author notes', 0b is mentioned as a suffix, and
it took me a while to understand that it was meant to be a prefix.
"For readability, we’ll also omit the 0b suffix outside of code
examples (e.g. instead of 0b0101, we’ll just use 0101)."
Thanks for the tutorial!
-suffix, +prefix. Thanks for the post-fix!
Isn't there a mistake in the solution to Quiz question 1) d)?
You're doing an AND operation instead of an OR.
It is correct, just recheck it.
It has both OR and AND operator, but as OR operator is in parenthesis, it is operated first and then its outcome is operated by AND operator with the rightmost operand.
The first operation is not an OR operation at all, it is an AND. It should be an OR. There is a mistake!
Yup. Mistake fixed. Thanks for pointing that out!
Shouldn't this print -126, since we're dealing with signed char? I get -2.
EDIT: nvm, just realized that it's getting value from two complement method. Using signed and absolute value, it appears that char's range is from -127 to 127 (255), while unsigned the range is from 0 to 255 (256)
Also, why doesn't this compile:
`0b1111'1110` doesn't fit into a char.
Despite you entering a binary representation, you're not directly setting the bits of `x`. Writing `0b1111'1110` is not different than writing `254`.
Thanks, got it.
(EDIT) I just looked at the next lesson and realized this is probably more simply done by using an unsigned char, but this still works. :-)
If anyone wants something to try in terminal to see how bitwise operations work on actual c++ datatypes, we can define a short with a binary value (even though they are saved as ints) and follow along with operations.
What happens if we bitwise AND between variables of different sizes?
Like
Does it start from the beginning of both?
Does it loop through the smaller one?
The smaller types are promoted to the larger type.
@oneByte -> int
@manyBytes -> long long
If both types are smaller than an int, they're promoted to an int.
int is of 32 bits normally.
so why ~3 is giving ans -4 instead of 4294967292. all 32 bits will be flipped if its is an integer?
It's a signed integer. Lesson 3.7
Did the site go down yesterday?
There seems to have been some DNS trouble. If learncpp is down and you want to learn, you can visit snapshots over at archive.org
http://web.archive.org/web/20190119083823/http://www.learncpp.com/
Yes, it went down quite a few times yesterday. The site apparently ran out of virtual memory and that was causing intermittent issues. I've given the server a reboot and it's been stable since. Sorry for the inconvenience.
Thank you, it’s running quite well now.
"We will talk more about operator overloading in a future section, including discussion of how to override operators for your own purposes."
Shouldn't it be overload instead of overrride?
Yes, thank you. Fixed.
I'd like to ask your comments on some uses of bitwise operators I found on the internet (link available upon request).
One of them is this form of the postfix initializer and incrementer:
To be fair to the obviously experienced programmer who provides this example for Javascript rather than C++, he calls this use of bitwise operators "evil" because it's compiler abuse just in case you forgot to initialize your incrementer variable.
Another use just punched me in the face, and it's for rounding a float into an integer:
I figure that shortcut is probably most useful in environments such as embedded programming, but it's my first exposure to memory conservation and I find it stimulating. With regard to C++ or C can you point me to some resources on programming for low-memory environments? I'm happy with just one link where I can learn the basics.
Finally, here's one of my own:
In a very old computer RPG, there were four basic classes: Cleric, Fighter, Magic-User, and Thief. You had a party of six characters, although you could try to win the game with fewer than six. Some races of characters could be two or three classes at the same time. Well, in order to have a reasonable chance at winning the game, you needed 10 character classes among your six characters. There's a way to select those randomly using bitwise logic:
Each character starts with a class value of 0.
Cleric: character | 1
Fighter: character | 2
Magic-User: character | 4
Thief: character | 8
You can code a function that pseudorandomly flips the bits of your six characters until you have your 10 starting character classes, and build your pseudorandom party of six characters that way. It can be really fun just to go with whatever the pseudorandom generator gives you, and there's no way within the game to make pseudorandom character class choices, so this is one way. Neat, eh?
Hi Ajalle!
> One of them is this form of the postfix initializer and incrementer [...]
Nope. If @incrementer is uninitialized and not 0, the line after won't magically turn it into a 1. The example works in JS, because uninitialized values in JS are have the value 'undefined'. '~undefined' is -1, so '-~undefined' is 1. Uninitialized variables in C++ have an unspecified value. Always initialize your variables!
> Another use just punched me in the face
Again, this works in JS, but not in C++. floats don't have an @operator|.
Is XOR the only bitwise operator that doesn't cause data loss?
Operation reversal can evaluate the original value?
If, yes, I think this should be noted here. If, no, please correct me.
Bitwise NOT can also be reversed by doing another NOT.
Yes, logical and simpler.
I think these worth to be noted.
Thanks
"Note: Many people find this lesson challenging." And yet you explained the concepts so clearly that it became trivial to understand :) Thanks!
Yes, He did that very well. Thumbs-UP.
I hope Alex can do learnrust.com soon.
I’d like to add something my high school math teacher taught me back in 1975.
OR turns bits *on*.
AND turns bits *off*.
XOR *toggles* bits.
(Edit: Sorry Alex, I see now you say this exact thing in the next chapter!)
TYPO:
"With an 8-bit value, 3 << 3 would be 24 because the 16-bit wouldn't be shifted off the end of the binary number.", at the "8-bit" and "16-bit"
Thanks for pointing this out. Fixed!