C++ Bitset::operator[] Function

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The C++ function std::bitset::operator[] returns the value or a reference of a bit at the specified position.

This function has two polymorphic variants: the first returns the bit present at the specified position and the other returns a reference to the position of the bit specified in the parameter (You can find the syntaxes of all the variants below). The operator can be used for reading and writing binary values to the bitset.

Note that the index parameter is 0-based, meaning the first bit has an index of 0 and goes up to the size of the bitset minus 1.

Syntax

Following is the syntax for std::bitset::operator[] −

bool operator[] (size_t pos) const;
or,
reference operator[] (size_t pos);

Parameters

pos − Position of the bit whose value is to be accessed.

Return value

Returns the bit value or the reference to the bit value (which allows writing to the requested bit).

Example 1

The following example shows the usage of std::bitset::operator[] function by returning the value of the bits.

We are creating a bitset object "b" of size "4" with the binary value "1001". We are then iterating through each bit of the bitset using a for loop and printing the value of each bit using operator[].

#include <iostream>
#include <bitset>
using namespace std;

int main(void) {
   bitset<4> b("1001");
   for (int i = 0; i < 4; ++i)
      cout << "In bitset b[" << i << "] = " << b[i] << endl;
   return 0;
}

Output

Let us compile and run the above program, this will produce the following result −

In bitset b[0] = 1
In bitset b[1] = 0
In bitset b[2] = 0
In bitset b[3] = 1

Example 2

The following example shows the usage of std::bitset::operator[] function by returning the reference of a bit at the specified position.

We are creating a bitset object "b" of size of "4" with all bits set to "0". We are then setting the 2nd and 4th bit of the bitset to 1 using operator[].

#include <iostream>
#include <bitset>
using namespace std;

int main(void) {
   bitset<4> b;
   cout << "Initial value of bitset = " << b << endl;
   b[1] = 1;
   b[3] = 1;
   cout << "Value of bitset after setting few bits = " << b << endl;
   return 0;
}

Output

If we run the above code it will generate the following output −

Initial value of bitset = 0000
Value of bitset after setting few bits = 1010

Example 3

Here, we are trying to check if the bit is set to 1 using operator[].

In the following example, we are creating a bitset object "bits" of size "8" with the value of "170" (which is 10101010 in binary). Then, we use the if statement to check if the 3rd bit (counting from 0) is set to 1 using the operator[].

#include <iostream>
#include <bitset>

int main() {
   std::bitset<8> bits(170); 
   if (bits[2]) {
      std::cout << "The 3rd bit is set to 1" << std::endl;
   } else {
      std::cout << "The 3rd bit is set to 0" << std::endl;
   }
   return 0;
}

Output

Following is an output of the above code −

The 3rd bit is set to 0

Example 4

Now, we are trying to flip a bit using operator[].

We are creating a bitset object "bits" of size "8" with the value 255 (which is 11111111 in binary). We are then using the flip() function in conjunction with operator[] on the 5th bit (counting from 0) to flip the bit from 1 to 0.

#include <iostream>
#include <bitset>

int main() {
   // initializes all bits to 1
   std::bitset<8> bits(255); 
   // flips the 5th bit from 1 to 0
   bits[4].flip(); 
   std::cout << "Value of bitset after flipping bit 4 = " << bits << std::endl;
   return 0;
}

Output

Output of the above code is as follows −

Value of bitset after flipping bit 4 = 11101111