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std::bitset<N>:: operator[]

From cppreference.net
C++
Utilities library
std::bitset
bool operator [ ] ( std:: size_t pos ) const ;
(1) (constexpr since C++11)
reference operator [ ] ( std:: size_t pos ) ;
(2) (constexpr since C++23)

Accesses the bit at position pos .

1) Returns the value of the bit.
2) Returns an object of type std::bitset::reference that allows modification of the value.

If pos < size ( ) is false , the behavior is undefined.

(until C++26)

If pos < size ( ) is false :

  • If the implementation is hardened , a contract violation occurs. Moreover, if the contract-violation handler returns under “observe” evaluation semantic, the behavior is undefined.
  • If the implementation is not hardened, the behavior is undefined.
(since C++26)

Contents

Parameters

pos - position of the bit to return

Return value

1) The value of the requested bit.
2) An object of type std::bitset::reference , which allows writing to the requested bit.

Exceptions

Throws nothing.

Example

Run this code 
#include <bitset>
#include <cstddef>
#include <iostream>
int main()
{
    std::bitset<8> b1{0b00101010}; // binary literal for 42
    for (std::size_t i = 0; i < b1.size(); ++i)
        std::cout << "b1[" << i << "]: " << b1[i] << '\n';
    b1[0] = true; // modifies the first bit through bitset::reference
    std::cout << "After setting bit 0, b1 holds " << b1 << '\n';
}

Output: 

b1[0]: 0
b1[1]: 1
b1[2]: 0
b1[3]: 1
b1[4]: 0
b1[5]: 1
b1[6]: 0
b1[7]: 0
After setting bit 0, b1 holds 00101011

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 11 C++98 1. the description was missing in the C++ standard
2. there was only the non-const overload 		1. description added
2. added the const overload
LWG 907 C++98 the behavior of reading the bit at pos was equivalent
to that of test(pos) , but test() may throw exceptions 		avoids mentioning test()

See also

accesses specific bit
(public member function)