bool operator[]( std::size_t pos ) const;
(until C++11)
constexpr bool operator[]( std::size_t pos ) const;
(since C++11)
reference operator[]( std::size_t pos );
(until C++23)
constexpr reference operator[]( std::size_t pos );
(since C++23)

Accesses the bit at position pos. The first version returns the value of the bit, the second version returns an object of type std::bitset::reference that allows modification of the value.

Unlike test(), does not throw exceptions: the behavior is undefined if pos is out of bounds.


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.




#include <iostream>
#include <bitset>
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';


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

See also

accesses specific bit
(public member function)

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