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std::bitset<N>::set - cppreference.com

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bitset& set();

 (1) (noexcept since C++11)
(constexpr since C++23)

bitset& set( std::size_t pos, bool value = true );

 (2) (constexpr since C++23)

Sets all bits to true or sets one bit to specified value.

1) Sets all bits to true.

2) Sets the bit at position pos to the value value.

Parameters

pos - the position (counting from 0, i.e. from least significant to most significant) of the bit to set
value - the value to set the bit to

Return value

*this

Exceptions

2) Throws std::out_of_range if pos does not correspond to a valid bit position.

Example

#include <bitset>
#include <cstddef>
#include <iostream>
 
int main()
{
    std::bitset<8> b;
    std::cout << b << '\n';
    std::cout << b.set() << '\n';
    std::cout << b.reset() << '\n';
    
    for (std::size_t i = 1; i < b.size(); i += 2)
        b.set(i);
    
    std::cout << b << '\n';
}

Output: 

00000000
11111111
00000000
10101010

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 186 C++98 the type of value was int corrected to bool
LWG 2250 C++98 the behavior was undefined if pos does
not correspond to a valid bit position 		always throws an
exception in this case

See also

sets bits to false
(public member function) [edit]
toggles the values of bits
(public member function) [edit]