NAME¶

std::bitset::test - std::bitset::test

Synopsis¶

bool test( std::size_t pos ) const; (constexpr since C++23)

Returns the value of the bit at the position pos (counting from 0).

Unlike operator[], performs a bounds check and throws std::out_of_range if pos does
not correspond to a valid position in the bitset.

Parameters¶

pos - position of the bit to return (counting from 0)

Return value¶

true if the requested bit is set, false otherwise.

Exceptions¶

std::out_of_range if pos does not correspond to a valid position within the bitset.

Example¶

// Run this code

#include <bit>
#include <bitset>
#include <cassert>
#include <iostream>
#include <stdexcept>

int main()
{
std::bitset<10> b1("1111010000");

std::size_t idx = 0;
while (idx < b1.size() && !b1.test(idx))
++idx;

assert(static_cast<int>(idx) == std::countr_zero(b1.to_ulong()));

if (idx < b1.size())
std::cout << "The first set bit is at index " << idx << '\n';
else
std::cout << "no set bits\n";

try
{
std::bitset<0B10'1001'1010> bad;
if (bad.test(bad.size()))
std::cout << "Expect unexpected!\n";
}
catch (std::out_of_range const& ex)
{
std::cout << "Exception: " << ex.what() << '\n';
}
}

Possible output:¶

The first set bit is at index 4
Exception: bitset::test: __position (which is 666) >= _Nb (which is 666)

See also¶

operator[] accesses specific bit
(public member function)
popcount counts the number of 1 bits in an unsigned integer
(C++20) (function template)
has_single_bit checks if a number is an integral power of 2
(C++20) (function template)