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std::bitset<N>::test

From cppreference.com
< cpp‎ | utility‎ | bitset
 
 
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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[] , it performs a bounds check.

[edit] Parameters

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

[edit] Return value

true if the requested bit is set, false otherwise.

[edit] Exceptions

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

[edit] 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)

[edit] 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 2250 C++98 the behavior was undefined if pos does
not correspond to a valid bit position
always throws an
exception in this case

[edit] See also

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

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