NAME¶

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

Synopsis¶

bitset(); (until
C++11)
constexpr bitset() noexcept; (since
C++11)
bitset( unsigned long val ); (until C++11)
constexpr bitset( unsigned long long val ) (since C++11)
noexcept;
template< class CharT, class Traits, class Alloc >

explicit bitset( const std::basic_string<CharT,
Traits, Alloc>& str,
typename std::basic_string<CharT,
Traits, Alloc>::size_type
pos = 0, (3) (constexpr since
typename std::basic_string<CharT, C++23)
Traits, Alloc>::size_type
n = std::basic_string<CharT,
Traits, Alloc>::npos,
CharT zero = CharT('0'),
(1)
CharT one = CharT('1') );
template< class CharT, class Traits > (2)

constexpr explicit bitset(
std::basic_string_view<CharT, Traits> str,
std::size_t pos = 0,
std::size_t n = (4) (since C++26)
std::size_t(-1),
CharT zero = CharT('0'),

CharT one = CharT('1')
);
template< class CharT >

explicit bitset( const CharT* str, (since C++11)
std::size_t n = std::size_t(-1), (5) (constexpr since
CharT zero = CharT('0'), C++23)

CharT one = CharT('1') );

Constructs a new bitset from one of several optional data sources:

1) Default constructor. Constructs a bitset with all bits set to zero.
2) Constructs a bitset, initializing the first (rightmost, least significant) M bit
positions to the corresponding bit values of val, where M is the smaller of
* N, and

* the number of bits in the value representation of unsigned long. (until C++11)
For typical implementations, it is 32 or 64.
* the number of bits in the value representation of unsigned long (since C++11)
long. For typical implementations, it is 64.

If M is less than N, the remaining bit positions are initialized to zeroes.
3) Constructs a bitset using the characters in the std::basic_string str. An
optional starting position pos and length n can be provided, as well as characters
denoting alternate values for set (one) and unset (zero) bits. Traits::eq() is used
to compare the character values.
The effective length of the initializing string is std::min(n, str.size() - pos).
If pos > str.size(), this constructor throws std::out_of_range. If any character
examined in str is not zero or one, this constructor throws std::invalid_argument.
4) Similar to (3), but uses a std::basic_string_view instead of a std::basic_string.
5) Similar to (3), but uses a const CharT* instead of a std::basic_string.
Equivalent to

bitset(n == std::size_t(-1)
? basic_string<CharT>(str)
: basic_string<CharT>(str, n), 0, n, zero, one)

This constructor may dynamically allocate memory, although (until C++26)
implementations generally avoid dynamic allocation.
This constructor does not dynamically allocate memory, as if only (since C++26)
std::basic_string_view is involved.

Parameters¶

val - number used to initialize the bitset
str - string used to initialize the bitset
pos - a starting offset into str
n - number of characters to use from str
one - alternate character for set bits in str
zero - alternate character for unset bits in str

Exceptions¶

3,4) std::out_of_range if pos > str.size(), std::invalid_argument if any character
is not one or zero.
5) std::invalid_argument if any character is not one or zero.

Notes¶

Feature-test macro Value Std Feature
__cpp_lib_constexpr_bitset 202207L (C++23) A more constexpr std::bitset, overloads
(3,5)
__cpp_lib_bitset 202306L (C++26) Interfacing std::bitset with
std::string_view, (4)

Example¶

// Run this code

#include <bitset>
#include <climits>
#include <iostream>
#include <string>

int main()
{
// empty constructor (1)
std::bitset<8> b1; // [0,0,0,0,0,0,0,0]

// unsigned long long constructor (2)
std::bitset<8> b2(42); // [0,0,1,0,1,0,1,0]
std::bitset<70> bl(ULLONG_MAX); // [0,0,0,0,0,0,1,1,1,...,1,1,1] in C++11
std::bitset<8> bs(0xfff0); // [1,1,1,1,0,0,0,0]

// string constructor (3)
std::string bit_string = "110010";
std::bitset<8> b3(bit_string); // [0,0,1,1,0,0,1,0]
std::bitset<8> b4(bit_string, 2); // [0,0,0,0,0,0,1,0]
std::bitset<8> b5(bit_string, 2, 3); // [0,0,0,0,0,0,0,1]

// string constructor using custom zero/one digits (3)
std::string alpha_bit_string = "aBaaBBaB";
std::bitset<8> b6(alpha_bit_string, 0, alpha_bit_string.size(),
'a', 'B'); // [0,1,0,0,1,1,0,1]

// char* constructor using custom digits (5)
std::bitset<8> b7("XXXXYYYY", 8, 'X', 'Y'); // [0,0,0,0,1,1,1,1]

std::cout << "b1: " << b1 << "\nb2: " << b2 << "\nbl: " << bl
<< "\nbs: " << bs << "\nb3: " << b3 << "\nb4: " << b4
<< "\nb5: " << b5 << "\nb6: " << b6 << "\nb7: " << b7 << '\n';
}

Possible output:¶

b1: 00000000
b2: 00101010
bl: 0000001111111111111111111111111111111111111111111111111111111111111111
bs: 11110000
b3: 00110010
b4: 00000010
b5: 00000001
b6: 01001101
b7: 00001111

Defect reports

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

DR Applied to Behavior as published Correct behavior
the values of the zero and one characters added parameters to
LWG 396 C++98 for overload (3) provide
were 0 and 1 (which do not correspond to values for these
'0' and '1') characters
M is the smaller of N and the value
CHAR_BIT * sizeof(unsigned long) consider the number
LWG 457 C++98 for overload (2), but unsigned long is of bits of the value
not representation instead
guaranteed to use all its bits to
represent its value

See also¶

set sets bits to true or given value
(public member function)
reset sets bits to false
(public member function)