std::bitset<N>::bitset

	(1)
`bitset();`		(until C++11)
`constexpr bitset() noexcept;`		(since C++11)
	(2)
`bitset( unsigned long val );`		(until C++11)
`constexpr bitset( unsigned long long val ) noexcept;`		(since C++11)
`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, typename std::basic_string<CharT, Traits, Alloc>::size_type n = std::basic_string<CharT, Traits, Alloc>::npos, CharT zero = CharT('0'), CharT one = CharT('1') );`	(3)	(constexpr since C++23)
`template< class CharT, class Traits > constexpr explicit bitset( std::basic_string_view<CharT, Traits> str, std::size_t pos = 0, std::size_t n = std::size_t(-1), CharT zero = CharT('0'), CharT one = CharT('1') );`	(4)	(since C++26)
`template< class CharT > explicit bitset( const CharT* str, std::size_t n = std::size_t(-1), CharT zero = CharT('0'), CharT one = CharT('1') );`	(5)	(since C++11) (constexpr since C++23)

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

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

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

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

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

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