std::bitset<N>::bitset
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bitset();
(1)
(noexcept since C++11) (constexpr 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 >
(3)
(constexpr since C++23)
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,
template< class CharT, class Traits >
(4)
(since C++26)
constexpr explicit bitset
( std::basic_string_view <CharT, Traits> str,
std::size_t pos = 0, std::size_t n = std::size_t (-1),
template< class CharT >
(5)
(since C++11) explicit bitset( const CharT* str, std::size_t n = std::size_t (-1),
(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 from an unsigned integer val.
Given the number of bits in the value representation of unsigned long(until C++11)unsigned long long(since C++11) as S:
- The first (rightmost, least significant) std::min (S, N) bit positions are initialized with the corresponding bit values of val.
- If S is less than N, the remaining bit positions are initialized to zeroes.
3) Constructs a bitset using the characters in 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).
5) Similar to (3), but uses a const CharT* instead of a std::basic_string .
Equivalent to bitset(n == std::basic_string <CharT>::npos
? std::basic_string <CharT>(str)
: std::basic_string <CharT>(str, n), 0, n, zero, one).
Equivalent to bitset(n == std::basic_string_view <CharT>::npos
? std::basic_string_view <CharT>(str)
: std::basic_string_view <CharT>(str, n), 0, n, zero, one).
[edit] 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
zero
-
alternate character for unset bits in str
one
-
alternate character for set bits in str
[edit] Exceptions
3,4) std::out_of_range if pos > str.size(), std::invalid_argument if any character is not one or zero.
[edit] 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)
|
[edit] 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
[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 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 | S was 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 |
| LWG 2250 | C++98 | the behavior was undefined if pos > str.size() is true | always throws an exception in this case |