NAME¶

std::basic_string::basic_string - std::basic_string::basic_string

Synopsis¶

basic_string(); (until explicit basic_string( const C++17) Allocator& alloc ); basic_string() noexcept(noexcept( Allocator() )) : (since
C++17) basic_string( Allocator() ) (until {} C++20)

explicit basic_string( const Allocator& alloc ) noexcept; constexpr basic_string() noexcept(noexcept( Allocator() )) :

basic_string( Allocator() ) (since {} C++20)

explicit constexpr basic_string( const Allocator& alloc ) noexcept; basic_string( size_type count, CharT ch, (until const Allocator& alloc = C++20) Allocator() ); constexpr basic_string( size_type count, CharT ch, (since const Allocator& alloc = C++20) Allocator() ); basic_string( const basic_string& other,
(until size_type pos, C++20)

const Allocator& alloc = Allocator() ); constexpr basic_string( const basic_string& other,
(since size_type pos, C++20)

const Allocator& alloc = Allocator() ); basic_string( const basic_string& other,

size_type pos, size_type (until count, C++20)

const Allocator& alloc = Allocator() ); constexpr basic_string( const basic_string& other,

size_type pos, size_type (since count, C++20)

const Allocator& alloc = Allocator() ); basic_string( const CharT* s, size_type count, (until const Allocator& alloc = C++20) Allocator() ); constexpr basic_string( const CharT* s, size_type (since count, C++20) const Allocator& alloc = Allocator() ); basic_string( const CharT* s, (until const Allocator& alloc = C++20) Allocator() ); constexpr basic_string( const CharT* s, (since const Allocator& alloc = C++20) Allocator() ); template< class InputIt >

basic_string( InputIt first, (until InputIt last, C++20)
(1) const Allocator& alloc = Allocator() ); template< class InputIt >

constexpr basic_string( (since InputIt first, InputIt last, C++20)

const Allocator& alloc = Allocator() ); basic_string( const (until basic_string& other ); (2) C++20) constexpr basic_string( (since const basic_string& other ); C++20) basic_string( const (since basic_string& other, (3) C++11) const Allocator& alloc ); (until
C++20) constexpr basic_string( (since const basic_string& other, C++20) const Allocator& alloc );
(since basic_string( basic_string&& (3) C++11) other ) noexcept; (until
C++20) constexpr basic_string( (since basic_string&& other ) C++20) noexcept; basic_string( basic_string&& (since other, C++11) const Allocator& alloc ); (4) (until
C++20) constexpr basic_string( (since basic_string&& other, C++20) const Allocator& alloc ); (5) basic_string( (since std::initializer_list<CharT> C++11) ilist, (until const Allocator& alloc = (6) C++20) Allocator() ); constexpr basic_string( std::initializer_list<CharT> (since ilist, C++20) const Allocator& alloc = Allocator() ); template< class (7) StringViewLike >
(7) (since explicit basic_string( const C++17) StringViewLike& t, (until
C++20) const Allocator& alloc = (8) Allocator() ); template< class StringViewLike > (8)

explicit constexpr (since basic_string( const C++20) StringViewLike& t, (9)

const Allocator& alloc = Allocator() ); template< class StringViewLike > (10)
(since basic_string( const C++17) StringViewLike& t, size_type (until pos, size_type n, C++20)

const Allocator& alloc = Allocator() ); template< class (11) StringViewLike >

constexpr basic_string( (since const StringViewLike& t, C++20) size_type pos, size_type n,

const Allocator& alloc = Allocator() ); basic_string( std::nullptr_t (12) (since ) = delete; C++23)

Constructs new string from a variety of data sources and optionally using user
supplied allocator alloc.

1) Default constructor. Constructs empty string (zero size and unspecified
capacity). If no allocator is supplied, allocator is obtained from a
default-constructed instance.
2) Constructs the string with count copies of character ch.
This constructor is not used for class template argument deduction if the Allocator
type that would be deduced does not qualify as an allocator.
(since C++17)
3) Constructs the string with a substring [pos, pos+count) of other. If count ==
npos, if count is not specified, or if the requested substring lasts past the end of
the string, the resulting substring is [pos, other.size()).
4) Constructs the string with the first count characters of character string pointed
to by s. s can contain null characters. The length of the string is count. The
behavior is undefined if [s, s + count) is not a valid range.
5) Constructs the string with the contents initialized with a copy of the
null-terminated character string pointed to by s. The length of the string is
determined by the first null character. The behavior is undefined if [s, s +
Traits::length(s)) is not a valid range (for example, if s is a null pointer).
This constructor is not used for class template argument deduction if the Allocator
type that would be deduced does not qualify as an allocator.
(since C++17)
6) Constructs the string with the contents of the range [first, last).

If InputIt is an integral type, equivalent to overload (2), as if by
basic_string(static_cast<size_type>(first), (until C++11)
static_cast<value_type>(last), alloc)
This constructor only participates in overload resolution if InputIt (since C++11)
satisfies LegacyInputIterator.

7) Copy constructor. Constructs the string with a copy of the contents of other.
8) Move constructor. Constructs the string with the contents of other using move
semantics. other is left in valid, but unspecified state.
9) Constructs the string with the contents of the initializer list ilist.
10) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT,
Traits> sv = t;, then initializes the string with the contents of sv, as if by
basic_string(sv.data(), sv.size(), alloc). This overload participates in overload
resolution only if std::is_convertible_v<const StringViewLike&,
std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const
StringViewLike&, const CharT*> is false.
11) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT,
Traits> sv = t;, then initializes the string with the subrange [pos, pos + n) of sv
as if by basic_string(sv.substr(pos, n), alloc). This overload participates in
overload resolution only if std::is_convertible_v<const StringViewLike&,
std::basic_string_view<CharT, Traits>> is true .
12) basic_string cannot be constructed from nullptr.

Parameters¶

alloc - allocator to use for all memory allocations of this string
count - size of the resulting string
ch - value to initialize the string with
pos - position of the first character to include
first, last - range to copy the characters from
s - pointer to an array of characters to use as source to initialize the
string with
other - another string to use as source to initialize the string with
ilist - std::initializer_list to initialize the string with
t - object (convertible to std::basic_string_view) to initialize the
string with

Complexity¶

1) constant
2-4) linear in count
5) linear in length of s
6) linear in distance between first and last
7) linear in size of other
8) constant. If alloc is given and alloc != other.get_allocator(), then linear
9) linear in size of ilist

Exceptions¶

3) std::out_of_range if pos > other.size()
8) Throws nothing if alloc == str.get_allocator()
11) std::out_of_range if pos is out of range

Throws std::length_error if the length of the constructed string would exceed
max_size() (for example, if count > max_size() for (2)). Calls to
Allocator::allocate may throw.

Notes¶

Initialization with a string literal that contains embedded '\0' characters uses the
overload (5), which stops at the first null character. This can be avoided by
specifying a different constructor or by using operator""s:

std::string s1 = "ab\0\0cd"; // s1 contains "ab"
std::string s2{"ab\0\0cd", 6}; // s2 contains "ab\0\0cd"
std::string s3 = "ab\0\0cd"s; // s3 contains "ab\0\0cd"

Example¶

// Run this code

#include <iostream>
#include <iomanip>
#include <cassert>
#include <iterator>
#include <string>
#include <cctype>

int main()
{
{
std::cout << "1) string(); ";
std::string s;
assert(s.empty() && (s.length() == 0) && (s.size() == 0));
std::cout << "s.capacity(): " << s.capacity() << '\n'; // unspecified
}

{
std::cout << "2) string(size_type count, charT ch): ";
std::string s(4, '=');
std::cout << std::quoted(s) << '\n'; // "===="
}

{
std::cout << "3) string(const string& other, size_type pos, size_type count): ";
std::string const other("Exemplary");
std::string s(other, 0, other.length()-1);
std::cout << quoted(s) << '\n'; // "Exemplar"
}

{
std::cout << "4) string(const string& other, size_type pos): ";
std::string const other("Mutatis Mutandis");
std::string s(other, 8);
std::cout << quoted(s) << '\n'; // "Mutandis", i.e. [8, 16)
}

{
std::cout << "5) string(charT const* s, size_type count): ";
std::string s("C-style string", 7);
std::cout << quoted(s) << '\n'; // "C-style", i.e. [0, 7)
}

{
std::cout << "6) string(charT const* s): ";
std::string s("C-style\0string");
std::cout << quoted(s) << '\n'; // "C-style"
}

{
std::cout << "7) string(InputIt first, InputIt last): ";
char mutable_c_str[] = "another C-style string";
std::string s(std::begin(mutable_c_str)+8, std::end(mutable_c_str)-1);
std::cout << quoted(s) << '\n'; // "C-style string"
}

{
std::cout << "8) string(string&): ";
std::string const other("Exemplar");
std::string s(other);
std::cout << quoted(s) << '\n'; // "Exemplar"
}

{
std::cout << "9) string(string&&): ";
std::string s(std::string("C++ by ") + std::string("example"));
std::cout << quoted(s) << '\n'; // "C++ by example"
}

{
std::cout << "α) string(std::initializer_list<charT>): ";
std::string s({ 'C', '-', 's', 't', 'y', 'l', 'e' });
std::cout << quoted(s) << '\n'; // "C-style"
}

{
// before C++11, overload resolution selects string(InputIt first, InputIt last)
// [with InputIt = int] which behaves *as if* string(size_type count, charT ch)
// after C++11 the InputIt constructor is disabled for integral types and calls:
std::cout << "β) string(size_type count, charT ch) is called: ";
std::string s(3, std::toupper('a'));
std::cout << quoted(s) << '\n'; // "AAA"
}

{
[[maybe_unused]]
auto zero = [] { /* ... */ return nullptr; };
// std::string s{ zero() }; // Before C++23: throws std::logic_error
// Since C++23: won't compile, see overload (12)
}
}

Possible output:¶

1) string(); s.capacity(): 15
2) string(size_type count, charT ch): "===="
3) string(const string& other, size_type pos, size_type count): "Exemplar"
4) string(const string& other, size_type pos): "Mutandis"
5) string(charT const* s, size_type count): "C-style"
6) string(charT const* s): "C-style"
7) string(InputIt first, InputIt last): "C-style string"
8) string(string&): "Exemplar"
9) string(string&&): "C++ by example"
α) string(std::initializer_list<charT>): "C-style"
β) string(size_type count, charT ch) is called: "AAA"

Defect reports

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

DR Applied to Behavior as published Correct behavior
there is no way to supply an there's a constructor for
LWG 2583 C++98 allocator for basic_string(str, pos, alloc)
basic_string(str, pos)
LWG 2193 C++11 the default constructor is made non-explicit
explicit
LWG 2946 C++17 string_view overload causes avoided by making it a template
ambiguity in some cases
two constructors may cause
LWG 3076 C++17 ambiguities in class constrained
template argument deduction

See also¶

assign assign characters to a string
(public member function)
operator= assigns values to the string
(public member function)
to_string converts an integral or floating point value to string
(C++11) (function)
to_wstring converts an integral or floating point value to wstring
(C++11) (function)
constructor constructs a basic_string_view
(C++17) (public member function of std::basic_string_view<CharT,Traits>)