std::basic_string<CharT,Traits,Allocator>::basic_string

 

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Feature test macros (C++20)

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Symbols index

External libraries

 

Strings library

Null-terminated strings

Byte strings

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Wide strings

Classes

basic_string

basic_string_view (C++17)

char_traits

 

std::basic_string

Member functions basic_string::basic_string basic_string::~basic_string basic_string::operator= basic_string::assign basic_string::assign_range (C++23) basic_string::get_allocator Element access basic_string::at basic_string::operator[] basic_string::front (DR*) basic_string::back (DR*) basic_string::data basic_string::c_str basic_string::operator  basic_string_view (C++17) Iterators basic_string::beginbasic_string::cbegin (C++11) basic_string::endbasic_string::cend (C++11) basic_string::rbeginbasic_string::crbegin (C++11) basic_string::rendbasic_string::crend (C++11) Capacity basic_string::empty basic_string::sizebasic_string::length basic_string::max_size basic_string::reserve basic_string::capacity basic_string::shrink_to_fit (DR*) Modifiers basic_string::clear basic_string::insert basic_string::insert_range (C++23) basic_string::erase basic_string::push_back basic_string::pop_back (DR*) basic_string::append basic_string::append_range (C++23) basic_string::operator+= basic_string::replace basic_string::replace_with_range (C++23) basic_string::copy basic_string::resize basic_string::resize_and_overwrite (C++23)   basic_string::swap

Search basic_string::find basic_string::rfind basic_string::find_first_of basic_string::find_first_not_of basic_string::find_last_of basic_string::find_last_not_of Operations basic_string::compare basic_string::starts_with (C++20) basic_string::ends_with (C++20) basic_string::contains (C++23) basic_string::substr Constants basic_string::npos Non-member functions operator+ swap(std::basic_string) erase(std::basic_string)erase_if(std::basic_string) (C++20)(C++20) I/O operator<<operator>> getline Comparison operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20) Numeric conversions stoistolstoll (C++11)(C++11)(C++11) stoulstoull (C++11)(C++11) stofstodstold (C++11)(C++11)(C++11) to_string (C++11) to_wstring (C++11) Literals operator""s (C++14) Helper classes hash<std::basic_string> (C++11) Deduction guides (C++17)

 

	(1)
basic_string() : basic_string(Allocator()) {}			(since C++11) (until C++17)
basic_string() noexcept(noexcept(Allocator()))     : basic_string(Allocator()) {}			(since C++17) (constexpr since C++20)
		(2)
explicit basic_string( const Allocator& alloc = Allocator() );				(until C++11)
explicit basic_string( const Allocator& alloc );				(noexcept since C++17) (constexpr since C++20)
basic_string( size_type count, CharT ch,               const Allocator& alloc = Allocator() );			(3)	(constexpr since C++20)
template< class InputIt > basic_string( InputIt first, InputIt last,               const Allocator& alloc = Allocator() );			(4)	(constexpr since C++20)
template< container-compatible-range<CharT> R > constexpr basic_string( std::from_range_t, R&& rg,                         const Allocator& = Allocator());			(5)	(since C++23)
basic_string( const CharT* s, size_type count,               const Allocator& alloc = Allocator() );			(6)	(constexpr since C++20)
basic_string( const CharT* s, const Allocator& alloc = Allocator() );			(7)	(constexpr since C++20)
basic_string( std::nullptr_t ) = delete;			(8)	(since C++23)
template< class StringViewLike > explicit basic_string( const StringViewLike& t,                        const Allocator& alloc = Allocator() );			(9)	(since C++17) (constexpr since C++20)
template< class StringViewLike > basic_string( const StringViewLike& t,               size_type pos, size_type count,               const Allocator& alloc = Allocator() );			(10)	(since C++17)
basic_string( const basic_string& other );			(11)	(constexpr since C++20)
basic_string( basic_string&& other ) noexcept;			(12)	(since C++11) (constexpr since C++20)
basic_string( const basic_string& other, const Allocator& alloc );			(13)	(since C++11) (constexpr since C++20)
basic_string( basic_string&& other, const Allocator& alloc );			(14)	(since C++11) (constexpr since C++20)
basic_string( const basic_string& other, size_type pos,               const Allocator& alloc = Allocator() );			(15)	(constexpr since C++20)
constexpr basic_string( basic_string&& other, size_type pos,                         const Allocator& alloc = Allocator() );			(16)	(since C++23)
basic_string( const basic_string& other,               size_type pos, size_type count,               const Allocator& alloc = Allocator() );			(17)	(constexpr since C++20)
constexpr basic_string( basic_string&& other,                         size_type pos, size_type count,                         const Allocator& alloc = Allocator() );			(18)	(since C++23)
basic_string( std::initializer_list<CharT> ilist,               const Allocator& alloc = Allocator() );			(19)	(since C++11) (constexpr since C++20)

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

1) The default constructor since C++11. Constructs an empty string with a default-constructed allocator.

If Allocator is not DefaultConstructible, the behavior is undefined.

2) The default constructor until C++11. Constructs an empty string with the given allocator alloc.

3) Constructs a string with count copies of character ch.

If CharT is not CopyInsertable into std::basic_string<CharT>, the behavior is undefined.	(since C++11)
This overload participates in overload resolution only if Allocator satisfies the requirements of Allocator.	(since C++17)

4) Constructs a string with the contents of the range [first, last). Each iterator in [first, last) is dereferenced exactly once.

If InputIt does not satisfy the requirements of LegacyInputIterator, overload (3) is called instead with arguments static_cast<size_type>(first), last and alloc.	(until C++11)
This overload participates in overload resolution only if InputIt satisfies the requirements of LegacyInputIterator. If CharT is not EmplaceConstructible into std::basic_string<CharT> from *first, the behavior is undefined.	(since C++11)

5) Constructs a string with the contents of the range rg. Each iterator in rg is dereferenced exactly once.

If CharT is not EmplaceConstructible into std::basic_string<CharT> from *ranges::begin(rg), the behavior is undefined.

6) Constructs a string with the contents of the range [s, s + count).

If [s, s + count) is not a valid range, the behavior is undefined.

7) Equivalent to basic_string(s, Traits::length(s), alloc).

This overload participates in overload resolution only if Allocator satisfies the requirements of Allocator.

(since C++17)

8) std::basic_string cannot be constructed from nullptr.

9) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT, Traits> sv = t;, then constructs a string 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.

10) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT, Traits> sv = t;, then constructs a string 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.

11-18) Constructs a string with (part of) the contents of other. If the type of other is basic_string&&, when the construction finishes, other is in a valid but unspecified state.

11) The copy constructor.

The allocator is obtained as if by calling std::allocator_traits<Allocator>::     select_on_container_copy_construction         (other.get_allocator()).

(since C++11)

12) The move constructor. The allocator is obtained by move construction from other.get_allocator().

13) Same as the copy constructor, except that alloc is used as the allocator.

If CharT is not CopyInsertable into std::basic_string<CharT>, the behavior is undefined.

14) Same as the move constructor, except that alloc is used as the allocator.

If CharT is not MoveInsertable into std::basic_string<CharT>, the behavior is undefined.

15,16) Constructs a string with the contents of the range [other.data() + pos, other.data() + other.size()).

17,18) Constructs a string with the contents of the range [other.data() + pos, other.data() + (pos + std::min(count, other.size() - pos))).

19) Equivalent to basic_string(ilist.begin(), ilist.end()).

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
rg	-	a container compatible range

Complexity

1,2) Constant.

3-7) Linear in the size of the string.

9-11) Linear in the size of the string.

12) Constant.

13) Linear in the size of the string.

14) Linear in the size of the string if alloc != other.get_allocator() is true, otherwise constant.

15-19) Linear in the size of the string.

Exceptions

10) std::out_of_range if pos is out of range.

14) Throws nothing if alloc == str.get_allocator() is true.

15-18) std::out_of_range if pos > other.size().

Throws std::length_error if the length of the constructed string would exceed max_size() (for example, if count > max_size() for (3)). Calls to Allocator::allocate may throw.

If an exception is thrown for any reason, these functions have no effect (strong exception safety guarantee).

Notes

Initialization with a string literal that contains embedded '\0' characters uses the overload (7), 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"

Feature-test macro	Value	Std	Feature
__cpp_lib_containers_ranges	202202L	(C++23)	Tagged constructor (5) to construct from container compatible range

Example

Run this code

#include <cassert>
#include <cctype>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <string>
 
int main()
{
    std::cout << "1)  string(); ";
    std::string s1;
    assert(s1.empty() && (s1.length() == 0) && (s1.size() == 0));
    std::cout << "s1.capacity(): " << s1.capacity() << '\n'; // unspecified
 
    std::cout << "3)  string(size_type count, CharT ch): ";
    std::string s2(4, '=');
    std::cout << std::quoted(s2) << '\n'; // "===="
 
    std::cout << "4)  string(InputIt first, InputIt last): ";
    char mutable_c_str[] = "another C-style string";
    std::string s4(std::begin(mutable_c_str) + 8, std::end(mutable_c_str) - 1);
    std::cout << std::quoted(s4) << '\n'; // "C-style string"
 
    std::cout << "6)  string(CharT const* s, size_type count): ";
    std::string s6("C-style string", 7);
    std::cout << std::quoted(s6) << '\n'; // "C-style", i.e. [0, 7)
 
    std::cout << "7)  string(CharT const* s): ";
    std::string s7("C-style\0string");
    std::cout << std::quoted(s7) << '\n'; // "C-style"
 
    std::cout << "11) string(string&): ";
    std::string const other11("Exemplar");
    std::string s11(other11);
    std::cout << std::quoted(s11) << '\n'; // "Exemplar"
 
    std::cout << "12) string(string&&): ";
    std::string s12(std::string("C++ by ") + std::string("example"));
    std::cout << std::quoted(s12) << '\n'; // "C++ by example"
 
    std::cout << "15) string(const string& other, size_type pos): ";
    std::string const other15("Mutatis Mutandis");
    std::string s15(other15, 8);
    std::cout << std::quoted(s15) << '\n'; // "Mutandis", i.e. [8, 16)
 
    std::cout << "17) string(const string& other, size_type pos, size_type count): ";
    std::string const other17("Exemplary");
    std::string s17(other17, 0, other17.length() - 1);
    std::cout << std::quoted(s17) << '\n'; // "Exemplar"
 
    std::cout << "19) string(std::initializer_list<CharT>): ";
    std::string s19({'C', '-', 's', 't', 'y', 'l', 'e'});
    std::cout << std::quoted(s19) << '\n'; // "C-style"
}

Possible output:

1)  string(); s1.capacity(): 15
3)  string(size_type count, CharT ch): "===="
4)  string(InputIt first, InputIt last): "C-style string"
6)  string(CharT const* s, size_type count): "C-style"
7)  string(CharT const* s): "C-style"
11) string(string&): "Exemplar"
12) string(string&&): "C++ by example"
15) string(const string& other, size_type pos): "Mutandis"
17) string(const string& other, size_type pos, size_type count): "Exemplar"
19) string(std::initializer_list<CharT>): "C-style"

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 301	C++98	overload (4) did not use the parameter alloc if InputIt is an integral type	use that parameter
LWG 438	C++98	overload (4) would only call overload (3) if InputIt is an integral type	calls overload (3) if InputIt is not an LegacyInputIterator
LWG 847	C++98	there was no exception safety guarantee	added strong exception safety guarantee
LWG 2193	C++11	the default constructor is explicit	made non-explicit
LWG 2583	C++98	there was no way to supply an allocator for overload (17)	added overload (15)
LWG 2946	C++17	overload (9) causes ambiguity in some cases	avoided by making it a template
LWG 3076	C++17	overloads (3,7) might cause ambiguities in class template argument deduction	constrained

See also

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