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

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Strings library

Null-terminated strings

Byte strings

Multibyte strings

Wide strings

Classes

basic_string

basic_string_view (C++17)

char_traits

 

std::basic_string

Member functions basic_string::basic_string basic_string::operator= basic_string::assign basic_string::get_allocator Element access basic_string::at basic_string::operator[] basic_string::front (C++11) basic_string::back (C++11) 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 (C++11)

Operations basic_string::clear basic_string::insert basic_string::erase basic_string::push_back basic_string::pop_back (C++11) basic_string::append basic_string::operator+= basic_string::compare basic_string::starts_with (C++20) basic_string::ends_with (C++20) basic_string::replace basic_string::substr basic_string::copy basic_string::resize 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 Constants basic_string::npos

Non-member functions operator+ swap(std::basic_string) operator""s (C++14) 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>= Numeric conversion (C++11) stoistolstoll stoulstoull stofstodstold to_string to_wstring Helper classes hash<std::string>hash<std::wstring>hash<std::u32string>hash<std::u16string>hash<std::u8string> (C++11)(C++11)(C++11)(C++11)(C++20) hash<std::pmr::string>hash<std::pmr::wstring>hash<std::pmr::u32string>hash<std::pmr::u16string>hash<std::pmr::u8string> (C++20)(C++20)(C++20)(C++20)(C++20) Deduction guides(C++17)

 

	(1)
basic_string(); explicit basic_string( const Allocator& alloc );			(until C++17)
basic_string() noexcept(noexcept( Allocator() )): basic_string( Allocator() ) {} explicit basic_string( const Allocator& alloc ) noexcept;			(since C++17)
basic_string( size_type count,               CharT ch,               const Allocator& alloc = Allocator() );		(2)
		(3)
basic_string( const basic_string& other,               size_type pos,               size_type count = std::basic_string::npos,               const Allocator& alloc = Allocator() );				(until C++17)
basic_string( const basic_string& other,               size_type pos,               const Allocator& alloc = Allocator() );				(since C++17)
basic_string( const basic_string& other,               size_type pos,               size_type count,               const Allocator& alloc = Allocator() );				(since C++17)
basic_string( const CharT* s,               size_type count,               const Allocator& alloc = Allocator() );			(4)
basic_string( const CharT* s,               const Allocator& alloc = Allocator() );			(5)
template< class InputIt > basic_string( InputIt first, InputIt last,               const Allocator& alloc = Allocator() );			(6)
basic_string( const basic_string& other );			(7)
basic_string( const basic_string& other, const Allocator& alloc );			(7)	(since C++11)
basic_string( basic_string&& other ) noexcept;			(8)	(since C++11)
basic_string( basic_string&& other, const Allocator& alloc );			(8)	(since C++11)
basic_string( std::initializer_list<CharT> ilist,               const Allocator& alloc = Allocator() );			(9)	(since C++11)
template < class T > explicit basic_string( const T& t, const Allocator& alloc = Allocator() );			(10)	(since C++17)
template < class T > basic_string( const T& t, size_type pos, size_type n,               const Allocator& alloc = Allocator() );			(11)	(since C++17)

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 basic_string(static_cast<size_type>(first), static_cast<value_type>(last), a)	(until C++11)
This constructor only participates in overload resolution if InputIt satisfies LegacyInputIterator.	(since C++11)

7) Copy constructor. Constructs the string with the 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 only participates in overload resolution if std::is_convertible_v<const T&, std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const T&, 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), a). This overload only participates in overload resolution if std::is_convertible_v<const T&, std::basic_string_view<CharT, Traits>> is true .

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"

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 2193	C++11	the default constructor is explicit	made non-explicit
LWG 2946	C++17	string_view overload causes ambiguity in some cases	avoided by making it a template
LWG 3076	C++17	two constructors may cause ambiguities in class template argument deduction	constrained

Example

Run this code

#include <iostream>
#include <cassert>
#include <iterator>
#include <string>
#include <cctype>
 
int main()
{
  {
    // string::string()
    std::string s;
    assert(s.empty() && (s.length() == 0) && (s.size() == 0));
  }
 
  {
    // string::string(size_type count, charT ch)
    std::string s(4, '=');
    std::cout << s << '\n'; // "===="
  }
 
  {
    std::string const other("Exemplary");
    // string::string(string const& other, size_type pos, size_type count)
    std::string s(other, 0, other.length()-1);
    std::cout << s << '\n'; // "Exemplar"
  }
 
  {
    // string::string(charT const* s, size_type count)
    std::string s("C-style string", 7);
    std::cout << s << '\n'; // "C-style"
  }
 
  {
    // string::string(charT const* s)
    std::string s("C-style\0string");
    std::cout << s << '\n'; // "C-style"
  }
 
  {
    char mutable_c_str[] = "another C-style string";
    // string::string(InputIt first, InputIt last)
    std::string s(std::begin(mutable_c_str)+8, std::end(mutable_c_str)-1);
    std::cout << s << '\n'; // "C-style string"
  }
 
  {
    std::string const other("Exemplar");
    std::string s(other);
    std::cout << s << '\n'; // "Exemplar"
  }
 
  {
    // string::string(string&& str)
    std::string s(std::string("C++ by ") + std::string("example"));
    std::cout << s << '\n'; // "C++ by example"
  }
 
  {
    // string(std::initializer_list<charT> ilist)
    std::string s({ 'C', '-', 's', 't', 'y', 'l', 'e' });
    std::cout << s << '\n'; // "C-style"
  }
 
  {
    // overload resolution selects string(InputIt first, InputIt last) [with InputIt = int]
    // which behaves as if string(size_type count, charT ch) is called
    std::string s(3, std::toupper('a'));
    std::cout << s << '\n'; // "AAA"
  }
}

Output:

====
Exemplar
C-style
C-style
C-style string
Exemplar
C++ by example
C-style
AAA

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)