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std::basic_string::basic_string

(1) 
explicit basic_string( const Allocator& alloc = Allocator() );
 (until C++14) 
basic_string() : basic_string( Allocator() ) {}
explicit basic_string( const Allocator& alloc );
 (since C++14)
(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& a = 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> init, 
              const Allocator& alloc = Allocator() );
 (9) (since C++11) 
explicit basic_string( std::basic_string_view<CharT, Traits> sv,
                       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).
2) Constructs the string with count copies of character ch. The behavior is undefined if count >= npos.
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, 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 does not point at an array of at least count elements of CharT, including the case when s is a null pointer.
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 does not point at an array of at least Traits::length(s)+1 elements of CharT, including the case when s is a null pointer.
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).
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 init.
10) Constructs the string with the contents of the string view sv, as if by basic_string(sv.data(), sv.size(), alloc)
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
init - std::initializer_list to initialize the string with
sv - std::basic_string_view 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 init

Exceptions

3) std::out_of_range if pos > other.size()
8) Throws nothing if alloc == str.get_allocator()

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

#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 characters to a string
(public member function)
assigns values to the string
(public member function)

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