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

From cppreference.com
< cpp‎ | string‎ | basic string

| (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)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/string/basic_string/basic_string&oldid=110321"