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

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< cpp‎ | string‎ | basic string

| (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() is true.

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 2235 | C++98 | s could be a null pointer value | the behavior is undefined in this case
LWG 2250 | C++98 | the behavior of overload (17) was undefined if pos > other.size() is true | always throws an exception in this case
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
LWG 3111 (P1148R0) | C++98 | the resolution of LWG issue 2235 made the behavior of basic_string(nullptr, 0) undefined | the behavior is well- defined in this case[1]

1. ↑ `[`nullptr`,` nullptr + 0`)` is a valid empty range, because adding zero to a null pointer value is also well-defined (the result is still a null pointer value).

### 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>`)

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