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

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< cpp‎ | string‎ | basic string
 
 
 
std::basic_string
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basic_string::basic_string
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(1)
basic_string();
(until C++17)
basic_string() noexcept(noexcept(Allocator()))
    : basic_string( Allocator() ) {}
(since C++17)
(constexpr since C++20)
explicit basic_string( const Allocator& alloc );
(2) (noexcept since C++17)
(constexpr since C++20)
basic_string( size_type count, CharT ch,
              const Allocator& alloc = Allocator() );
(3) (constexpr since C++20)
basic_string( const basic_string& other, size_type pos,
              const Allocator& alloc = Allocator() );
(4) (constexpr since C++20)
constexpr basic_string( basic_string&& other, size_type pos,
                        const Allocator& alloc = Allocator() );
(5) (since C++23)
basic_string( const basic_string& other,

              size_type pos, size_type count,

              const Allocator& alloc = Allocator() );
(6) (since C++23)
constexpr basic_string( basic_string&& other,

                        size_type pos, size_type count,

                        const Allocator& alloc = Allocator() );
(7) (since C++23)
basic_string( const CharT* s, size_type count,
              const Allocator& alloc = Allocator() );
(8) (constexpr since C++20)
basic_string( const CharT* s, const Allocator& alloc = Allocator() );
(9) (constexpr since C++20)
template< class InputIt >

basic_string( InputIt first, InputIt last,

              const Allocator& alloc = Allocator() );
(10) (constexpr since C++20)
basic_string( const basic_string& other );
(11) (constexpr since C++20)
basic_string( const basic_string& other, const Allocator& alloc );
(12) (since C++11)
(constexpr since C++20)
basic_string( basic_string&& other ) noexcept;
(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( std::initializer_list<CharT> ilist,
              const Allocator& alloc = Allocator() );
(15) (since C++11)
(constexpr since C++20)
template< class StringViewLike >

explicit basic_string( const StringViewLike& t,

                       const Allocator& alloc = Allocator() );
(16) (since C++17)
(constexpr since C++20)
template< class StringViewLike >

basic_string( const StringViewLike& t, size_type pos, size_type n,

              const Allocator& alloc = Allocator() );
(17) (since C++17)
(constexpr since C++20)
basic_string( std::nullptr_t ) = delete;
(18) (since C++23)
template< container-compatible-range<CharT> R >

constexpr basic_string( std::from_range_t, R&& rg,

                        const Allocator& = Allocator());
(19) (since C++23)

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

1,2) Default constructor. Constructs empty string (of zero size and unspecified capacity). If no allocator is supplied, allocator is obtained from a default-constructed instance.
3) 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)
4-7) Constructs the string with a substring [pospos + 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 [posother.size()). If other is an rvalue reference, it is left in a valid but unspecified state.(since C++23)
8) 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 [ss + count) is not a valid range.
9) 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 [ss + 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)
10) Constructs the string with the contents of the range [firstlast).

If InputIt is an integral type, equivalent to overload (2), as if by basic_string(static_cast<size_type>(first),
             static_cast<value_type>(last), alloc)
.

(until C++11)

This constructor only participates in overload resolution if InputIt satisfies LegacyInputIterator.

(since C++11)
11,12) Copy constructor. Constructs the string with a copy of the contents of other.
13,14) Move constructor. Constructs the string with the contents of other using move semantics. other is left in valid, but unspecified state.
15) Constructs the string with the contents of the initializer list ilist.
16) 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 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.
17) 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 [pospos + n) of sv 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.
18) std::basic_string cannot be constructed from nullptr.
19) Constructs the string with the values contained in the range rg.

Contents

[edit] 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

[edit] Complexity

1,2) Constant.
3-8) Linear in count.
9) Linear in length of s.
10) Linear in distance between first and last.
11,12) Linear in size of other.
13) Constant.
14) Constant. If alloc is given and alloc != other.get_allocator(), then linear.
15) Linear in size of ilist.
16) Linear in size of t.
17) Linear in size of assigned substring, i.e. std::min(std::size(t) - pos, n).
19) Linear in size of rg.

[edit] Exceptions

4-7) std::out_of_range if pos > other.size().
13) Throws nothing.
14) Throws nothing if alloc == str.get_allocator().
17) 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 (3)). Calls to Allocator::allocate may throw.

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

[edit] Notes

Initialization with a string literal that contains embedded '\0' characters uses the overload (9), 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 (19) to construct from container compatible range

[edit] Example

#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 << "2) string(size_type count, CharT ch): ";
    std::string s2(4, '=');
    std::cout << std::quoted(s2) << '\n'; // "===="
 
    std::cout << "3) string(const string& other, size_type pos, size_type count): ";
    std::string const other3("Exemplary");
    std::string s3(other3, 0, other3.length() - 1);
    std::cout << std::quoted(s3) << '\n'; // "Exemplar"
 
    std::cout << "4) string(const string& other, size_type pos): ";
    std::string const other4("Mutatis Mutandis");
    std::string s4(other4, 8);
    std::cout << std::quoted(s4) << '\n'; // "Mutandis", i.e. [8, 16)
 
    std::cout << "5) string(CharT const* s, size_type count): ";
    std::string s5("C-style string", 7);
    std::cout << std::quoted(s5) << '\n'; // "C-style", i.e. [0, 7)
 
    std::cout << "6) string(CharT const* s): ";
    std::string s6("C-style\0string");
    std::cout << std::quoted(s6) << '\n'; // "C-style"
 
    std::cout << "7) string(InputIt first, InputIt last): ";
    char mutable_c_str[] = "another C-style string";
    std::string s7(std::begin(mutable_c_str) + 8, std::end(mutable_c_str) - 1);
    std::cout << std::quoted(s7) << '\n'; // "C-style string"
 
    std::cout << "8) string(string&): ";
    std::string const other8("Exemplar");
    std::string s8(other8);
    std::cout << std::quoted(s8) << '\n'; // "Exemplar"
 
    std::cout << "9) string(string&&): ";
    std::string s9(std::string("C++ by ") + std::string("example"));
    std::cout << std::quoted(s9) << '\n'; // "C++ by example"
 
    std::cout << "a) string(std::initializer_list<CharT>): ";
    std::string sa({'C', '-', 's', 't', 'y', 'l', 'e'});
    std::cout << std::quoted(sa) << '\n'; // "C-style"
 
    // before C++11, overload resolution selects string(InputIt first, InputIt last)
    // [with InputIt = int] which behaves *as if* string(size_type count, CharT ch)
    // after C++11 the InputIt constructor is disabled for integral types and calls:
    std::cout << "b) string(size_type count, CharT ch) is called: ";
    std::string sb(3, std::toupper('a'));
    std::cout << std::quoted(sb) << '\n'; // "AAA"
 
//  std::string sc(nullptr); // Before C++23: throws std::logic_error
                             // Since C++23: won't compile, see overload (18)
//  std::string sc(0); // Same as above, as literal 0 is a null pointer constant
 
    auto const range = {0x43, 43, 43};
#ifdef __cpp_lib_containers_ranges
    std::string sc(std::from_range, range); // tagged constructor (19)
    std::cout << "c) string(std::from_range, range) is called: ";
#else
    std::string sc(range.begin(), range.end()); // fallback to overload (12)
    std::cout << "c) string(range.begin(), range.end()) is called: ";
#endif
    std::cout << std::quoted(sc) << '\n'; // "C++"
}

Possible output:

1) string(); s1.capacity(): 15
2) string(size_type count, CharT ch): "===="
3) string(const string& other, size_type pos, size_type count): "Exemplar"
4) string(const string& other, size_type pos): "Mutandis"
5) string(CharT const* s, size_type count): "C-style"
6) string(CharT const* s): "C-style"
7) string(InputIt first, InputIt last): "C-style string"
8) string(string&): "Exemplar"
9) string(string&&): "C++ by example"
a) string(std::initializer_list<CharT>): "C-style"
b) string(size_type count, CharT ch) is called: "AAA"
c) string(std::from_range, range) is called: "C++"

[edit] 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 (10) did not use the parameter
alloc if InputIt is an integral type
use that parameter
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 is no way to supply an allocator
for basic_string(str, pos)
there is a constructor for
basic_string(str, pos, alloc)
LWG 2946 C++17 overload (16) 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

[edit] See also

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