std::basic_...basic_string(3)

💖 NAME 💖

std::basic_string::basic_string — yep, the constructor itself, aka std::basic_string::basic_string. This page is from the C++ Standard Libary manual as std::basic_...basic_string(3) (we are keeping the manual name exactly as it appears, bestie 💅).

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✨ Synopsis ✨

okay so here’s the whole constructor buffet. Deep breath, babe — every overload is here, no skips, no cap 🫶:

basic_string();
// (1) (until C++17)

basic_string() noexcept(noexcept(Allocator()))
    : basic_string( Allocator() ) {}
// (1) (since C++17) (constexpr since C++20) (noexcept since C++17)

explicit basic_string( const Allocator& alloc );
// (2) (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 energy. Constructs an 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. Also, 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 [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()). 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 [s, s + count) is not a valid range. That’s undefined behavior, bestie — do not play with it.
• (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 [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)
• (10) — Constructs the string with the contents of the range [first, last). 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 [pos, pos + 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.

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🌸 Parameters 🌸

line up, bestie — here’s what all the little names mean 💖:

• 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

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🔥 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.

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💀 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).

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🎀 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

• __cpp_lib_containers_ranges — 202202L — (C++23) — Tagged constructor (19) to construct from container compatible range

✨ EXAMPLE ✨

okay bestie, this is the part where we actually run the thing and watch every single constructor do its lil moment on stage 💅 the whole main() is literally a vibe check for each overload, numbered 1) through c) like a tracklist. no cap, run it yourself.

// 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 << "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++"
}

quick notes before you run off and compile, bestie 🫶

• line 1) prints s1.capacity() and it is unspecified — so your number is not gonna match mine and that is fine, it's literally in the comments.
• the commented-out std::string sc(nullptr); line: before C++23 it throws std::logic_error, and since C++23 it won't compile at all, see overload (18). 💀
• the commented-out std::string sc(0); line: same deal, because literal 0 is a null pointer constant.
• the #ifdef __cpp_lib_containers_ranges dance picks the tagged constructor (19) when ranges are available, otherwise it falls back to overload (12).

🎬 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++"

🌸 reminder that the s1.capacity() value here (15) is just one possible output — it's unspecified, so don't come for me if yours says something else.

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📜 DEFECT REPORTS

these are the glow-up receipts ✨ the following behavior-changing defect reports were applied retroactively to previously published C++ standards — meaning the standard got a patch note and the old behavior was, respectfully, not it.

• LWG 301, applied to C++98 — behavior as published: overload (10) did not use the parameter alloc if InputIt is an integral type → correct behavior: use that parameter 💅
• LWG 847, applied to C++98 — behavior as published: there was no exception safety guarantee → correct behavior: added strong exception safety guarantee 🫶
• LWG 2193, applied to C++11 — behavior as published: the default constructor is explicit → correct behavior: made non-explicit
• LWG 2583, applied to C++98 — behavior as published: there is no way to supply an allocator for basic_string(str, pos) → correct behavior: there is a constructor for basic_string(str, pos, alloc) 🎀
• LWG 2946, applied to C++17 — behavior as published: overload (16) causes ambiguity in some cases → correct behavior: avoided by making it a template
• LWG 3076, applied to C++17 — behavior as published: two constructors may cause ambiguities in class template argument deduction → correct behavior: constrained ⭐

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👀 SEE ALSO

the whole friend group of related members, go say hi:

• 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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and that's the whole page, bestie, we ate and left zero crumbs 🍓 this lil reference card lives at http://cppreference.com, it's the 2024.06.10 edition of the std::basic_...basic_string(3) page, and yes the title is literally abbreviated like that — iconic, no notes 💜 go construct something gorgeous fr fr 🦋