deductiongu...basic_string(3)

💅 deductiongu...basic_string(3) 💅

okay so this page is from the C++ Standard Libary manual, section (3), and it's literally all about deductionguidesforstd::basic_string ✨ no cap, we're doing deduction guides today bestie.

---

🎀 NAME 🎀

deductionguidesforstd::basic_string — deductionguidesforstd::basic_string

yes bestie, it's the same string twice, that's the whole name 💀

---

✨ SYNOPSIS ✨

          Defined in header <string>
          template< class InputIt, class Alloc = std::allocator<

                                       typename
          std::iterator_traits<InputIt>::value_type> >
          basic_string( InputIt, InputIt, Alloc = Alloc() )
              -> basic_string<typename
       (1) (since C++17)
          std::iterator_traits<InputIt>::value_type,
                              std::char_traits<

                                  typename
          std::iterator_traits<InputIt>::value_type>, Alloc>;
          template< class CharT,

                    class Traits,
                    class Alloc = std::allocator<CharT> >
       (2) (since C++17)
          explicit basic_string( std::basic_string_view<CharT, Traits>,
          const Alloc& = Alloc() )

              -> basic_string<CharT, Traits, Alloc>;
          template< class CharT,

                    class Traits,
                    class Alloc = std::allocator<CharT>> >
          basic_string( std::basic_string_view<CharT, Traits>,
       (3) (since C++17)
                        typename /* see below */::size_type,
                        typename /* see below */::size_type,
                        const Alloc& = Alloc() )

              -> basic_string<CharT, Traits, Alloc>;
          template< ranges::input_range R,

                    class Alloc = std::allocator<ranges::range_value_t<R>> >
          basic_string( std::from_range_t, R&&, Alloc = Alloc() )
       (4) (since C++23)
              -> basic_string<ranges::range_value_t<R>,

                                 std::char_traits<ranges::range_value_t<R>>,
          Alloc>;

• (1) — this guide exists so std::basic_string can deduce from an iterator range. It only participates in overload resolution if InputIt satisfies LegacyInputIterator and Alloc satisfies Allocator.
• (2,3) — these guides exist so std::basic_string can deduce from a std::basic_string_view. The size_type parameter type in (3) refers to the size_type member type of the type deduced by the deduction guide. These overloads participate in overload resolution only if Alloc satisfies Allocator.
• (4) — this guide exists so std::basic_string can deduce from a std::from_range_t tag and an input_range.

Note: the extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum integral types do not qualify as input iterators. Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum the member type Alloc::value_type must exist and the expression std::declval<Alloc&>().allocate(std::size_t{}) must be well-formed when treated as an unevaluated operand. 🫶

---

📝 NOTES 📝

Guides (2,3) are needed because the std::basic_string constructors for std::basic_string_views are made templates to avoid causing ambiguities in existing code, and those templates do not support class template argument deduction.

---

📝 NOTES 📝 (feature-test macro edition)

• __cpp_lib_containers_ranges — value 202202L, Std (C++23), feature: Ranges-aware construction and insertion; overload (4).

---

🍓 EXAMPLE 🍓

       // Run this code

        #include <cassert>
        #include <string>
        #include <vector>

        int main()
        {
            std::vector<char> v = {'a', 'b', 'c'};
            std::basic_string s1(v.begin(), v.end()); // uses deduction guide
       (1)
            assert(s1 == "abc");

        #if __cpp_lib_containers_ranges >= 202202L
            std::vector<wchar_t> v4{0x43, 43, 053, 0x32, 0x33};
            std::basic_string s4(std::from_range, v4); // uses deduction guide
       (4)
            assert(s4 == L"C++23");
        #endif
        }

---

🕵️ DEFECT REPORTS 🕵️

The following behavior-changing defect reports were applied retroactively to previously published C++ standards:

• DR: LWG 3075 — Applied to: C++17 — Behavior as published: deduction from basic_string_view was unsupported (exacerbated by LWG issue 2946) — Correct behavior: deduction guides added.

---

okay that's the whole page bestie 💖 this came from http://cppreference.com, dated 2024.06.10, and the footer title is deductiongu...basic_string(3) — slay 🦋