最終更新日時(UTC): 2023年10月14日 06時59分12秒
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function template
<string>

# std::operator<

    namespace std {
      // operator<=>により、以下の演算子が使用可能になる (C++20)
      template <class CharT, class Traits, class Allocator>
      bool
        operator<(const basic_string<CharT, Traits, Allocator>& a,
                  const basic_string<CharT, Traits, Allocator>& b);          // (1) C++03
      template <class CharT, class Traits, class Allocator>
      bool
        operator<(const basic_string<CharT, Traits, Allocator>& a,
                  const basic_string<CharT, Traits, Allocator>& b) noexcept; // (1) C++14
      template <class CharT, class Traits, class Allocator>
      constexpr bool
        operator<(const basic_string<CharT, Traits, Allocator>& a,
                  const basic_string<CharT, Traits, Allocator>& b) noexcept; // (1) C++20

      template <class CharT, class Traits, class Allocator>
      bool
        operator<(const CharT* a,
                  const basic_string<CharT, Traits, Allocator>& b);          // (2) C++03
      template <class CharT, class Traits, class Allocator>
      constexpr bool
        operator<(const CharT* a,
                  const basic_string<CharT, Traits, Allocator>& b);          // (2) C++20

      template <class CharT, class Traits, class Allocator>
      bool
        operator<(const basic_string<CharT, Traits, Allocator>& a,
                  const CharT* b);                                         // (3) C++03
      template <class CharT, class Traits, class Allocator>
      constexpr bool
        operator<(const basic_string<CharT, Traits, Allocator>& a,
                  const CharT* b);                                         // (3) C++20
    }

## 概要

`basic_string`において、左辺が右辺より小さいかの判定を行う。

## 戻り値

- (1) `a.compare(b) < 0`
- (2) `b.compare(a) > 0`
- (3) `a.compare(b) < 0`

## 例

    #include <iostream>
    #include <string>

    int main()
    {
      std::string a = "aaa";
      std::string b = "bbb";

      std::cout << std::boolalpha;
      std::cout << (a < b) << std::endl;
    }

### 出力

    true

## 参照

- LWG2064 - More `noexcept` issues in `basic_string`
- P0980R1 Making `std::string` constexpr
- P1614R2 The Mothership has Landed
  - C++20での三方比較演算子の追加と、関連する演算子の自動導出