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最終更新日時: 2026年08月25日 12時11分17秒 (JST)
Akira Takahashi が更新

function template

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++98
  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++98
  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++98
  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オブジェクトの非等値比較を行う。

要件

(3) パラメータbが、Traits::length(b) + 1の要素数を持つCharT文字型の配列を指していること

戻り値

(1) !(a == b)
(2) b != a
(3) a.compare(b) != 0

例

#include <iostream>
#include <string>

int main()
{
  std::string a = "abc";
  std::string b = "abcd";

  if (a != b) {
    std::cout << "not equal" << std::endl;
  }
  else {
    std::cout << "equal" << std::endl;
  }
}

出力

not equal

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

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