如何使 map::find 操作不区分大小写?

2022-01-07 00:00:00 dictionary string c++ case-insensitive stl

map::find 方法是否支持不区分大小写的搜索?我有一张地图如下:

Does the map::find method support case insensitive search? I have a map as follows:

map<string, vector<string> > directory;

并希望以下搜索忽略大小写:

and want the below search to ignore case:

directory.find(search_string);

推荐答案

默认情况下没有.您必须提供一个自定义比较器作为第三个参数.以下代码段将帮助您...

It does not by default. You will have to provide a custom comparator as a third argument. Following snippet will help you...

  /************************************************************************/
  /* Comparator for case-insensitive comparison in STL assos. containers  */
  /************************************************************************/
  struct ci_less : std::binary_function<std::string, std::string, bool>
  {
    // case-independent (ci) compare_less binary function
    struct nocase_compare : public std::binary_function<unsigned char,unsigned char,bool> 
    {
      bool operator() (const unsigned char& c1, const unsigned char& c2) const {
          return tolower (c1) < tolower (c2); 
      }
    };
    bool operator() (const std::string & s1, const std::string & s2) const {
      return std::lexicographical_compare 
        (s1.begin (), s1.end (),   // source range
        s2.begin (), s2.end (),   // dest range
        nocase_compare ());  // comparison
    }
  };

像 std::map< 一样使用它std::string, std::vector, ci_less >myMap;

注意:std::lexicographical_compare 有一些细节.如果您考虑语言环境，字符串比较并不总是那么简单.如果有兴趣，请参阅 clc++ 上的 this 线程.

NOTE: std::lexicographical_compare has some nitty-gritty details. String comparison isn't always straightforward if you consider locales. See this thread on c.l.c++ if interested.

更新:在 C++11 中 std::binary_function 已被弃用，因为类型是自动推导出来的，所以没有必要.

UPDATE: With C++11 std::binary_function is deprecated and is unnecessary as the types are deduced automatically.

  struct ci_less
  {
    // case-independent (ci) compare_less binary function
    struct nocase_compare
    {
      bool operator() (const unsigned char& c1, const unsigned char& c2) const {
          return tolower (c1) < tolower (c2); 
      }
    };
    bool operator() (const std::string & s1, const std::string & s2) const {
      return std::lexicographical_compare 
        (s1.begin (), s1.end (),   // source range
        s2.begin (), s2.end (),   // dest range
        nocase_compare ());  // comparison
    }
  };

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