c++ - 无序映射初始化失败

Question

感谢您的任何意见。我有一个大型数据集，我正在尝试操作。我将事件元素保存在列表中，并在它们变为非事件状态时将其删除。我想在某些数据结构中保持所有元素处于事件状态和非事件状态。目前正在尝试使用 map 或 unordered_map，但欢迎提出任何建议。

我正在编译

clang++ -std=c++11 -Wall -Wextra

尝试映射时:

#include <map>
std::map <class1, std::string> fullMap;
//and later...
for (std::list<class1>::iterator x = l.begin(); x != l.end(); x++)
{
    fullMap[(*x)] =  s
}

输出读取:

error: invalid operands to binary expression ('const class1' and 'const class1') { return __x < __y; }

尽管我已经为 class1 重载了小于运算符。此错误源于 map 的重载括号运算符。为了规避，我尝试存储在 unordered_map 中。

#include <unordered_map>
std::unordered_map <class1, std::string> fullMap;

程序在 fullMap 初始化时失败，更令人困惑:

/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.4/include/g++-v4/bits/hashtable_policy.h:830:23: error: implicit instantiation of undefined template 'std::hash' bool __use_ebo = !__is_final(_Tp) && __is_empty(_Tp)> ^

/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.4/include/g++-v4/bits/hashtable_policy.h:1073:15: note: in instantiation of default argument for '_Hashtable_ebo_helper<1, std::hash >' required here private _Hashtable_ebo_helper<1, _H1>, ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.4/include/g++-v4/bits/hashtable_policy.h:1403:12: note: in instantiation of template class 'std::__detail::_Hash_code_base >, std::__detail::_Select1st, std::hash, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, true>' requested here : public _Hash_code_base<_Key, _Value, _ExtractKey, _H1, _H2, _Hash, ^

/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.4/include/g++-v4/bits/hashtable.h:175:14: note: in instantiation of template class 'std::__detail::_Hashtable_base >, std::__detail::_Select1st, std::equal_to, std::hash, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Hashtable_traits >' requested here : public __detail::_Hashtable_base<_Key, _Value, _ExtractKey, _Equal, ^

/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.4/include/g++-v4/bits/unordered_map.h:100:18: note: in instantiation of template class 'std::_Hashtable >, std::allocator > >, std::__detail::_Select1st, std::equal_to, std::hash, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits >' requested here _Hashtable _M_h; ^

main.cpp:34:44: note: in instantiation of template class 'std::unordered_map, std::hash, std::equal_to, std::allocator > > >' requested here std::unordered_map fullMap; ^

/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.4/include/g++-v4/bits/functional_hash.h:58:12: note: template is declared here struct hash;

我试图只将代码缩减为相关的 block ，但如果需要更多信息，请告诉我。感谢阅读，我们将不胜感激。

//
//  class1.hpp
//  class
//
//  Created by Roach on 9/3/16.
//  Copyright © 2016 Roach. All rights reserved.
//

#ifndef class1_hpp
#define class1_hpp

#include <iostream>
#include <sstream>
#include <iomanip>
#include <ctime>


class class1
{
public:
  class1 ();
  class1 (const class1& t); // copy constructor
  ~class1 (); // destructor
  class1& operator = (const class1& t); // assignment operator
  bool operator == (const class1& t); // comparison operator
  void setSetting2 (std::string t);
  void setSetting1 (std::string p);
  void setSetting3 (double d);
  void setSetting4 (double d);
  std::tm getTime () const;
  std::string getSetting2 () const;
  double getSetting3 () const;
  double getSetting4 () const;
  std::string getSetting1 () const;
  void setSetting3End (double d);
  void setSetting4End (double d);
  double getSetting3End () const;
  double getSetting4End () const;
  double getSetting3flag () const;
  double getSetting4flag () const;
  double getSetting3final () const; // in pips
  double getSetting4final () const; // in pips
  void processList (class1::class1 t);
  void setNew ();
  //void dump (std::ostream& os) const;

private:
  std::string setting1;
  double setting4;
  double setting3;
  std::tm setting2;
  double setting4End_;
  double setting3End_;
  bool setting4Flag_;
  bool setting3Flag_;
  double setting4final_; // in pips
  double setting3final_; // in pips
};
// stream extraction operator
std::ostream& operator << (std::ostream& os, const class1& s);
std::istream& operator >> (std::istream& is, class1& t);

endif /* class1_hpp */

以下是我重载的 less than 运算符(我知道它不是最简洁或最有效的):

bool class1::operator< (const class1& t)
{
  if (this->time_.tm_year < t.time_.tm_year) {return true;}
  else if (this->time_.tm_year > t.time_.tm_year) {return false;}
  else if (this->time_.tm_mon < t.time_.tm_mon) {return true;}
  else if (this->time_.tm_mon > t.time_.tm_mon) {return false;}
  else if (this->time_.tm_mday < t.time_.tm_mday) {return true;}
  else if (this->time_.tm_mday > t.time_.tm_mday) {return false;}
  else if (this->time_.tm_hour < t.time_.tm_hour) {return true;}
  else if (this->time_.tm_hour > t.time_.tm_hour) {return false;}
  else if (this->time_.tm_min < t.time_.tm_min) {return true;}
  else if (this->time_.tm_min > t.time_.tm_min) {return false;}
  else if (this->time_.tm_sec < t.time_.tm_sec) {return true;}
  else {return false;}
}

Answer 1

问题是 std::map<key_type, value_type>需要正确定义的 operator<对于 key_type ，在这种情况下，您的 operator<不是 const已指定，因此它与 std::map 不兼容因为这个数据结构要求比较器不以任何方式改变关键对象。因此解决方案是标记class1::operator<作为const .

第二个错误指​​出没有应用哈希函数对象与 std::unordered_map 一起使用，这将需要以下框架:

auto class1_hasher = [](const class1& c) -> std::size_t { return {some hash based on c}; }
std::unordered_map<class1, std::string, decltype(class1_hasher)> um;