Zroach Zroach - 1 year ago 118
C++ Question

Failure on initialization of unordered map

and thanks for any input. I have a large dataset I am trying to manipulate. I am holding active elements in a list, and removing them when they become inactive. I want to hold all elements active and inactive in some data structure. Currently trying a map or an unordered_map, but am welcome to any suggestions.

I am compiling with

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

When trying map:

#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

output reads:

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

Even though I have overloaded the less than operator for class1.
This error originates at the overloaded bracket operators for map.
To circumvent I tried storing in an unordered_map.

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

and the program fails at the initialization of fullMap with the even more confusing:

error: implicit instantiation of undefined template
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,

note: in instantiation of template class
'std::__detail::_Hashtable_base >, std::__detail::_Select1st, std::equal_to, std::hash,
std::__detail::_Hashtable_traits >' requested here
: public __detail::_Hashtable_base<_Key, _Value, _ExtractKey, _Equal,

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::hash, std::equal_to,
std::allocator > > >' requested here std::unordered_map fullMap;

note: template is declared here
struct hash;

I tried to cut the code down only to the relevant chunks, but let me know if more information is needed. Thanks for reading, any help is appreciated.

// 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
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;

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 */

The following is my overloaded less than operator (I know it isn't the most succinct or efficient):

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 Source

The issue is that std::map<key_type, value_type> requires a properly defined operator< for key_type, in this case your operator< is not const specified so it is incompatible with std::map as this data structure requires that the comparator not alter the key object in any way. Thus the solution is to mark class1::operator< as const.

The second error notes that no hash function-object has been applied for use with std::unordered_map, this would require the following framework:

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;
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