What line is causing the 'operator==' error

Question

This program allows for a user to add an items storage location and search for an item by name to find it later. I am getting a weird error when I try and run my code. Everything was working fine until I added in a search function for the vector. Any ideas on what is going wrong since I don't see a line item issue?

The red section of GDB error report says this:

/tmp/ccngGZSU.o: In function `bool __gnu_cxx::__ops::_Iter_equals_val<item const>::operator()<__gnu_cxx::__normal_iterator<item*, std::vector<item, std::allocator<item> > > >(__gnu_cxx::__normal_iterator<item*, std::vector<item, std::allocator<item> > >)':
main.cpp:(.text._ZN9__gnu_cxx5__ops16_Iter_equals_valIK4itemEclINS_17__normal_iteratorIPS2_St6vectorIS2_SaIS2_EEEEEEbT_[_ZN9__gnu_cxx5__ops16_Iter_equals_valIK4itemEclINS_17__normal_iteratorIPS2_St6vectorIS2_SaIS2_EEEEEEbT_]+0x2b): undefined reference to `operator==(item const&, item const&)'
collect2: error: ld returned 1 exit status

The code is only 150 lines long, so 199:17 doesn't exist as far as I know.

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

struct item {
  string name;

  string color;

  string size;

  string location;
};

void additem()

{
  int num;

  cout << "How any items do you wish to add? " << endl;
  cin >> num;

  vector<item> items(num);

  item hold;

  for (vector<item>::size_type i = 0; i < items.size(); i++) {
    cout << "What is the item name? " << endl;
    cin >> hold.name;

    cout << "What is the item color? " << endl;
    cin >> hold.color;

    cout << "What is the item size? " << endl;
    cin >> hold.size;

    cout << "What is the item location? " << endl;
    cin >> hold.location;

    items[i] = hold;
  }
}

void search()

{
  item s;

  typedef vector<item> itemsearch;
  typedef itemsearch::iterator itemiterator;

  itemsearch items;

  cout << "Enter the item name to search for. " << endl;
  cin >> s.name;
  cout << "Enter the item color to search for. " << endl;
  cin >> s.color;
  cout << "Enter the item size to search for. " << endl;
  cin >> s.size;

  itemiterator i;
  i = find(items.begin(), items.end(), s);

  if (i != items.end()) {
    cout << "Item Found!" << endl;
    cout << "The location is . . ." << endl;
    cout << s.location << endl;
  } else {
    cout << "Item Not Found" << endl;
  }
}

void menu()

{
  int menu;

  while (menu != 3) {
    cout << "Please select an option from the menu to begin." << endl;

    cout << "1. Add Item/s " << endl;
    cout << "2. Search for Item " << endl;
    cout << "3. Exit Program " << endl;

    cin >> menu;

    if (menu == 1)

    {
      cout << "You selected to add an item." << endl;
      additem();
    }

    if (menu == 2)

    {
      cout << "You selected to search for an item." << endl;
      search();
    }

    if (menu == 3)

    {
      cout << "Exiting Prgram." << endl;
      cout << ". . . . ." << endl;

      exit;
    }
  }
}

int main()

{
  cout << "Welcome to Brandon's Box" << endl;
  cout << "This program will allow you to add details about an item and where "
          "it is stored."
       << endl;

  menu();
}

Answer 1

Error messages for the algorithm library are indeed terrible. I hope with concepts we get better error messages for the standard library in the future.

Let's look at the documentation of std::find. There it says that the signature of std::find is

template <class InputIterator, class T>
   InputIterator find (InputIterator first, InputIterator last, const T& val);

with first and last being input iterators and val being the value you want to search for. The important part in the documentation is that val must be a type supporting comparisons with the elements pointed by InputIterator using operator== . Your struct item does support that. Add the code below as a free function (after struct item definition) and the code will compile.

bool operator==(const item& first, const item& second) {
    // Implement-me as appropriate
    return false;
}

When you finish the implementation of bool operator==(const item& first, const item& second) then std::find will do what you want.

---

And as @drescherjm commented, you probably want to change menu = number in the ifs to menu == number.