Question

I have a problem with overloading of the < operator.
I have this class:

WordEntry.h:

class WordEntry
{
public:
    WordEntry(string word);
    ~WordEntry();

    bool operator<(const WordEntry otherWordEntry);

    string getWord();

private:
    string _word;
};

WordEntry.cpp(I removed constructor & destructor):

string WordEntry::getWord()
{
   return _word;
}


bool WordEntry::operator<(WordEntry otherWordEntry)
{
   return  lexicographical_compare(_word.begin(),_word.end(),otherWordEntry.getWord().begin(),otherWordEntry.getWord().end());
}

Everything is fine when I’m using it in main.cpp like that:

    WordEntry w1("Der");
    WordEntry w2("das");

    if (w1.operator<(w2)) {
       cout << "w1 > w2";
    }
    else 
    {
       cout << "w2 > w1";
    }

But when I call sort() on a vector with WordEntry objects, I’ll get the error message

Invalid operands to binary expression (‘const WordEntry’ and ‘const
WordEntry’)

and it points to stl_algo.h.

Does anyone knows what’s going on here?

Answer 1

Right now the argument to < is const but the member is not. This means a < comparison between 2 const WordEntry& objects will fail because it can’t bind to <. You need to make the member and the argument both const

bool operator<(const WordEntry& otherWordEntry) const;

bool WordEntry::operator<(const WordEntry& otherWordEntry) const {
  ...
}

Note: As pointed out in the comments you should also pass WordEntry by reference