std::set error: no match for »operator<« [duplicate]

Question

I have a class (extract):

class Package
{
private:
        string s_package_name;
        float f_boottime;
        float f_bytesize;
        list <Package> l_depends;
        list <Package> l_selects;
}

I'd like to generate many objects and list them in a unique "array", therefore i took set I add the objects like this:

set<Package> l_tempSet;   
Package PackageA(1, 11, "what a package");
Package PackageB(2, 22, "what a 2nd package");

l_tempSet.insert(PackageA);
l_tempSet.insert(PackageB);

When compiling I receive an error Message:

no match for »operator<« (operand types are »const Package« and »const Package«)

When clicking into the Error Message it points me to the set.h to the line where unique is called and I think this is the error.

Is it true that C++ isn't capable to "unique" objects (like in this example) into lists and sets and can just handle easy data types like int, float etc. ? Or did I went wrong somewhere, please help me I'm not sure where the error is exactly.

Thanks for your suppport

Answer 1

The problem is, that a c++ std::set is orderd, therefore it needs to sort the elements by evaluating the operator<.

Basically there are three solutions to your problem:

1. Implement the operator< for your class (see more here)

    bool operator<(const Package& other) const
    {
        //return true if this < other
    }
   

2. Use an unsorted set like std::unordered_set, but then you'll have to implement the equality operator and a hash function.

3. Implement a comparator function and pass it to the set as second template parameter:

    bool smaller (const Package& left, const Package& right) 
    {
        //return true if left < right;
    }
    std::set<Package,std::function<bool(const Package&, const Package&)>> newSet (std::function<bool(const Package&, const Package&)>(smaller));
   

Answer 2

You have to define operator< for your class. Otherwise, set cannot compare objects it's supposed to store, thus cannot check their uniqueness.

Answer 3

When you use std::set, internal implementation may use operator < to arrange objects into buckets, RBTree etc.(Because std::set is ordered) So it needs operator <. Compiler doesn't provide comparision operator by default, so you need to implement 1 by yourself.

class Package
{
private:
        string s_package_name;
        float f_boottime;
        float f_bytesize;
        list <Package> l_depends;
        list <Package> l_selects;
public:
        bool operator <(const Package &t) {
          /* your logic */
        }
}