Declaring priority_queue for user-defined class

Question

I have a class named Customer which is overloading the < operator:

bool Customer::operator<(const Customer &other) {
    return this->price < other.price;
}

but when I try to initialize the priority queue I get pages of errors.

Customer c1(10,5,12,30);// last parameter is price
Customer c2(10,5,12,2);
priority_queue<Customer , vector<Customer> , less<Customer> > barQ;
barQ.push(c2);
barQ.push(c1);
cout<<barQ.top().price;

Then, i accidentally found out that when i initialize it in following way:

Customer c1(10,5,12,30);
Customer c2(10,5,12,2);
priority_queue<Customer* , vector<Customer*> , less<Customer*> > barQ;
barQ.push(&c2);
barQ.push(&c1);
cout<<barQ.top()->price;

I got no errors and it works well. So my question is that what's the difference between Customer & Customer*? I thought it should work when I declare it with Customer, NOT Customer* , why it works otherwise?

Answer 1

The signature of std::less::operator() is (taken from cppreference):

bool operator()( const T& lhs, const T& rhs ) const;   // (until C++14)
constexpr bool operator()( const T& lhs, const T& rhs ) const; //(since C++14)

Note the it takes both parameters as const, thus it can only call a const operator<:

bool Customer::operator<(const Customer &other) const {  // <--- add const here
    return this->price < other.price;
}

Your second code is not really doing what you want it to do, because it uses the built-in operator< for pointers.