Declaring priority_queue for user-defined class - c++

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.

Answer 2

you are missing const in operator overload
bool operator<(const Customer &other) const {
return this->price < other.price;
}
you can also use your own comparator instead of std::less. Here is how we can write it.
template<typename type>
struct mycomp {
bool operator()(const type & first, const type & second) const {
return first.price < second.price;
}
};
struct Customer {
int price;
};
int main(){
Customer c1{3};// last parameter is price
Customer c2{2};
priority_queue<Customer , vector<Customer> , mycomp<Customer> > barQ;
barQ.push(c2);
barQ.push(c1);
cout<<barQ.top().price;
return 0;
}
set::less<> doesn't gives an error with Customer* pointer because pointers comparisons can happen as int and it doesn't look for the custom implementation. which is not the case with Customer.