STABLE_PARTITION problems: no matching function to call to "swap" - c++

Question

somehow I can't use stable_partition algorithm on
vector<pair<Class, string>>.
I can re-organize the code to get what I want, but for me (as I am new to C++) it's more "WHY" and not "HOW" question. will be glad if you clarify this behavior:
First, class Date (you can omit it and come look at it later):
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <set>
#include <vector>
using namespace std;
class Date {
public:
Date(int new_year, int new_month, int new_day) {
year = new_year; month = new_month; day = new_day;
}
int GetYear() const {return year;}
int GetMonth() const {return month;}
int GetDay() const {return day;}
private:
int year, month, day;
};
bool operator<(const Date& lhs, const Date& rhs) {
return vector<int>{lhs.GetYear(), lhs.GetMonth(), lhs.GetDay()} <
vector<int>{rhs.GetYear(), rhs.GetMonth(), rhs.GetDay()};
}
bool operator==(const Date& lhs, const Date& rhs) {
return vector<int>{lhs.GetYear(), lhs.GetMonth(), lhs.GetDay()} ==
vector<int>{rhs.GetYear(), rhs.GetMonth(), rhs.GetDay()};
}
SO THIS IS THE CLASS WITH THE TROUBLE:
class Database {
public:
void Add(const Date& date, const string event){
storage.push_back(make_pair(date, event));
set_dates.insert(date);
}
void Print(ostream& s) const{
for(const auto& date : set_dates) {
// TROUBLE IS HERE:
auto it = stable_partition(begin(storage), end(storage),
[date](const pair<Date, string> p){
return p.first == date;
});
};
}
private:
vector<pair<Date, string>> storage;
set<Date> set_dates;
};
When compiled, it returns a lot of problems of same kind:
I've tried the same code on vector<pair<int, string>> (used stable_partition with lambda {return p.first == _int; } and it worked.
Would appreciate your help

Answer 1

The std::stable_partition function is supposed to modify the vector. However, you are calling it in a const member function, so storage is const there. This can't work.
Solution: Don't make Print const, or use std::stable_partition on a copy of storage. Neither is a great solution, so you should probably rethink your design.

Answer 2

You need to define overloading operator= for Date class as well. It will work if you do that stuff.
class Date {
public:
Date(int new_year, int new_month, int new_day) {
year = new_year; month = new_month; day = new_day;
}
// Need to define overloading operator=
Date& operator=(const Date& rhs)
{
}
int GetYear() const {return year;}
int GetMonth() const {return month;}
int GetDay() const {return day;}
private:
int year, month, day;
};