Using vector with derived classes

Question

I'm trying to use a vector for derived class Employee but I got en error:

class Company : public Employee, public TesterEmployee, public DeveloperEmployee {
private:
   std::vector<Employee*> _table;
public:
    friend std::vector<Employee*>& operator+=(const Employee* emp) {
        _table.push_back(emp->clone());
        return *this;
    }
};

The error is:

error: 'std::vector<Employee*>& operator+=(const Employee*)' must have an argument of class or enumerated type

Answer 1

+= is a binary operator. The function

friend std::vector<Employee*>& operator+=(const Employee* emp) {
    _table.push_back(emp->clone());
    return *this;
}

defines a non-member function that can take only one argument. Hence, it cannot be used in an expression like:

a += b;

What you need is:

Company& operator+=(const Employee* emp) {
    _table.push_back(emp->clone());
    return *this;
}

Answer 2

In response to the other question about the same code (that you deleted https://stackoverflow.com/questions/30717107/operator-overloading-in-my-class-with-error ):

Q and the problem :

 error: invalid operands of types 'Company*' and 'DeveloperEmployee*' to binary 'operator+'| 

What does it mean?

It means you're trying to write Java/C# code in C++. Don't do that.

• In particular you're abusing OOP for the sake of OOP. A Company is-not-a Employee (see Liskov ), and company + employee doesn't make sense.

• Lose the new .

• Lose the base class.

• Probably lose the clone based dynamic value semantics.

Finally, your problem is, you're using operator+= which you didn't overload.

Live On Coliru

#include <vector>
#include <iostream>

struct Employee {
    int getID() const { return _id; }

  private:
    int _id = generate_id();

    static int generate_id() {
        static int next = 0;
        return ++next;
    }
};
struct DeveloperEmployee : Employee {
    DeveloperEmployee(std::string const& descr, std::string const& project)
        : _description(descr), _project(project) { }

  private:
    std::string _description;
    std::string _project;

    friend std::ostream& operator<<(std::ostream& os, DeveloperEmployee const& de) {
        return os << "id:" << de.getID() << " descr:'" << de._description << "' project:'" << de._project << "'"; 
    }
};

class Company {
  private:
    std::vector<Employee> _table;
    std::string _des;

  public:
    Company &operator+=(const Employee &emp) {
        _table.push_back(emp);
        return *this;
    }
    void setDescription(std::string des) { _des = des; }
};


int main() {
    Company company;
    DeveloperEmployee a("description", "project");

    int id = a.getID();
    std::cout << a << std::endl; // Developer ID = 2, project = hw5

    company += a;
}