我应该如何在 C++ 中重载运算符 +

Question

I should be overloading the operator + for adding two squads that will return as a result a new squad with the number of members that is the sum of the number of members of both squads, and other attributes are taken from the squad with larger number of members. I am trying it but the result does not make any sense! And I don't know whether the mistake is in the part where I am overloading the + operator, or it is in the function that shows which tour has most members?!

Here is what all the exercise says: "Mountain squad Problem 2 (0 / 30)

Write a class for mountain squad, that keeps information for the name of the squad (dynamically allocated array of chars), number of tours (integer) and number of members (integer). For this class implement:

•operator + for adding two squads that will return as a result a new squad with number of members that is sum of the number of members of both squads, and other attributes are taken from the squad with larger number of members.

•operators >, < for comparison by the number of members

•operator << for printing the info on SO.

Write a function that accepts array of mountain squads and the size of the array and prints the squad with max number of members. "

#include <iostream>
#include <string.h>
using namespace std;

class MSquad{
private:
char *name;
int tours;
int members;
void copy(const MSquad &toCopy){
    name = new char[strlen(toCopy.name) + 1];
    strcpy(name, toCopy.name);
    tours = toCopy.tours;
    members = toCopy.members;
}


public:
MSquad(char *n = "unknown", int nT = 0, int nM = 0){
    name = new char[strlen(n + 1)];
    strcpy(name, n);
    tours = nT;
    members = nM;
}

MSquad(const MSquad &toCopy){
    copy(toCopy);   
}
~MSquad(){
    delete [] name;
}

const MSquad &operator=(const MSquad &right){
    if(&right != this){ // avoiding self- assignment
        delete [] name;
        copy(right);
    }

    return *this;
}

MSquad &operator+(const MSquad &right) const{

    members = members + right.members;
    if(right.members > members){
        name = new char[strlen(right.name) + 1];
        strcpy(name, right.name);
        tours = right.tours;
        //members = members + right.members;
    }
    return *this;

}

bool operator>(const MSquad &right){
    return members > right.members;
}

bool operator<(const MSquad &right){
    return members < right.members;
}

friend ostream &operator<<(ostream &output, const MSquad &right);
friend void mostMembers(MSquad *squads, int size);


};

ostream &operator<<(ostream &output, const MSquad &right){
output << "Name: " << right.name;
output << " Tours: " << right.tours;
output << " Members: " << right.members << endl;
return output;
}
void mostMembers(MSquad squads[], int size){
int max = squads[0].members;
int j = 0;
for(int i = 1; i < size; i++){
    if(squads[i].members >= max){
        max = squads[i].members;
        j = i;
    }
}

cout << "The max number of members is in squad in: " << squads[j] << endl;
}


int main()
{               
MSquad squads[3];
MSquad s;
for (int i=0;i<3;i++)
{
    char name[100];
    int tours;
    int members;
    cin>>name;
    cin>>tours;
    cin>>members;       
    squads[i] = MSquad(name, tours, members);

}

s = squads[0] + squads[1];
cout<<s;

mostMembers(squads, 3);

return 0;
}

Answer 1

The problem is that you're not understanding what operator + is supposed to return. According to your assignment:

•operator + for adding two squads that will return as a result a new squad with number of members that is sum of the number of members of both squads,

In your code, you're returning a MSquad& for operator + . This is not correct, as you should be returning a new MSquad object that is made up of the current object and the passed-in object.

However, all is not lost, and we can keep the code you have now. What's saved us is that you've written a user-defined copy constructor and destructor, so we can apply an easy fix.

The way we can fix this is to do the following (this was not in your requirements, but let's use the code you do have now):

MSquad &operator+=(const MSquad &right) const
{
    members = members + right.members;
    if(right.members > members){
        name = new char[strlen(right.name) + 1];
        strcpy(name, right.name);
        tours = right.tours;
        //members = members + right.members;
    }
    return *this;
}

MSquad operator+(const MSquad &right) const
{
   MSquad temp(*this); // create a temporary copy of the current object 
   return temp += right;  // use += above on passed-in object and return object.
}

We've taken your existing code, made it operator+= , and just created an operator + using this function. So we killed two birds with one stone. We created an operator += , which is supposed to return a reference to the current object, and wrote operator + using operator += as a helper function. Also, note that we created the temporary of the current object by taking advantage of the copy constructor.

In general, when you overload the operators such as + it is a better practice to overload op= first, and call it from operator op .

---

Please note that there are many other issues with your code involving mismanagement of pointers and memory leaks. For example, the operator + function (now operator += in the answer) has a memory leak in that you didn't delete [] the previous name allocation. However I leave it up to you to fix these issues.

Answer 2

Did you try going thru with the debugger?

Or, better yet - solve your problem on paper - then translate it into the code.