Operator Overloading question

[Sira Raven]

For whatever reason, I'm having issues overloading the << operator. Here's what I have - any help would be appreciated.

//Set.h
class Set{
.
.
.
}
ostream operator<<(ostream& out, const Set& s);
 
//Set.cpp
#include "Set.h"
.
.
.
void Set::print(ostream &out){
//sends 'out' a nicely formatted version of the set
}
.
.
.
ostream operator<< (ostream& out, const Set& s){
    s.print(out);
    return out;
}
 

Whenever I try to compile this, I get an error saying
"error C2662: 'Set::print' : cannot convert 'this' pointer from 'const Set' to 'Set &'"

I'm unsure what to do about this - any advice?

[dmjpro]

I cant' figure out how to delete this....

Where is your problem?

Kind regards,
Dmjpro.

[Sira Raven]

Where is your problem?

Kind regards,
Dmjpro.

Oh - sorry, I've posted my problem now.

Any suggestions??

[dmjpro]

For whatever reason, I'm having issues overloading the << operator. Here's what I have - any help would be appreciated.

//Set.h
class Set{
.
.
.
}
ostream operator<<(ostream& out, const Set& s);
 
//Set.cpp
#include "Set.h"
.
.
.
void Set::print(ostream &out){
//sends 'out' a nicely formatted version of the set
}
.
.
.
ostream operator<< (ostream& out, const Set& s){
    s.print(out);
    return out;
}
 

Whenever I try to compile this, I get an error saying
"error C2662: 'Set::print' : cannot convert 'this' pointer from 'const Set' to 'Set &'"

I'm unsure what to do about this - any advice?

Using code tags, post whole code.

Kind regards,
Dmjpro.

[Sira Raven]

Using code tags, post whole code.

Kind regards,
Dmjpro.

Alright:

//SetGE.h

#include<iostream>
using namespace std;
 
class SetGE {
public:
    SetGE();
    void add(int);
    void add(int[], int);
    void remove(int);
    bool elementOf(int) const;
    SetGE setUnion(const SetGE&) const;
    SetGE intersection(const SetGE&) const;
    SetGE difference(const SetGE&) const;
    void makeEmpty();
    bool isEmpty();
    bool subset(const SetGE&) const;
    bool equals(const SetGE&) const;
    void print(ostream&) const;
 
private:
    int set[100];
    int size;
};
 
SetGE operator+(const SetGE& lhs, const SetGE& rhs);
SetGE operator-(const SetGE& lhs, const SetGE& rhs);
bool operator<=(const SetGE& lhs, const SetGE& rhs);
bool operator==(const SetGE& lhs, const SetGE& rhs);
ostream operator<<(ostream& out, const SetGE& s);
 

//SetGE.cpp

#include "SetGE.h"
#include <ostream>
 
SetGE::SetGE(){
    size = 0;
}
 
void SetGE::add(int i) {
    if (!elementOf(i)){//only adds if that value isn't already in the set
        set[size] = i;
        size++;
    }
}
 
void SetGE::add(int a[], int n){
    for (int i=0; i<n; i++){
        add(a[i]);
    }
}
 
void SetGE::remove(int i){
    int index = -1;
    for (int j=0; j<size; j++){
        if (set[j] == i){
            index = j;
            break;
        }
    }
    if (index != -1){ //if i was found in the set
        for (int j=index+1; j<size; j++)
            set[j-1] = set[j];
        size--;
    }
}
 
bool SetGE::elementOf(int i) const{
    for (int j=0; j<size; j++)
        if (set[j] == i)
            return true;
    return false;
}
 
SetGE SetGE::setUnion(const SetGE& s2) const{
    SetGE result;
    for (int i=0; i<size; i++)
        result.add(set[i]);
    for (int i=0; i<s2.size; i++)
        result.add(s2.set[i]);
    return result;
}
 
SetGE SetGE::intersection(const SetGE& s2) const{
    SetGE result;
    for (int i=0; i<size; i++){
        for (int j=0; j<s2.size; j++){
            if (set[i] == s2.set[j])
                result.add(set[i]);
        }
    }
    return result;
}
 
SetGE SetGE::difference(const SetGE& s2) const{
    SetGE result;
    for (int i=0; i<size; i++)
        result.add(set[i]);
    for (int i=0; i<s2.size; i++)
        result.remove(s2.set[i]);
    return result;
}
 
void SetGE::makeEmpty(){
    size = 0;
}
 
bool SetGE::isEmpty(){
    return size==0;
}
 
bool SetGE::subset(const SetGE& s2) const{
    for (int i=0; i<s2.size; i++)
        if (!elementOf(s2.set[i]))
            return false;
    return true;
}
 
bool SetGE::equals(const SetGE& s2) const{
    if (subset(s2) && s2.size == size)//couldn't figure out how to get s2.subset(this) to work
        return true;
    return false;
}
 
void SetGE::print(ostream &out) const{
    out<<"{";
    for (int i=0; i<size-1; i++) //prints # followed by comma
        out<<set[i]<<",";
 
    if (size != 0)    //final # isn't followed by a comma
        out<<set[size-1]<<"}";
    else
        out<<"}";
}
 
SetGE operator+ (const SetGE& lhs, const SetGE& rhs){
    return lhs.setUnion(rhs);
}
 
SetGE operator- (const SetGE& lhs, const SetGE& rhs){
    return lhs.difference(rhs);
}
 
bool operator<= (const SetGE& lhs, const SetGE& rhs){
    return rhs.subset(lhs);
}
 
bool operator== (const SetGE& lhs, const SetGE& rhs){
    return lhs.equals(rhs);
}
 
ostream operator<< (ostream &out, const SetGE& s){
    s.print(out);
    return out;
}
 

[dmjpro]

Use Code Tags...It will be code=cpp

ostream operator<< (ostream &out, const SetGE& s){
//make it...ostream operator<< (ostream &out, SetGE& s)
    s.print(out);
    return out;
}
 

I think it will work.

Kind regards,
Dmjpro.

[Sira Raven]

Use Code Tags...It will be code=cpp

ostream operator<< (ostream &out, const SetGE& s){
//make it...ostream operator<< (ostream &out, SetGE& s)
    s.print(out);
    return out;
}
 

I think it will work.

Kind regards,
Dmjpro.

removing the const (as well as making print const) gives me the following crazy error:

1>c:\program files\microsoft visual studio 8\vc\include\ostream(587) : error C2248: 'std::basic_ios<_Elem,_Traits>::basic_ios' : cannot access private member declared in class 'std::basic_ios<_Elem,_Traits>'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>
1> ]
1> c:\program files\microsoft visual studio 8\vc\include\ios(151) : see declaration of 'std::basic_ios<_Elem,_Traits>::basic_ios'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>
1> ]
1> This diagnostic occurred in the compiler generated function 'std::basic_ostream<_Elem,_Traits>::basic_ostream( const std::basic_ostream<_Elem,_Traits> &)'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>
1> ]

And I have no idea what to do with that.

[RRick]

You have a couple of issues with this code. Let's first take a look at the object and then we'll take a look at overloading the operator<<.

It looks like you're dealing with 2 objects: Set and SetGE. Is SetGE supposed to be derived from Set or is it a stand alone object? I'm going to assume SetGE is standalone.

The second problem you have is that you don't declare operator<< as a friend and return by value instead of reference. This might be the reason for the private access error. You're code should look like:

friend ostream &  operator<<( ostream &, const SetGE &);

Finally, if you pass a const SetGE, then you have to make sure all the object methods called are also const.

[Sira Raven]

You have a couple of issues with this code. Let's first take a look at the object and then we'll take a look at overloading the operator<<.

It looks like you're dealing with 2 objects: Set and SetGE. Is SetGE supposed to be derived from Set or is it a stand alone object? I'm going to assume SetGE is standalone.

The second problem you have is that you don't declare operator<< as a friend and return by value instead of reference. This might be the reason for the private access error. You're code should look like:

friend ostream &  operator<<( ostream &, const SetGE &);

Finally, if you pass a const SetGE, then you have to make sure all the object methods called are also const.

Ah, that does seem to fix the problem - thanks! Now, though, I'd like to know a bit more 'bout why.

To answer your first question 'Set' on its own doesn't actually exist. SetGE is the name of the class - I just though 'Set' made more sense when I wasn't pasting the full code. Sorry 'bout that.

It actually seems to work even without declaring it as a friend. To get the code to work, just returning the ostream my reference did the trick. However, while I understand the importance of passing the ostream by reference, why do I need to return it by reference as well?

[weaknessforcats]

However, while I understand the importance of passing the ostream by reference, why do I need to return it by reference as well?

It is not required to return the ostream by reference. However, unless you return the ostream&, then code like this won't work:

SetGE obj;
cout << "The result is " << obj << endl;
 

As far as the friend goes, that is not required unless the function requires access to the private data variables.