Programming Forums > Application Development > C++

JD-Salinger · Apr 30th, 2008, 5:59 AM

I just need some little explanation about this snippet, i did some little experiment in the class "value" and when i compiled the source file, error pops out, even though in my own logic, there is nothing wrong with it, i came with this book and im trying to learn copy constructors and operator overloading and i did some experimenting on the code:

This is the original code:

 c++ Syntax (Toggle Plain Text) 
#include  <iostream>
#include  <sstream>
 
using namespace std;
 
class Value
{
    public:
        Value()
        {
            cout<<" Default constructor\n";
            _numString=new char[1];
            _numString[0]= '\0';
        }
 
        Value(int i)
        {
            cout<< "Construction/conversion from int "<<i<<endl;
 
            stringstream buffer;
            buffer<< i <<ends; //terminate sring
            Init(buffer.str().c_str());
            Display();
        }
 
        Value(Value const& v)
        {
            cout<< " Copy constructor("
                << v._numString<< ")\n";
            Init(v._numString);
            Display();
        }
 
        Value& operator= (Value const& v)
        {
            cout<<"operator=("
                <<v._numString
                <<")\n";
 
            if(_numString != v._numString)
            {
                cout<<"not equal"<<endl;
                delete _numString;
                Init(v._numString);
            }
            Display();
            return *this;
        }
 
        friend Value operator+ (Value const& v1,Value const& v2)
        {
            cout<<"  operator+ ("<<v1._numString<< ", "
                <<v2._numString <<")\n";
 
            stringstream buffer;
            buffer<<v1._numString<< " + "<<v2._numString<<ends;
            Value result;
            result.Init(buffer.str().c_str());
            cout<<" Returning by value\n";
            return result;
        }
 
    private:
 
        void Init(char const* buf)
        {
            int len=strlen(buf);
            _numString=new char[len + 1];
            strcpy(_numString,buf);
        }
 
        void Display()
        {
            cout<< "\t" <<_numString<<endl;
        }
 
        char* _numString;
};
#include  <iostream>
#include  <sstream>

using namespace std;

class Value
{
    public:
        Value()
        {
            cout<<" Default constructor\n";
            _numString=new char[1];
            _numString[0]= '\0';
        }

        Value(int i)
        {
            cout<< "Construction/conversion from int "<<i<<endl;

            stringstream buffer;
            buffer<< i <<ends; //terminate sring
            Init(buffer.str().c_str());
            Display();
        }

        Value(Value const& v)
        {
            cout<< " Copy constructor("
                << v._numString<< ")\n";
            Init(v._numString);
            Display();
        }

        Value& operator= (Value const& v)
        {
            cout<<"operator=("
                <<v._numString
                <<")\n";

            if(_numString != v._numString)
            {
                cout<<"not equal"<<endl;
                delete _numString;
                Init(v._numString);
            }
            Display();
            return *this;
        }

        friend Value operator+ (Value const& v1,Value const& v2)
        {
            cout<<"  operator+ ("<<v1._numString<< ", "
                <<v2._numString <<")\n";

            stringstream buffer;
            buffer<<v1._numString<< " + "<<v2._numString<<ends;
            Value result;
            result.Init(buffer.str().c_str());
            cout<<" Returning by value\n";
            return result;
        }

    private:

        void Init(char const* buf)
        {
            int len=strlen(buf);
            _numString=new char[len + 1];
            strcpy(_numString,buf);
        }

        void Display()
        {
            cout<< "\t" <<_numString<<endl;
        }

        char* _numString;
};

now what I did is that on line 50 i replaced it to
inline Value operator+ (Value const& v1,Value const& v2)
then the error is "must take zero to one argument"

and i also replaced line 50 to
Value& operator+ (Value const& v1,Value const& v2)
and still same error....

im wondering what's the explanation behind this, i just copied line 34... Thanks

hammerhead · Apr 30th, 2008, 7:08 AM

I am not sure but you can try rewriting your code from line 50 as keep in mind to remove the friend keyword

 c++ Syntax (Toggle Plain Text) 
Value operator+ (Value const& v2)
        {
            cout<<"  operator+ ("<<_numString<< ", "
                <<v2._numString <<")\n";
 
            stringstream buffer;
            buffer<<_numString<< " + "<<v2._numString<<ends;
            Value result;
            result.Init(buffer.str().c_str());
            cout<<" Returning by value\n";
            return result;
        }
Value operator+ (Value const& v2)
        {
            cout<<"  operator+ ("<<_numString<< ", "
                <<v2._numString <<")\n";
 
            stringstream buffer;
            buffer<<_numString<< " + "<<v2._numString<<ends;
            Value result;
            result.Init(buffer.str().c_str());
            cout<<" Returning by value\n";
            return result;
        }

grumpy · Apr 30th, 2008, 7:16 AM

operator+() can be a member function of the class that accepts one argument, so "x = a + b" is effectively "x = a.operator+(b)" or a non-member, so "x = a + b" is effectively "x = operator+(a, b);".

The member function version can only accept one argument. The non-member version can only accept two.

The friend declaration within the class declares the non-member version as a friend of the class, which accepts two arguments. Removing the friend keyword makes the declaration into a member function, which is only allowed to accept one argument. However, your declaration has two .... hence the error.

The inline keyword is unrelated to your error.

JD-Salinger · May 1st, 2008, 12:08 AM

Quote:

Originally Posted by grumpy

operator+() can be a member function of the class that accepts one argument, so "x = a + b" is effectively "x = a.operator+(b)" or a non-member, so "x = a + b" is effectively "x = operator+(a, b);".

The member function version can only accept one argument. The non-member version can only accept two.

The friend declaration within the class declares the non-member version as a friend of the class, which accepts two arguments. Removing the friend keyword makes the declaration into a member function, which is only allowed to accept one argument. However, your declaration has two .... hence the error.

The inline keyword is unrelated to your error.

wow!!!!.... c++ is really complicated... have to read more... and get some experience in real life programming....Tnx Grumpy... you are really a guru

JD-Salinger · May 1st, 2008, 1:36 AM

anway that means that operator+() can only be used to accept one argument? so everytime you overload an operator you'll always think that it must accept one argument?

grumpy · May 1st, 2008, 5:45 AM

Addition is a binary operation: it acts on two objects and returns a result.

The only difference between implementing an operator+() as a member function or as a non-member function is how the two objects are passed to the function.

Unrelated to the topic: operator+(), like operator-() can also be a unary operation.

JD-Salinger · May 1st, 2008, 8:00 AM

thanks again grumpy.... now i get it......

Apr 30th, 2008, 5:59 AM	#1
JD-Salinger Unknown Join Date: Apr 2008 Location: unknown Posts: 59 Rep Power: 1	copy constructors explanation and o.overloading I just need some little explanation about this snippet, i did some little experiment in the class "value" and when i compiled the source file, error pops out, even though in my own logic, there is nothing wrong with it, i came with this book and im trying to learn copy constructors and operator overloading and i did some experimenting on the code: This is the original code: c++ Syntax (Toggle Plain Text) #include <iostream> #include <sstream> using namespace std; class Value { public: Value() { cout<<" Default constructor\n"; _numString=new char[1]; _numString[0]= '\0'; } Value(int i) { cout<< "Construction/conversion from int "<<i<<endl; stringstream buffer; buffer<< i <<ends; //terminate sring Init(buffer.str().c_str()); Display(); } Value(Value const& v) { cout<< " Copy constructor(" << v._numString<< ")\n"; Init(v._numString); Display(); } Value& operator= (Value const& v) { cout<<"operator=(" <<v._numString <<")\n"; if(_numString != v._numString) { cout<<"not equal"<<endl; delete _numString; Init(v._numString); } Display(); return this; } friend Value operator+ (Value const& v1,Value const& v2) { cout<<" operator+ ("<<v1._numString<< ", " <<v2._numString <<")\n"; stringstream buffer; buffer<<v1._numString<< " + "<<v2._numString<<ends; Value result; result.Init(buffer.str().c_str()); cout<<" Returning by value\n"; return result; } private: void Init(char const buf) { int len=strlen(buf); _numString=new char[len + 1]; strcpy(_numString,buf); } void Display() { cout<< "\t" <<_numString<<endl; } char* _numString; }; #include <iostream> #include <sstream> using namespace std; class Value { public: Value() { cout<<" Default constructor\n"; _numString=new char[1]; _numString[0]= '\0'; } Value(int i) { cout<< "Construction/conversion from int "<<i<<endl; stringstream buffer; buffer<< i <<ends; //terminate sring Init(buffer.str().c_str()); Display(); } Value(Value const& v) { cout<< " Copy constructor(" << v._numString<< ")\n"; Init(v._numString); Display(); } Value& operator= (Value const& v) { cout<<"operator=(" <<v._numString <<")\n"; if(_numString != v._numString) { cout<<"not equal"<<endl; delete _numString; Init(v._numString); } Display(); return this; } friend Value operator+ (Value const& v1,Value const& v2) { cout<<" operator+ ("<<v1._numString<< ", " <<v2._numString <<")\n"; stringstream buffer; buffer<<v1._numString<< " + "<<v2._numString<<ends; Value result; result.Init(buffer.str().c_str()); cout<<" Returning by value\n"; return result; } private: void Init(char const buf) { int len=strlen(buf); _numString=new char[len + 1]; strcpy(_numString,buf); } void Display() { cout<< "\t" <<_numString<<endl; } char* _numString; }; now what I did is that on line 50 i replaced it to `inline Value operator+ (Value const& v1,Value const& v2)` then the error is "must take zero to one argument" and i also replaced line 50 to `Value& operator+ (Value const& v1,Value const& v2)` and still same error.... im wondering what's the explanation behind this, i just copied line 34... Thanks __________________ ------------------------------------------------------------------------- I thought what I'd Do was, I'd pretend to be one of those deaf mutes ------------------------------------------------------------------------

Apr 30th, 2008, 7:08 AM	#2
hammerhead Newbie Join Date: Apr 2008 Location: India Posts: 3 Rep Power: 0	Re: copy constructors explanation and o.overloading I am not sure but you can try rewriting your code from line 50 as keep in mind to remove the friend keyword c++ Syntax (Toggle Plain Text) Value operator+ (Value const& v2) { cout<<" operator+ ("<<_numString<< ", " <<v2._numString <<")\n"; stringstream buffer; buffer<<_numString<< " + "<<v2._numString<<ends; Value result; result.Init(buffer.str().c_str()); cout<<" Returning by value\n"; return result; } Value operator+ (Value const& v2) { cout<<" operator+ ("<<_numString<< ", " <<v2._numString <<")\n"; stringstream buffer; buffer<<_numString<< " + "<<v2._numString<<ends; Value result; result.Init(buffer.str().c_str()); cout<<" Returning by value\n"; return result; }

Apr 30th, 2008, 7:16 AM	#3
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,198 Rep Power: 5	Re: copy constructors explanation and o.overloading operator+() can be a member function of the class that accepts one argument, so "x = a + b" is effectively "x = a.operator+(b)" or a non-member, so "x = a + b" is effectively "x = operator+(a, b);". The member function version can only accept one argument. The non-member version can only accept two. The friend declaration within the class declares the non-member version as a friend of the class, which accepts two arguments. Removing the friend keyword makes the declaration into a member function, which is only allowed to accept one argument. However, your declaration has two .... hence the error. The inline keyword is unrelated to your error.

May 1st, 2008, 1:36 AM	#5
JD-Salinger Unknown Join Date: Apr 2008 Location: unknown Posts: 59 Rep Power: 1	Re: copy constructors explanation and o.overloading anway that means that operator+() can only be used to accept one argument? so everytime you overload an operator you'll always think that it must accept one argument? __________________ ------------------------------------------------------------------------- I thought what I'd Do was, I'd pretend to be one of those deaf mutes ------------------------------------------------------------------------

May 1st, 2008, 5:45 AM	#6
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,198 Rep Power: 5	Re: copy constructors explanation and o.overloading Addition is a binary operation: it acts on two objects and returns a result. The only difference between implementing an operator+() as a member function or as a non-member function is how the two objects are passed to the function. Unrelated to the topic: operator+(), like operator-() can also be a unary operation.

May 1st, 2008, 8:00 AM	#7
JD-Salinger Unknown Join Date: Apr 2008 Location: unknown Posts: 59 Rep Power: 1	Re: copy constructors explanation and o.overloading thanks again grumpy.... now i get it...... __________________ ------------------------------------------------------------------------- I thought what I'd Do was, I'd pretend to be one of those deaf mutes ------------------------------------------------------------------------

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Programming Forums > Application Development > C++

copy constructors explanation and o.overloading