Needing to take Class Object from User.

[ScottyYDg]

Hello, the program I am writing takes an intiial menu selection, between class object Double and Integer, and then adds/subs/mul/divs based upon what the user has selected. I believe I have most of the code, except the ability to take the objects from the user. Obviously, cin won't work, and I am unsure about using istream/ostream. Here is a sample of the code. Is there a simple way to do this?

  void doubleAdd()
{
	Double d, d3, d4;
	cout << "Enter your first double." << endl;
//Input from user (d)
	cout << "Enter your second double." << endl;
//Input from user (d4)
	d3 = d.add(d4);
	cout << d3.toDouble() << endl;
	m.waitKey();
}

[mbozzi]

Tell the compiler what it means to extract a Double from an input stream, by defining the function

std::istream& operator>> (std::istream& is, Double& d);

This will allow std::cin (which is a sort of std::istream) to work with objects of type Double. This is a simple example of generic programming, one of the primary strengths of C++.

If you post your class, we'd be able to help you write it.

[ScottyYDg]

#ifndef DOUBLE
#define DOUBLE
#include "Integer.h"

class Double
{
private:
	double data;
	double data2;
public:
	void equals(double d);
	Double add(const Double &d);
	Double sub(const Double &d);
	Double mul(const Double &d);
	Double div(const Double &d);
	double toDouble() const;

	// Overloaded Functions
	Double add(double d);
	Double sub(double d);
	Double mul(double d);
	Double div(double d);
	double getFirstD()const { return this->data; }
	double getSecondD()const { return this->data2; }

	// Constructors
	Double();
	Double(double d);
	Double(const Double &d);
	Double(const Integer &i);
};

#endif 

Sorry, here's my class for Double. Where would I put the stream overload at? it's own place in the header?

[dhayden]

Since the data members of Double are private and can't be set outside the constructor, you have to do this indirectly. First, create Double::read() to read from a stream:

class Double
{
...
public:
    std::istream & read(std::istream &is) {
        return is >> data >> data2;
    };
...

Then outside class Double, you can define the >> operator and have it call read();

std::istream &operator>> (std::istream& is, Double& d)
{
    return d.read(is);
}

[ScottyYDg]

Would a friend function work as well?

[icy1]

Yep, though if you had a SetData() public function, you wouldn't need to have the istream as a friend. Example with friend:

 
#include <iostream>

using namespace std;

class MyClass
{
public:
    MyClass(double d) : d_(d)
    {
    }
    const double& Data() const
    {
        return d_;
    }
    // In order to change private values directly
    friend istream& operator>>(istream& is, MyClass& m);
    
private:
    double d_;
};

// Outputting custom class to ostream 
ostream& operator<<(ostream& os, const MyClass& m)
{
    os << m.Data();
    return os;
}

// Inputting custom class to istream
istream& operator>>(istream& is, MyClass& m)
{
    is >> m.d_;
    return is;
}

int main() 
{
    MyClass m(45.67);
    cout << "MyClass is set to " << m << endl;
    cout << "Enter new value for data: ";
    cin >> m;
    cout << "MyClass is now set to " << m << endl;
}

MyClass is set to 45.67
Enter new value for data:  32.3
MyClass is now set to 32.3

[ScottyYDg]

What about if I needed to take in two values? Like in the case of my program, I'm taking two numbers and sending them to my programs in order to add/sub/etc based upon what the user chose in a menu?

[icy1]

The istream stuff for a custom class is usually for imitating the constructor for that class. Like if I had a Time(int hours, int minutes) constructor, I could overload and read in a space-separated Time like:

is >> t.hours_ >> t.minutes_;
return is;

You could make a complex istream setup, but that wouldn't be intuitive to the user of that class.