Re: Problem overloading extraction operator

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Mon, 07 May 2007 01:08:20 +0200

Message-ID:

<5a75bdF2nodtrU1@mid.individual.net>

* EM.Bateman@gmail.com:

Working on Visual Studio .Net I've implemented a class:

#ifndef CONTRIBUTOR_H
#define CONTRIBUTOR_H

enum Gender {male=1, female, unk};

OK so far.

#include <iostream>
#include <iomanip>

Instead of these heavy-weight headers, consider using the light-weight
<iosfwd> in a header file.

#include <string>
using namespace std;

Never have "using namespace std" in a header file.

class Contributor
{
friend ostream& operator << (ostream& output, Contributor& RHS);
friend istream& operator >> (istream& input, Contributor& RHS);

Reserve all uppercase for macros. See this group's FAQ as well as
Bjarne Stroustrup's FAQs.

public:
Contributor();
Contributor(string Name, double Contribution, Gender MF, int IDKey);

Unrelated to C++ but, what does it matter whether a Contributor has a
penis, a vagina, or perhaps nothing? I'd remove that. Unless the
sexual organs really matter for the contribution (whatever it is). If,
for example, the gender information is used to establish a proper
honorific in a computer generated thank-you letter, it would be much
better design-wise to store the proper honorific. It would also yield
simpler code; see below.

     Contributor(const Contributor &copy);
    Contributor& operator=(const Contributor& RHS);
    bool operator== (const Contributor& RHS);
    ~Contributor() {cout << "In Contributor Destructor" << endl;}

private:
    std::string Name;
    double Contribution;
    Gender MF;
    int IDKey;
};

Note: with the data members you have there's no need to define a copy
constructor and assignment operator; the compiler generated ones will do
nicely.

Then I tried to overload the extraction operator thus:

istream& operator >> (istream& input, Contributor& RHS)
{
    cout << "\nEnter a new contributor's name: ";
    std::getline (cin, RHS.Name);
    cout << "Enter the amount of their contribution: ";
    cin >> RHS.Contribution;
    cout << "What is the gender of the contributor? ";
    cin >> RHS.MF;
    cout << "Assign an ID Key to the contributor: ";
    cin >> RHS.IDKey;

    return input;
}

Consider how you would test your class with prepared data in a file.
Consider how you'd use it in a batch job (silent, no user interaction).

But I keep getting this error: error C2679: binary '>>' : no operator
found which takes a right-hand operand of type 'Gender' (or there is
no acceptable conversion)

I've got gender defined in my constructors as well as the overloading
implementation. I don't understand why I'm getting this error. Can
someone help?

Gender is defined as an enum type. You can provide an extractor for it.
  That function (example below) /defines/, implicitly, the textual
representation of genders, here as simple decimal integers:

   istream& operator>>( istream& input, Gender& g )
   {
       int genderVal;
       input >> genderVal;
       if( input.fail() ) { throw std::runtime_error( "Ouch ouch" ); }
       // Here perhaps check the value and throw exception if not OK.
       // Assuming it is OK:
       g = static_cast<Gender>( genderVal );
       return input;
   }

Note that this would be very much easier it you stored a preferred
honorific (string) instead of a gender (enum).

Hth.,

- Alf

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