Printing objects in C++

Overloading operators with methods is a great way of providing a shorthand using the operator syntax for common things you want to do with objects. In other words - it's much easier to write:
  c = a + b
than
  c = a.addition(b)
ans it's also much easier for the maintainance programmer later too - a short and sweet "+" is pretty obvious.

In C++ (which I have been teaching for the last couple of days), you can define an operator within a class simply be defining a method with an appropriate name - a predefined name, in fact, such as operator+. There's an example of that from a course I ran earlier this year - [here].

It gets a little trickier when you want to define how an object is to be output when you pass it to an output stream. You've probably seen code like:
  cout << varname << "kgs" << endl
and had it work well for you. And if varname is the name of a standard type such as an integer or a float, it works seemlessly and without the need for much thought. But what if varname is one of your own objects?

Your initial reaction may well be to overload the << operator (actually the left shift operator), but unfortuanatley the object you want to run it on appears to the right of the <<, and so that overriding doesn't work out. What you need to do instead is to provide a method that will work on cout. Here's how ...

On a C++ course

cout is a an object of type ostream, so you define an extra function that takes an output stream parameter, and a parameter of whatever object type you have. You call the function operator<< so that it works when you call the << operator. And you have it return the incoming ostream parameter. This latter action is what allows the chaining of a whole series of << operators on a line.

Code:

  ostream &operator<<(ostream &os, Person &p) {

    p.print(&os);

    return os; 

    }

You an then simply implement the print method in your class:

  void Person::print(ostream *os) {

    *os << "A Person "  << weight << " " << height << endl;

    }

Demonstration source code for those methods include in a class [here]. The header file (including the function template needed) is [here]. And the new logic is called from a sample test program [here].

Illustrations - lunchtime at Well House Manor on the C++ course that was running there yesterday. We have a selection of lunches during the week, tailored to suit the delegates on each particular week. Yesterday was a "healthy option" lunch - very often that's much more popular that a traditional heavy meal which can take up a lot of valuable learning time, and may leave some of us a little too sleepy and mellow for some of the more challenging subjects.
(written 2011-08-13) 3834

Associated topics are indexed under

C235 - C and C based languages - I/O in C++
  [3810] Reading files, and using factories to create vectors of objects from the data in C++ - (2012-07-21)
  [3807] Reading (and writing) files in C++ - (2012-07-18)
  [3252] C++ - unknown array size, unknown object type. Help! - (2011-04-17)
  [3124] C++ - putting the language elements together into a program - (2011-01-08)
  [1675] Comparing Objects in C++ - (2008-06-13)
  [1478] Some new C programming examples - files, structs, unions etc - (2007-12-19)

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Templates in C++ - defining a family pattern of methods / functions
Eating out in Melksham - where we like for lunch.
Adding the pieces together to make a complete language - C
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