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For 2023 - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Templates in C++ - defining a family pattern of methods / functions

In C, function names need to be unique. If you try to compile or load two functions of the same name, even with a different number / type of parameters, you'll get an error rather than a runnable file out of your compile / load process. See [here].

In C++, you can have methods (functions) in each class (namespace) of the same name - and indeed this is the whole basis of polymorphism, where the piece of code to be run when the program's in operation is selected dynmaically based on the type of data on which it's run. This can even change from one iteration of a loop to another, so that if you have an array that contains fishing rods and skis, you can call different "getlength" pieces of code on each of them without having to use any if or switch type constructs. And in C++, you can also gave multiple methods / functions of the same name, each with a different number / type of parameters, and the compiler, loader and runtime library will sort them out for you, calling the right one an the right time. Example [here], with minimal changes from the C example above.

If you require a whole family of functions in C++, you may use a function template. There's an example [here] on our web site.

Templates are essentially patterns - you can say that there is a whole pattern of methods which is available if you call a function of a certain name, with differing data types in use. The example that I've linked to above defines a function called "exchange" which swaps over two variables ... and needs a third variable internally of the same type to act as a temporary store.

Sample template definition:

  template<typename T>
    void exchange(T& x, T& y)
      T tmp = x;
      x = y;
      y = tmp;

Templates can have several differing types in their call, and you can even define multiple templates of the same name, with a generic one initially then exception(s) where a particular type is passed in as a parameter. In an extended examle [here], we have modified our "exchange" template to give it functionally different behaviour via an alternative template if it's called with bool parameters.

Also in that example, we've used a template to give a variable return type too, and to force the incoming parameters to be specified / cast too:

  double gloucester = differ<float, long int, double>(boat,length);

Looking at that notation, you'll spot that standard library code is in fact template based too, with the C++ vector, for example, using a template to allow the vector to hold data of a type of your choosing.
(written 2011-08-12, updated 2011-08-13)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
C238 - C and C based languages - Templates
  [1478] Some new C programming examples - files, structs, unions etc - (2007-12-19)
  [3245] Collections in C and C++ - arrays, vectors and heap memory blocks - (2011-04-12)
  [3252] C++ - unknown array size, unknown object type. Help! - (2011-04-17)
  [3509] Operator Overloading, Exceptions, Pointers, References and Templates in C++ - new examples from our courses - (2011-11-06)
  [3810] Reading files, and using factories to create vectors of objects from the data in C++ - (2012-07-21)
  [3982] Using a vector within an object - C++ - (2013-01-19)

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What costs 8.20 from Melksham, or 22.30 via Chippenham?
For programmers who use Internet Explorer as their browser
Printing objects in C++
Plenty to do in Melksham
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
Do university courses teach the right things for life at work later on?
Loops - a comparison of goto, while and for
Are people who walk into Melksham being asked to subsidise parking?
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