C++ / G4MICE Course Session 3 Introduction to Classes Pointers and References Makefiles Standard Template Library

Classes A class is a way of making a new variable type that is very powerful. It does not just contain data, but also comes with functions (methods) that can be called. This allows us to define a new type for a 3 vector (for example) have it do more than just store 3 numbers, it can tell you the magnitude, direction, dot product with another vector, etc, etc. 2

Constructor When you make a new int or double the compiler just finds sufficient memory and if you have set an initial value in the code, that value is copied to this new memory location. A class is more sophisticated and so we have the ability to specify exactly what should happen when an instance of our class is created. This code is called the constructor. 3

Destructor Similarly, when a simple type is no longer needed that memory is freed up, but nothing else is done. In a class, we can define specific code that should happen just as the instance of that class is being destroyed. This code is called the destructor. 4

Functions and Variables In addition to the constructor and destructor, a class can have any number of functions. It can also have variables of either the basic types, or of other classes. We typically store internal information in the variables and then provide functions (often called methods) that use this information to perform whatever tasks the class is meant to do. 5

Public and Private Methods and variables can be declared as being public or private. If something is public, it means that code outside of the class can see it and call it (if it is a function) or use it in an expression (if it is a variable). If something is declared to be private, then it is only visible to other code that is in that class. 6

Methods and Variables What we usually do is to have the variables that are part of a class private. We then make some (or all) of the methods of the class public. This means that the code that uses a class can call a well defined method that should perform a certain task without any need to know how that task is performed. 7

A Very Simple Class class MyClass { public : MyClass() {};// this is the constructor ~MyClass() {};// this is the destructor void increment() { val++; } // this function increments the value void decrement() { val--; } // this function decrements the value int value() { return val; } // this function returns the value private : int val; }; 8

Inline Functions In the previous example, the code for each function was provided in the class declaration. This is known as an inline function. It is a good idea to do this when the function is very short. For more complicated code, it should be compiled separately (we will see this shortly). 9

Simple Class Example Download the file simpleClass.cc and compile it. Run it and check that you understand what is happening. Uncomment the line that attempts to use the variable _val and try to compile. 10

Pointers Pointers are effectively a different type and refer to the address in memory of a variable. double x = 5; double* y = & x; y holds the address in memory where the variable x is located. You can de-reference a pointer using * e.g. std::cout << *y << std::endl; 11

References Are like pointers, except: –They do not need to be dereferenced –They must always refer to something (i.e. they cannot be NULL, like a pointer!) A reference IS whatever it is referring to. double x = 5; double& y = x; y = 9; 12

Pointers & References Download the pointers.cc and references.cc files and compile them. Run them and check that you understand what is happening. In the pointers.cc file, uncomment the code at the end, recompile and see what happens when you run. 13

Makefiles When we start making G4MICE applications, we will use a tool that generates Makefiles for us. For now, I will provide some Makefiles for the remaining exercises. A Makefile allows you to specify what actions are required to compile the code and how one file depends on another. Use the example Makefile for this session. 14

STL Standard Template Library: – We have already seen one class from the library, string. Another useful tool from the STL is the vector template. This template allows you to create a vector (array) of any type. 15

vector std::vector numbers; Defines a vector (container) of integers called numbers. size() – return the number of elements in the vector [] – refers to the ith element (numbering starts at 0, e.g. numbers[10] push_back( x ) – add an element to the end of the vector. 16

Vector Example The Makefile should already have built the vector example for you (if you used the tarball). If not, download the code and compile it. Run the vector example to understand how the vector template works. 17

More STL We don’t have enough time to go into the STL in any detail, but it is worth spending some time to learn what it can provide. There are many different templates that provide useful ways of storing information as well as algorithms such as sorting routines. 18