testing, casting & implicit conversion, and methods vs functions

This past week in OOP we discussed how to test private and protected members of a class in C++ without having to change the implementation, casting and implicit conversion and how to turn it off in C++, and we looked at methods vs functions.

The first discussion was motivated by the project which was due on wednesday and the fact that it is nice to be able to test internal state but many of the solutions involved making the class a friend of the tester which required some work and portrayed an object model that was not entirely true, and furthermore might need maintenance in the future. The better solution was to use compiler directives to expose all of the members and as such the state, the code for which would look something like:

#define private public
#define protected public
#define class struct

#include "darwin.h" //the header file we wanted to include in the tester

With reference to casting we covered the fact that in C++ we can typecast an int to a double, which we can also do in Java, however we can also typecast a double to an int(with a loss of precision) which we can’t do in Java. Prof Downing also introduced that fact that the one arg constructor was an implicit conversion function from the type of the argument to the type of the object, in some cases though this is undesirable so by defining the constructor with the keyword ‘explicit’ we can force this implicit conversion to not happen, we then sacrifice the nice syntax of <Type> x = <value>; and we are forced into using the otherwise equivalent <Type> x(<value>); syntax. In C++ we can additionally define functions for other types to allow conversion of the user defined type to the other type, the syntax for which is as follows:

operator <some_type_here>(){...} //will always return a value of type <some_type_here>

In cases where these implicit conversions might cause problems the compiler will prefer the non-converted function.

Progressing through the list we come to the comparison of functions vs methods and when to use which. Functions belong to no class or rather they exist in the file/global scope whereas methods are bound to a class. Methods normally my first choice coming from Java but when deciding which to use one should ponder the following questions: “does the public interface provide all the access we need to perform the function?”, “do we need to define the operation with a different class on the left hand side(example: ostreams’  << and  istreams’ >> operators)?”, and “do we want the operation to be symetric?”. If you answered yes to any or all of the above then it might be worthwhile to define the code block as a function(using friend if necessary).


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s