Monday, 8 May 2017

Software Development : Get Constant in C++

What does "const" mean to you?  Does it just mean a value can not be changed?  If so, you may want to read on!

Const is one of those things, which though not unique to C++ is given more meaning when you leverage C++ fully, const allows you to not only define a value as invariant, but also to instruct other programmers coming to your code how a value should be used, how when it is passed as a parameter it should be treated and ultimately how to protect data from needless alteration, de-synchronisation or simple corruption.

Const is therefore your friend, and if you've come to C++ from C, Java, C#, Python or one of the other myriad of languages which don't treat const with as much relevance as C++ you may want to read more than I can say on the topic. 

Bjarne Stroustrop (the inventor of C++) and other authors on the topic (notably Scott Meyers & Herb Sutter) explain in much more detail than I ever could, but for brevity here are two examples of const from my own coding standard which I implore you to digest.

1. Initialise Const Values in the constructor ONLY....

Any instance of the "Ex" class can now access it's "Pi" value, and we communicate to all viewers that the value is a constant and only edited in one place.

2. Where-ever possible pass values as constant references....

We know for certain that the "p_Radius" parameter being passed is NOT to be changed by the function, this is important when you are thinking about letting code document itself, and easily accomplished with "const", and especially annoying when using a language lacking const!

Whatever you do, use const consistently, this needs no specific explanation here, however if you begin following my second rule, you must stick to it!  Don't change half way through a program, if you import a third party API which is not using const in the same way, abstract that third party interface away with a series of your own shims, to make it correct for your usage (the compiler will optimise all these layers away) to leave you with consistent and more easily maintainable code.

You can find must much more about const-correctness elsewhere, I'd start here.

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