Tell, Don’t Ask in C

I’m preparing a keynote on “Timeless Design Principles”, with the aim of demonstrating how the principles I try to instil in developers on my Codemanship courses could have been applied just as readily 30 years ago or even 50 years ago in programming languages of the time.

In 1989, C ruled the world. A common misconception among inexperienced developers is that design principles like S.O.L.I.D. and Tell, Don’t Ask only apply to OO languages like C++.

Nothing could be further from the truth, though. Let’s start with Tell, Don’t Ask.

Consider this simple C function that calculates a quote for a fitted carpet:

The quote() function has to ask for the room’s dimensions, and then has to ask for the carpet’s price pr square metre and whether it should round up to the nearest square metre.

Although Room and Carpet aren’t classes, as far as I’m concerned this is still Feature Envy. Room and Carpet are completely unencapsulated. The carpet_quote module knows how the area of a room is calculated, and it knows how to calculate the price of the carpet in that room. If those details change, carpet_quote breaks. Or, more simply, carpet_quote knows too much.

A good first step to fixing that would be to move those two pieces of logic into their own functions.

Now our quote() function knows a lot less. But the carpet_quote module still knows it all. So the next step would be to move the area() and price() functions to the modules where the Room and Carpet structs are declared.

carpet_quote now knows less about the details, which are neatly encapsulated inside carpet and room.

The data is still accessible from the outside, though. So we’ve got a little more work to do to complete this refactoring. At the moment, the Room and Carpet structs are declared in completeness in the header files room.h and carpet.h, so any module can create and set their field values directly.

In our client code, we instantiate these “objects” directly, setting their field values externally. This gives me the screaming heebie-jeebies. (Every bit a smuch as “data classes in Python, or JSON objects in JavaScript.)

How can we hide the data of a Room and a Carpet inside their respective modules? Luckily, C gives a mechanism: partial declarations. We can partially declare a struct in a .h file, defining its type but omitting its data.

Then we can declare the struct with all its data fields in the .c file, so that they can only be accessed internally. Then we add a factory method – essentially a “constructor” – for that type.

Now the only way a client can get a handle on an instance of the struct is via its module, and it can’t access the data directly.

 

Author: codemanship

Founder of Codemanship Ltd and code craft coach and trainer

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