I’ve been trying to design an interface for a data class I’m writing. This class stores styles for characters, for example whether the character is bold, italic or underlined. But also the font-size and the font-family. So it has different types of member variables. The easiest way to implement this would be to add getters and setters for every member variable, but this just feels wrong to me. It feels way more logical (and more OOP) to call style.format(BOLD, true) instead of style.setBold(true). So to use logical methods instead of getters/setters.
But I am facing two problems while implementing these methods: I would need a big switch statement with all member variables, since you can’t access a variable by the contents of a string in C++. Moreover, you can’t overload by return type, which means you can’t write one getter like style.getFormatting(BOLD) (I know there are some tricks to do this, but these don’t allow for parameters, which I would obviously need).
However, if I would implement getters and setters, there are also issues. I would have to duplicate quite some code because styles can also have a parent styles, which means the getters have to look not only at the member variables of this style, but also at the variables of the parent styles.
Because I wasn’t able to figure out how to do this, I decided to ask a question a couple of weeks ago. See Object Oriented Programming: getters/setters or logical names. But in that question I didn’t stress it would be just a data object and that I’m not making a text rendering engine, which was the reason one of the people that answered suggested I ask another question while making that clear (because his solution, the decorator pattern, isn’t suitable for my problem). So please note that I’m not creating my own text rendering engine, I just use these classes to store data.
Because I still haven’t been able to find a solution to this problem I’d like to ask this question again: how would you design a styles class like this? And why would you do that?
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Part 1
This is a good design question. You are correct in detecting code smell concerning getters and setters. They generally indicate a design problem exposing the implementation details of your object.
Try to think in terms of what your objects should do – Tell, Don’t Ask:
- http://pragprog.com/articles/tell-dont-ask
- http://c2.com/cgi/wiki?TellDontAsk
Your first problem may be that you are trying to design “data classes”. Rather than worry about the data (implementation details), think about the functionality. Again, what should your objects do? In your case, what do you want to do with character styles? Who (software-wise) cares about character styles? What do they need to do?
Hopefully that gets you started. Test driven development helps with these kinds of design problems. It forces you to think in terms of function not data.
On the contrary, if all you need is a data container, write a C-style struct class and go to town. I wouldn’t recommend that, maintaining it will be a $@#%#.
Good luck!
Part 2
What you want is either a simple data class – skip the getters and setters completely, or you want to create a platform independent abstraction that places a facade (Facade Pattern) in front of the rendering and style setting. It simply provides an interface for setting styles and rendering. Your platform specific implementation does the dirty work (using NSTextView in your example).
The benefit of the simple data class is that it is initially simple to write. Its drawback is that you will have a hard time avoiding a giant tangle of if-else statements. You will also be lacking a clear place to make you platform specific rendering calls that use the styles. As the system’s complexity grows, you may find it more diffcult to decide where implementation details go.
The facade is a more abstracted approach. The benefits are that it is more flexible and can be reused if you decide to port to another platform. Its drawbacks are more upfront development time.
The facade’s public interface will provide what you need to set and remove styles as well as initiate rendering when the time comes.
The details of how you want to set styles are up to you. Use what ever system feels best. Simple getters and setters or a generic set and get that uses a dictionary internally works too (see boosts ptree if you are using C++). You could even take all (or default) styles at construction time. You could decide to not even expose mutators at that point. Your call. Perhaps you decide it is important to data drive the styles you support and use a configuration plus factory system (We can add more detail later if that is important to you). In fact different implementatios of the facade could provide different ways of approaching the problem. You could prototype a few and choose what works best.
The platform specific implementation of you facade abstraction will use the platform specific rendering system (NSTextView in your case) and the styles you have set to make the appropriate calls to the system. Simply inject the platform specific classes at construction (Dependency Injection), implement your render() method and you should be good to go.
Part 3
If your system design allows, you could take all styles for a particular element at construction time. This could allow you to avoid getters and setters completely if you chose to make your element immutable. Your would then have a simple, clean and possibly immutable abstraction in front of your character styles system. Immutable state generally leads to fewer bugs but does require you to operate under the premise that you cannot change things willy-nilly.
Taking this a step further, a configuration file might define the different style setups you have. Again this would require prior knowledge of what styles you are setting (similar to constructing with the styles above). Give the type of style you are seeking, say “heading”, you might fetch the configuration for headings which specifies a larger, bold font.
These are just some ideas off the top of my head. Without further requirements gathering and use cases it will be tough to get more specific.
Hope that helps. Good luck!
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I smell an attribution list. I agree you should set format and get format, except I would use a list. A bit mask even if your attributes allow. Otherwise just keep an array of formatter objects which use IOC to format there field if possible.
forgive me, I don’t know c++
Public abstract class IFormatter
{
. public abstract void FormatText(TextField* fieldToFormat);
}
Public class BoldFormatter : IFormatter
{...}
Public class TextField
{
. public void AddFormatter(IFormatter* formatterToAdd)
. {...}
. public IFormatter[] GetFormatters()
. {...}
. public void Render()
. {
. foreach(IFormatter formatter in formatters)
. { formatter.FormatText(this); }
}
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If all they do is store data, then why bother with any kind of getter and setter? You have no invariants to enforce, no logical reason to control the data, and no reason to process it nor prevent access. The class’s reason to be is “Store data”. So make it store some data and leave it at that.
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