Programming Style in Large Scale C++ Applications

Question

Recently I've been browsing source code of large applications written in C++ to learn a bit but I couldn't help but notice that most if not all use a lot of IFDEFs and class-less functions (where they could have been class members/methods).

An example is Qt Creator IDE source code:

http://qt.gitorious.org/qt-creator/qt-creator/blobs/master/src/app/main.cpp

If you notice, it has a bunch of functions around the main() function; one of them, for example, is:

static inline void toHtml(QString &t)

the above method is defined outside while I think it could have been part of some utility class maybe.

Also, I think IFDEFs make the programs messy (again my opinion).

My question is: is it just an accepted practice in C++ world to mix procedural and OO code like that?

Answer 1

It is a rather subjective question - there are many schools of practice for writing C++ code.

C++ affords you the ability to pick and choose what paradigms you want to use for writing your code. The language allows you to use procedural, object-oriented, generic, and functional constructs (especially with C++11 features).

You don't necessarily need to build utility classes (unlike, say, in Java, where everything has to be in a class), as static functions in a class may not be required and can just be standalone free functions.

Conditional compilation is a necessity because of disparate platforms. I worked in defense, and have written code using Qt, boost, libraries for video games, and have worked on a variety of platforms including Linux, HP-UX, AIX, Windows, IRIX, and Mac OSX - platforms are different enough that code written for one does not necessarily work on a different platform. Some easy examples are:

• Endianess - Some architectures (like x86) are little endian, but many other architectures (like SPARC) are big endian. Networking is also reliant on big endian addressing. When you write code across different architectures, sometimes you need to conditionally compile in code that supports such endian differences.

• Different library calls - Windows uses winsock as its networking API, and it's similar to BSD Sockets used by Unix and Unix-derivatives. However, it is different enough that the function calls aren't similar. Additionally, there are library calls that exist in some flavors of Unix that don't exist in other flavors, or derivatives, so conditional compilation is needed for that too.

• Compiler differences or versioning differences - Different versions of compilers, or different compilers altogether, may support functionality/libraries that aren't supported by other compilers.

Even if you use a library like Qt, which aims to be cross-platform, there is still conditional compilation happening, as you found out. These cross-platform libraries are there to reduce your own needs to do conditional compilation.

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Additionally, coding styles and standards may be influencing how code is written.

• http://qt-project.org/wiki/Coding-Conventions - These are Qt's coding conventions for developers and contributors to the library.

• Boost Coding Conventions - The Boost library's conventions for contributors.

• Compare with the Joint Strike Fighter C++ Coding Standards - I have not had to read this, but it's vastly different to most standards I've had to follow and very stringent in regards to what C++ features can and can't be used, and how to structure code.

Just more anecdotes from my past - C++ can be written in many ways as influenced by internal standards. For example, some organizations restrict the use of the C++ Standard Library, especially in regards to using the standard containers (like std::vector and std::list) - this may be due to the environment involved (like embedded systems).

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Ultimately, it does come down to this: there are many ways to write C++, and it is acceptable to mix and match different paradigms in a single C++ program and code base, because there may be better/simpler ways to express something in one paradigm than to be locked into using a single pattern for an entire code base.

If you want to look at large C++ code bases that have high code quality, I will say that Boost and Qt are among the better examples.

Generally and widely accepted conventions and idioms for C++ programming are provided in books like Effective C++ and More Effective C++

Answer 2

Sometimes the "class" part of a utility class does nothing more than act as a pseudo-namespace. There is no instance state needed, just parameters passed into a function. For example the Math class in C#. In that case you may as well just have free functions in a namespace.

Sometimes behavior should be a property of the "environment" not the object. For example a save() function may be layer/environment specific and should be a free function in that layer. The object may visit many layers (physical or logical) where saving may not be relevant.

Answer 3

While I agree with others that the question is subjective, the two examples you give are viewed very differently by the C++ community:

• #ifdef sparkled around C++ code is a bad practice. System-dependent code is best kept isolated in separate files and handled by the build system. Otherwise you get the mess that you saw in the cpp file you sent a link to.
• Free functions in C++ are a good thing. Instead of giving lengthy explanation here, I suggest reading Scott Meyer's classic text on the topic: How Non-Member Functions Improve Encapsulation

Answer 4

It is generally accepted in many worlds. In fxCop for C# for example, it will flag you for not making methods static when they use no member variables. The idea is that a class should only define enough of a public interface to protect its invariants; the rest is the responsibility of the consumer, who knows best how the class will be consumed.

I'm not sure I would go so far as to consider it universally accepted, but it is a common practice.

Answer 5

Like what birryree said, this is indeed a subjective question. There are many ways to program in a large scale application. The type of application can also play a big part on the way you want to program your application.

It is good to look into Component-based programming if the project is really big scaled. This however should also be considered in a case-by-case basis to see if your project really needs it.

Google Component based programming for more info.

Here is the wiki page for it: http://en.wikipedia.org/wiki/Component-based_software_engineering