The V C++ GUI Framework User Guide:Tutorial:C++ Coding Style

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The V C++ Coding Style Guidelines

I have developed the following guidelines for writing C++ code over my long career as a programmer. All of V has been written using these guidelines, and I believe that using them is a big first step leading to readable, portable, and reliable code. Of course, just following these guidelines won't automatically give you that, but I think they are still necessary.

Readability

The ultimate goal of style guidelines is to help you to write code that is readable. While this means code that is readable by you, it mostly means code that is readable by others. Remember, code has a life of its own! No matter how small the project may seem, or how temporary, most code ends up being used and reused much longer than you might think. The real cost of software is often in the long term maintenance. While you may end up maintaining your own code, often it will be someone else. Even if it is you, after a few months, or even weeks, you will have likely forgotten just exactly what you were doing when you wrote the code to begin with.

The point of this is to emphasize the importance of producing readable code. Generally, readable code is inviting to look at. It is visually pleasing, just as a well designed book is well laid out and visually pleasing to look at. Your code should have plenty of visual attributes that make it easy to read. This means lots of whitespace, consistent indentation, abundant, well formatted comments, and visual separation of important sections of code. Much of the structure of your code should be visually obvious without having to read it. Many of the following guidelines are intended to help you produce readable code.

Naming

It is critical to choose meaningful names for your variables and functions. Avoid short, two or three letter names unless those names are really meaningful. While you may want to use short, abbreviated names to avoid typing, this habit will make your code more difficult to read later. While you should avoid short names, consistently using names that are too long can present problems, too. This can lead to code that must be split across multiple lines because the names are too long. Even so, it is probably better to trend to overly long names than short, abbreviated names.

Names should use both upper and lower case letters, using a case change to indicate word breaks. For example, a name like maxLength is more readable than MAXLENGTH, maxlength, or even max_length. In general, using mixed case is better than using underscores. Underscores are better used to indicate special classes of variables (see Class Definitions below).

Files

Each C++ module should be split into two files - the .h header file which contains class definitions and variable declarations, and a .c or .cpp1 file that contains source code for the functions.

Generally, each class will have its own .h and .cpp files. Utility helper functions that go with a class can be included in the same file as the class. Other functions that do not go with a class should be collected into logical groups and kept in a separate file. In general, files should not be much larger than twenty to forty thousand characters long.

Include Files

Include files or header files (.h) files must each have a #define statement that prevents problems caused by multiple inclusion. The standard way to do this is:

//
//  myclass.h - header file for myclass class definition
//
#ifndef MYCLASS_H              // Check to prevent
#define MYCLASS_H              // multiple inclusion

   ... definitions go here

#endif                         // last line of file

Files are included from the source file by placing the #include statement near the beginning of the source file, starting in column one.

#include "myclass.h"      // includes start in column 1

Function Definitions

All functions should have a prototype definition for use by others. For class methods, this will be part of the class declaration. For other functions, this should also be in a .h file. The parameters of all prototypes should include both the type and the name of each parameter since the name often conveys extra useful information.

The body of each method or function should use this convention:

//==================>>> myClass::myMethod <<<===================
  void myClass::myMethod(const int size)
  {
    // An introductory block of comments explaining the purpose
    // and interface to this function. You can also include an
    // author and modification history here if appropriate.

    ... declare variables used throughout the function

    ... body of function
  }

Each function should include the separator line to visually separate the body of the function from others in the same file. The preferred indentation for the function name is two spaces, with the enclosing { and } braces on separate lines, also indented two spaces. An acceptable alternative style is to have these lines start in the first column.

Following the opening { should come an introduction to the function. Variables required by the entire function follow the opening comments. Following that is the body of the function. Make liberal use of whitespace for visual separation.

Indentation

The preferred indentation scheme is based on groups of four spaces, with braces indented two additional spaces. It is acceptable to keep braces lined up with the outer statement rather than indenting two extra, but all braces must be on a line by themselves. This spacing works well with standard eight character tab stops - your code will either be indented on even tab stops, or on tab stops plus four.

Except for the most trivial cases of short, related assignment statements, each statement should be on a separate line. The body of loops and conditional statements should always use braces - never use a simple statement. There are two reasons for this. First, using braces on separate lines adds whitespace, which adds to the readability. Second, code is inevitably modified, and by always using braces, you will be more likely to add a statement in the proper place. As a special case for initializer loops with no code body, it is acceptable to use just a semicolon rather than braces.

The old K&R style of placing the opening brace at the end of the line is not acceptable. Most importantly, you lose the visual impact of lined up braces when you do this. It also tends to compress the code, and extra whitespace really helps make code more readable.

When calling functions that require long, complex argument lists, it is often advisable to place each argument on a separate line accompanied by an explanatory comment.

Use a blank between keywords and the associated left paren: if (test). Don't put a space for function calls: function(param);. Don't use parens for the returned value of a return statement. This helps to visually distinguish a return from a function call.

The following code demonstrates indentation for various C++ statements:

//=====================>>> sample <<<=====================
  int sample(int action)
  {
    // This meaningless sample demonstrates indentation.
    // The code should not considered to do anything useful
    // other than demonstrating indentation.

    char* name;                 // explain each variable
    int set;                    // with useful comment

    if (action)                 // indent 4, space after if
      {                         // the { in 2
        set = doSomething(action);
      }                         // always use {}'s
    else
      {
        set = SomethingElse(action);
      }

    switch (set)                // example for switch
      {
        case 1:                 // case in +4 from switch
          {                     // always use braces for cases!
            getName1(name); 
            break;
          }

        case 2:                 // Try to comment each case
          {
            int temp = len(name);  // try to declare as needed
            fixName(temp,name);
            break;
          }

        default:                // Good idea to have default
            break;
      }

    // Prefix some blocks with comments like this
    // to describe what a section of code does
    // Note that 'char* cp' is preferred to 'char *cp'.
    // Take advantage of C++ scope rules, and declare
    // variables (e.g., len) as close to use as possible.
    for (int len = 0, char* cp = name ; *cp ; ++cp)
      {
        ++len;                 // all loops use {}'s
        tryThat(set,cp);       // and meaningful comments!
      }

    while (IsStillOK(name))    // indent like for
      {
        Complex(name,          // a complex function call
            set,               // can explain each parameter
            len);              // for easier maintenance
      }

    int status = (checkName(name)) ? len    // sample ?:
                                   : len / 2;

    return status;             // no parens on return
  }

Comments

It is difficult to over comment your code. Comments are one of the most helpful things you can do to make your code easier to maintain. A 1 to 1 ratio of comments to code should be considered a bare minimum, with a ratio of more comments than code probably a better thing.

I claim it is almost impossible to have too many comments. A few expert programmers may disagree with this philosophy, and say that well written code can be self-commenting. The problem is that this is not really true. Assume, for example, that you are using a standard software library, such as Xt or V. You may know the library backwards and forwards, and it may seem perfectly clear to you what some code is doing. But assume that someone else will be maintaining the code later. They may not know the library as well, and what is obvious and self-commenting to you will be gibberish to them. A few well placed comments explaining what you are doing will be very helpful.

In order to write really effective comments, you must comment as you write the code! Do not go back and add comments after the code is written. You can go back and improve and expand your comments, but you should comment as you go. A few seconds taken to add a comment as you write the code can save many minutes or even hours later.

Comments should be meaningful and correct. If you change code, be sure you change the comments to correspond! If you are in the habit of commenting as you write, this will not be so hard.

Make the layout of your comments visually pleasing. Use whitespace to separate sections of code. Line up block comments near the left, and try to keep short per line comments lined up on the right without going too far right. Line them up on a tab stop if possible.

Above all, remember that what seems obvious to you at the moment is likely to be forgotten even a week or two later. And keep in mind that someone else is likely to modify or study your code later. Don't keep secrets. If you had to look something up, or have other information that might make the code more understandable, put that in a comment. If you are doing something tricky or obscure (which you should avoid, but sometimes can't avoid), explain what is going on! You might be teaching a valuable trick to whoever is working with your code later!

My own code has more comments than almost any other code I've seen. Time after time, when someone else has had to use or maintain my code, I've gotten feedback that it is very easy to understand and modify. I attribute much of this positive feedback to the abundant comments found in my code.

Class Definitions

The standards for class definitions are based on keeping braces on separate lines, and on not using implicit assumptions. Thus, a class will have braces on separate lines, either indented two (the preferred style), or lined up with the class statement.

There should always be all three public, protected, and private sections in that order, even if a section is empty. This order assumes it is more useful to have the public stuff at the top for easier readability. And even if a section is empty, that conveys information about the definition of the class. The prototypes for member functions should include both the type and name (e.g., int OnOrOff).

There should almost never be public access to class variables. Provide methods to access and set variables of the class. You may find it helpful to prefix class variables (especially private class variables) with an underscore (_variableName) to indicate the variable is private to the class.

The following example shows indentation and layout of a class definition. Note the visual separator for public, protected, and private, and the alignment with the braces.

    class myClass : public superClass    // name here
      {
        friend int FriendFunction(int ival);  // friends at top

      public:    //-------------------------- public

        myClass();          // constructor and destructor
        virtual ~myClass(); // first

        // simple methods can be inline
        int getVal() { return _val; }
        virtual void service(int iParam);    // prototype

      protected:  //------------------------- protected

        // even an empty section conveys information

      private:    //------------------------- private

        int _val;           // _ for class variables
      };

C++ Language Features

With C++, it is preferred to use const definitions of symbolic values rather than #defines.

Use const parameters whenever possible.

Declare variables as you need them, preferably inside a code block, rather than all at the top of a function. This makes your code much more maintainable, and helps avoid errors introduced by bad reuse of a variable, especially in loops.

For each new operation, there must be a corresponding delete operation. These new and delete pairs will often be found in the constructor and destructor for your objects.

Always define copy constructors and an assignment operator for each class if they use pointers and dynamic memory allocation using new. Some of the biggest problems in C++ code involves objects with pointers to dynamically allocated space. You should use either deep copy semantics or reference counts to avoid creating objects with dangling pointers.

When using V use the debug macros as much as possible. It is especially helpful to use UserDebug statements in constructors and destructors.

Software Portability

Always remember that your code might someday be ported to a different system. Keep this in mind when writing your code. These guidelines will help to make your code more portable.

Don't use nonstandard or nonportable language features. For example, templates are not yet universally portable. Avoid using them.

Use restrictive names when naming files. The most conservative approach is to use single case names limited to 8 characters for the name part, and 3 for the extension. This should get better as time goes by, but for now this is still a pretty good idea.

If you must use system calls, abstract them and isolate them in a single place.

Don't go behind the back of V to access X directly.

Avoid conversions that are Big and Little Endian dependent. If you need them, isolate them.

Footnotes:

(1) The naming conventions for C++ source files has not really been standardized yet. Common alternatives for .cpp include .C and .cxx.