by Vadim Zeitlin

This guide is intended for people who are (or intending to start) writing code for wxWindows class library.

The guide is separated into two parts: the first one addresses the general compatibility issues and is not wxWindows-specific. The advises in this part will hopefully help you to write programs which compile and run on greater variety of platforms. The second part details the wxWindows code organization and its goal it to make wxWindows as uniform as possible without imposing too many restrictions on the programmer.

Acknowledgements: This guide is partly based on C++ portability guide by David Williams.

General C++ Rules

• New or not widely supported C++ features
1. Don't use C++ templates
2. Don't use C++ exceptions
3. Don't use RTTI
4. Don't use namespaces
5. Don't use STL
6. Don't declare variables inside for()
7. Don't use nested classes


• Other compiler limitations
1. Use ternary operator ?: carefully
2. Don't use initializers with automatic arrays
3. Always have at least one constructor in a class with destructor


• General recommendations
1. No C++ comments in C code>
2. No global variables with constructor
3. Turn on all warnings and eradicate them
4. Don't rely on sizeof(int) == 2...
5. No assignments in conditional expressions
6. Use #if 0 rather than comments to temporarily disable blocks of code
7. Avoid overloaded virtual functions
8. Don't use extra semi-colons on top level


• Unix/DOS differences
1. Use .cpp for C++ source file extension
2. Don't use backslash ('\\') in #includes
3. Avoid carriage returns in cross-platform code
4. Use only lower letter filenames
5. Terminate the files with a new-line
6. Avoid globals differing by case only


• Style choices
1. Naming conventions: use m_ for members
2. Don't use void for functions without arguments
3. Don't use const for non pointer/reference arguments

wxWindows Rules

• Files location and naming conventions
1. File locations
2. Include guards
3. Precompiled headers


• File layout and indentation
1. wxWindows standard header
2. Indent your code with 4 spaces (no tabs!)
3. Order of parts in a class declarations


• More about naming conventions
1. Use wx or WX prefix for all public symbols
2. Use WXDLLEXPORT with all classes/functions in wxMSW/common code
3. Use Set/Get prefixes for accessors
4. wxNAMING_CONSTANTS


• Miscellaneous
1. Use forward declarations whenever possible
2. Use debugging macros

General C++ Rules

• New or not widely supported C++ features

The usage of all features in this section is not recommended for one reason: they appeared in C++ relatively recently and are not yet supported by all compilers. Moreover, when they're supported, there are differences between different vendor's implementations. It's understandable that you might love one (or all) of these features, but you surely can write C++ programs without them. Where possible, workarounds to compensate for absence of your favourite C++ abilities are indicated.

Just to suppress any doubts that there are compilers which don't support these new features, you can think about Win16 (a.k.a. Win 3.1) compilers, none of which supports any feature from the list below.

1. Don't use C++ templates

Besides the reasons mentioned above, template usage also makes the program compile much slower (200%-300% is not uncommon) and their support even in the compilers which have had it for a long time is far from perfect (the best example is probably gcc).

Workaround: The things you would like to use templates for are, most commonly, polymorphic containers (in the sense that they can contain objects of any type without compromising C++ type system, i.e. using void * is out of question). wxWindows provides dynamic arrays and lists which are sufficient in 99% of cases - please don't hesitate to use them. Lack of template is not a reason to use static arrays or type-less (passing by void *) containers.

2. Don't use C++ exceptions

The C++ exception system is an error-reporting mechanism. Another reasons not to use it, besides portability, are the performance penalty it imposes (small, but, at least for current compilers, non-zero), and subtle problems with memory/resource deallocation it may create (the place where you'd like to use C++ exceptions most of all are the constructors, but you need to be very careful in order to be able to do it).

Workaround: there is no real workaround, of course, or the exceptions wouldn't have been added to the language. However, there are several rules which might help here:

1. Every function returns an integer (or at least boolean) error code.

   There is no such thing as a function that never fails - even if it can't fail now, it might do it later, when modified to be more powerful/general. Put the int or bool return type from the very beginning!

2. Every function you call may fail - check the return code!

   Never rely on the function's success, always test for a possible error.

3. Tell the user about the error, don't silently ignore them.

   Exceptions are always caught and, normally, processed when they're caught. In the same manner, the error return code must always be processed somehow. You may choose to ignore it, but at least tell the user that something wrong happened using wxLogError or wxLogWarning functions. All wxWindows functions (must) log the error messages on failure - this can be disabled by using wxLogNull object before calling it.

   Examples:

   • Wrong:

     void ReadAddressBookFile(const wxString& strName)
     {
       wxFile file;
     
       if ( !file.Open(strFile) )
         return;
     
       ...process it...
     }
             

   • Correct:

     // returns false if the address book couldn't be read
     bool ReadAddressBookFile(const wxString& strName)
     {
       wxFile file;
     
       if ( !file.Open(strFile) ) {
         // wxFile logged an error because file couldn't be opened which
         // contains the system error code, however it doesn't know what
         // this file is for and an error message "can't open $GLCW.ADB"
         // can be quite confusing for the user. Here we say what we mean.
         wxLogError("Can't read address book from '%s'!",
     	       strName.c_str());
         return false;
       }
     
       ...process it...
     
       return true;
     }
     	

     or, if it's not an error if file doesn't exist (here we could just check its existence, but let's suppose that there is no wxFile::Exists()) we can also write:

     // returns false if address book file doesn't exist
     bool ReadAddressBookFile(const wxString& strName)
     {
       wxFile file;
     
       // start a block inside which all log messages are suppressed
       {
         wxLogNull noLog;
         if ( !file.Open(strFile) )
           return false;
       }
     
       ...process it...
     
       return true;
     }
             

3. Don't use RTTI

RTTI stands for Run-Time Type Information and there is probably no other reason not to use it except the portability issue and the fact that it adds sizeof(void *) bytes to any class having virtual functions (at least, in the implementations I'm aware of).

Workaround: use wxWindows RTTI system which allows you to do almost everything which the new C++ RTTI, except that, of course, you have to use macros instead of the (horrible looking, BTW) dynamic_cast.

4. Don't use namespaces

This topic is subject to change with time, however for the moment all wxWindows classes/functions live in the global namespace.

Workaround: None.

5. Don't use STL

STL is the new C++ standard library, proposing all kinds of template containers and generic algorithm implementations. Templates are the heart (and almost everything else) of the library, so its usage is out of question. Besides, even with the compilers which do support templates, STL has many of its own problems, there are many "not 100% standard compatible" vendor implementations, none of existing debuggers understands its complicated data structures, ... the list can go on (almost) forever.

Workaround: Use wxString, dynamic arrays and lists and other wxWindows classes. wxString has many of the most often used functions of std::string STL class (typedef to be precise).

6. Don't declare variables inside for()

The scope of a variable declared inside for() statement changed several years ago, however many compilers still will complain about second declaration of i in the following code:

  for ( int i = 0; i < 10; i++ ) {
    ...
  }

  ...

  for ( int i = 0; i < 10; i++ ) {
    ...
  }

Even if it's perfectly legal now.

Workaround: write this instead:

  int i;
  for ( i = 0; i < 10; i++ ) {
    ...
  }

  ...

  for ( i = 0; i < 10; i++ ) {
    ...
  }

7. Don't use nested classes

Nested classes are, without doubt, a very good thing because they allow to hide "private" (in the sense that they're used only inside the library) classes and, generally, put the related things together.

Unfortunately, some compilers have trouble understanding them, so we must sacrifice the ideals of software design to get a working program in this case.

Workaround: instead of

  // in the header
  class PublicLibClass {
  ...
  private:
    class PrivateLibClass { ... } m_object;
  };

you can try the following:

  // in the header
  class PrivateLibClass; // fwd decl
  class PublicLibClass {
  ...
  private:
   class PrivateLibClass *m_pObject;
  };

  // in the .cpp file
  class PrivateLibClass { ... };

  PublicLibClass::PublicLibClass()
  {
    m_pObject = new PrivateLibClass;

    ...
  }

  PublicLibClass::~PublicLibClass()
  {
    delete m_pObject;
  }

A nice side effect is that you don't need to recompile all the files including the header if you change the PrivateLibClass declaration (it's an example of a more general interface/implementation separation idea).



• Other compiler limitations

This section lists the less obvious limitations of the current C++ compilers which are less restrictive than the ones mentioned in the previous section but are may be even more dangerous as a program which compiles perfectly well on some platform and seems to use only standard C++ featurs may still fail to compile on another platform and/or with another compiler.

1. Use ternary operator ?: carefully

The ternary operator ?: shouldn't be used with objects (i.e. if any of its operands are objects) because some compilers (notable Borland C++) fail to compile such code.

Workaround: use if/else instead.

    wxString s1, s2;

    // Borland C++ won't compile the line below
    wxString s = s1.Len() < s2.Len() ? s1 : s2;

    // but any C++ compiler will compile this
    wxString s;
    if ( s1.Len() < s2.Len() )
        s = s1;
    else
        s = s2;

2. Don't use initializers with automatic arrays

The initializers for automatic array variables are not supported by some older compilers. For example, the following line

    int daysInMonth[12] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

will fail to compile with HP-UX C++ compiler.

Workaround: either make the array static or initialize each item separately: in the (stupid) example above, the array should be definitely declared as static const (assuming that the leap years are dealt with elsewhere somehow...) which is ok. When an array is really not const, you should initialize each element separately.

3. Always have at least one constructor in a class with destructor

It is a good rule to follow in general, but some compilers (HP-UX) enforce it. So even if you are sure that the default constructor for your class is ok but it has a destructor, remember to add an empty default constructor to it.



• General recommendations

While the recommendations in the previous section may not apply to you if you're only working with perfect compilers which implement the very newest directives of C++ standard, this section contains compiler- (and language-) independent advice which must be followed if you wish to write correct, i.e. working, programs. It also contains some C/C++ specific remarks in the end which are less important.

1. No C++ comments in C code>

Never use C++ comments in C code - not all C compilers/preprocessors understand them. Although we're mainly concerned with C++ here, there are several files in wxWindows sources tree which are compiled with C compiler. Among them are include/wx/setup.h and include/wx/expr.h. Another thing related to C vs C++ preprocessor differences is that some old C preprocessors require that all directives start in the first column (while it's generally allowed to have any amount of whitespace before them in C++), so you should start them in the beginning of the line in files which are compiled with C compiler.

2. No global variables with constructors

In C++, the constructors of global variables are called before the main() function (or WinMain() or any other program entry point) starts executing. Thus, there is no possibility to initialize anything before the constructor call. The order of construction is largely implementation-defined, meaning that there is no guarantee that one global object will be initialized before another one (except if they are both defined in the same translation unit, i.e. .cpp file). Most importantly, no custom memory allocation operators are installed at the moment of execution of global variables constructors, so a (less restrictive) rule is that you should have no global variables which allocate memory (or do anything else non-trivial) in the constructor. Of course, if an object doesn't allocate memory in its constructor right now, it may start making it later, so you can only be sure about this if you don't use any variables of object (as opposed to simple: int, ...) types. Example: currently, wxString doesn't allocate memory in its default constructor, so you might think that having a global (initially) empty wxString is safe. However, if wxString starts allocating some minimal amount of memory in its default constructor (which doesn't look unreasonable), you would have all kinds of problems with new and delete operators (overloaded in wxWindows), especially because the first new called is the standard one (before wxWindows overloads them) and delete will be the overloaded operator.

3. Turn on all warnings and eradicate them

Give the compiler a chance to help you - turn on all warnings! You should always use the maximum available warning level of your compiler and understand and correct each of them. If, for whatever reasons, a compiler gives a warning on some perfectly legal line of code and you can't change it, please insert a comment indicating it in the code. Most oftenly, however, all compiler warnings may be avoided (not suppressed!) with minimal changes to your code.

4. Don't rely on sizeof(int) == 2...

You should never assume any absolute constraints on data type sizes. Currently, we have 16-bit, 32-bit and 64-bit machines and even inside each class data type sizes are different. A small table illustrates it quite well:

Architecture/OS	sizeof(short)	sizeof(int)	sizeof(long)	sizeof(void *)
i386/Windows 3.1	2	2	4	2 or 4
i386/Windows 95	2	4	4	4
Merced/Win64	2	4	4	8
Alpha/Linux	???	???	???	???

5. No assignments in conditional expressions

Although close to the heart of many C programmers (I plead guilty), code like classical if ( (c = getchar()) != EOF ) is bad because it prevents you from enabling "assignment in conditional expression" warning (see also above) which is helpful to detect common mistypes like if ( x = 2 ) instead of if ( x == 2 ).

6. Use #if 0 rather than comments to temporarily disable blocks of code

If you have to temporarily disable some code, use

  #if 0 // VZ: I think this code is unneeded, it probably must be removed
    ...
  #endif // 0

instead of

  /*
    ...
  */

The reason is simple: if there are any /* ... */ comments inside ... the second version will, of course, miserably fail.
7. Avoid overloaded virtual functions

You should avoid having overloaded virtual methods in a base class because if any of them is overriden in a derived class, then all others must be overriden as well or it would be impossible to call them on an object of derived class. For example, the following code:

    class Base
    {
    public:
        virtual void Read(wxFile& file);
        virtual void Read(const wxString& filename);
    };

    class Derived : public Base
    {
    public:
        virtual void Read(wxFile& file) { ... }
    };

    ...

    Derived d;
    d.Read("some_filename");    // compile error here!

will fail to compile because the base class function taking filename is hidden by the virtual function overriden in the derived class (this is known as [virtual] function name hiding problem in C++).

The standard solution to this problem in wxWindows (where we have such situations quite often) is to make both Read() functions not virtual and introduce a single virtual function DoRead(). Usually, it makes sense because the function taking a filename is (again, usually) implemented in terms of the function reading from a file anyhow (but making only this functions not virtual won't solve the above problem!).

So, the above declarations should be written as:

    class Base
    {
    public:
        void Read(wxFile& file);
        void Read(const wxString& filename);

    protected:
        virtual void DoRead(wxFile& file);
    };

    class Derived : public Base
    {
    protected:
        virtual void DoRead(wxFile& file) { ... }
    };

This technique is widely used in many of wxWindows classes - for example, wxWindow has more than a dozen of DoXXX() functions which allows to have many overloaded versions of commonly used methods such as SetSize()
8. Don't use extra semi-colons on top level

Some compilers don't pay any attention to extra semicolons on top level, as in

  class Foo { };;

while others complain loudly about it. Of course, you would rarely put 2 semicolons yourself, but it may happen if you're using a macro (IMPLEMENT_something, for example) which already has a ';' inside and put another one after it.


• Unix/DOS differences

Two operating systems supported by wxWindows right now are (different flavours of) Unix and Windows 3.1/95/NT (although Mac, OS/2 and other ports exist/are being developed as well). The main differences between them are summarized here.

1. Use .cpp for C++ source file extension

There is, unfortunately, no standard exceptions for C++ source files. Different people use .C, .cc, .cpp, .cxx, .c++ and probably several others I forgot. Some compilers don't care about extension, but there are also other ones which can't be made to compile any file with "wrong" extension. Such compilers are very common in DOS/Windows land, that's why the .cpp extension is the least likely to cause any problems - it's the standard one under DOS and will probably be accepted by any Unix compiler as well (any counter examples?). The extension for the header files is .h.

2. Don't use backslash ('\\') in #includes

Although it's too silly to mention, please don't use backslashes in #include preprocessor statement. Even not all Windows compilers accept it, without speaking about all other ones.

3. Avoid carriage returns in cross-platform code

This problem will hopefully not arise at all, with CVS taking care of this stuff, however it's perhaps not useless to remember that many Unix compilers (including, but not limited to, gcc) don't accept carriage returns (= = '\r') in C/C++ code.

4. Use only lower case filenames

DOS/Windows 3.1 isn't case sensitive, Windows 95/NT are case preserving, but not case sensitive. To avoid all kinds of problems with compiling under Unix (or any other fully case-sensitive OS), please use only lower case letters in the filenames.

5. Terminate the files with a new-line

While DOS/Windows compilers don't seem to mind, their Unix counterparts don't like files without terminating new-line. Such files also give a warning message when loaded to vim (the Unix programmer's editor of choice :-)), so please think about terminating the last line.

6. Avoid globals differing by case only

The linker on VMS is case-insensitive. Therefore all external variables and functions which differ only in case are not recognized by the linker as different, so all externals should differ in more than the case only: i.e. GetId is the same as .



• Style choices

All wxWindows specific style guidelines are specified in the next section, here are the choices which are not completely arbitrary, but have some deeper and not wxWindows-specific meaning.

1. Naming conventions: use m_ for members

It's extremely important to write readable code. One of the first steps in this direction is the choice of naming convention. It may be quite vague or strictly define the names of all the variables and function in the program, however it surely must somehow allow the reader to distinguish between variable and functions and local variables and member variables from the first glance.

The first requirement is commonly respected, but for some strange reasons, the second isn't, even if it's much more important because, after all, the immediate context usually allows you to distinguish a variable from a function in C/C++ code. On the other hand, you cannot say what x in the following code fragment is:

  void Foo::Bar(int x_)
  {
    ...

    x = x_;

    ...
  }

It might be either a local variable (unluckily the function is too long so you don't see the variable declarations when you look at x = x_ line), a member variable or a global variable - you have no way of knowing.

The wxWindows naming convention gives you, the reader of the code, much more information about x. In the code above you know that it's a local variable because:

1. global variables are always prefixed with g_
2. member variables are always prefixed with m_
3. static variables are always prefixed with s_

Examples:

  extern int g_x; // of course, 'x' is not the best name for a global...

  void Bar()
  {
    int x;
  }

  class Foo {
    public:
      void SetX(int x) { m_x = x; }
    private:
      int m_x;
  };

As you see, it also solves once and for all the old C++ programmer's question: how to call SetX() parameter? The answer is simple: just call it x because there is no ambiguity with Foo::m_x.

The prefixes can be combined to give ms_ and gs_ for static member (a.k.a. class) variables and static global variables.

The convention is, of course, completely worthless if it is not followed: nothing like being sure that x is a local variable in the code fragment above and discovering later the following lines in the header:

  class Foo {
    ...
    int x;  // I don't like wxWindows naming convention
  };

Please do use these prefixes, they make your code much easier to read. Also please notice that it has nothing to do with the so-called Hungarian notation which is used in wxMSW part of wxWindows code and which encodes the type of the variable in its name - it is actually quite useful in C, but has little or no sense in C++.
2. Don't use void for functions without arguments

In ANSI C, void Foo() takes an arbitrary number of arbitrarily typed arguments (although the form void Foo(...) is preferred) and void Foo(void) doesn't take any arguments. In C++, however, the situation is different and both declarations are completely equivalent. As there is no need to write void in this situation, let's not write it - it can only be confusing and create an impression that it really means something when it's not at all the case.

3. Don't use const for non pointer/reference arguments

In both C and C++ an argument passed by value cannot be modified - or, more precisely, if it is modified in the called function, only the local copy is really changed, not the caller's variable. So, semantically speaking, there is no difference between void Foo(int) and void Foo(const int). However, the const keyword is confusing here, adds nothing to the code and even cannot be removed if Foo() is virtual and overridden (because the names are mangled differently). So, for arguments passed by value you shouldn't use const.

Of course, it doesn't apply to functions such as void PrintMessage(const char *text) where const is mandatory.