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wxWidgets - Cross-Platform GUI Library

Coding Guidelines

This guide is intended for people who are (or intending to start) writing code for inclusion into the wxWidgets library. The guidelines here do not need to be followed when writing applications using wxWidgets as they typically don't have to deal with all the platforms that the library itself supports, notably there is usually absolutely no reason to avoid the use of exceptions or the standard library in the application code.

Also notice that some parts of the existing code may not follow these guidelines, because it could have been written before the guidelines were put in place. However this is not a reason to not follow them for any new code. In other words, in case of a conflict between this document and the existing code, this document should be considered authoritative.

Physical Files Organization

Use .cpp for C++ source file extension

Use .cpp extension for the C++ files and .h for the headers.

File locations

All headers are found under the directory include/wx, both the public and private ones. All source file are under the directory src.

Both of these directories contain port-specific subdirectories, such as include/wx/msw or src/gtk. If a class has only a single implementation for all platforms, it is declared in a single include/wx/foo.h header and implemented in a single src/common/foo.cpp file, i.e. src/common subdirectory should be used for platform-independent code when there is no platform-specific version.

A more common case is to have several different versions of the class for different platforms. In this case, the header include/wx/foo.h is still the only header which can be directly included but it may and typically does include platform-specific include/wx/port/foo.h headers internally (where port is one of msw, gtk, osx, ...). The corresponding implementation files are src/port/foo.cpp and the common, not platform-specific, part, if any, is in src/common/foocmn.cpp.

Additionally, a GUI class can have a "generic" implementation, that is one written in wxWidgets itself instead of using the platform-specific API. Such implementation is declared in include/wx/generic/foo.h file (which is still never included directly, but only from include/wx/foo.h) and defined in src/generic/foog.cpp (notice the extra "g" at the end, to avoid confusion with the native implementation called foo.cpp). In some cases, you may notice that a class has only the generic implementation, which seems to contradict this explanation, but actually just means that we plan to add one or more native implementations in the future -- and this just hasn't happened yet.

Sometimes it can be useful to have a private header, i.e. one used only in the implementation of wxWidgets but not by the outside code using the library. Such files should be placed under include/wx/private or include/wx/port/private for port-specific ones.

Reference API documentation is in the files under interface/wx directory. The rest of the manual is found under docs/doxygen.

Standard file header

All wxWidgets files should start with the following standard header:

/////////////////////////////////////////////////////////////////////////////
// Name:        src/common/foo.cpp
// Purpose:     Implementation of wxFoo
// Author:      Your Name <your@email.address>
// Created:     2014-02-07
// Copyright:   (c) 2014 wxWidgets development team
// Licence:     wxWindows licence
/////////////////////////////////////////////////////////////////////////////

Notice that the date should be in ISO format (YYYY-MM-DD). The licence must always be specified as wxWindows licence (not "wxWidgets licence") while the copyright depends on the actual person or organization having contributed the code.

Include guards

Any header include/wx/foo/bar.h must wrap its contents (i.e. everything after the standard header comment) in an include guard:

#ifndef _WX_FOO_BAR_H_
#define _WX_FOO_BAR_H_

// ... all header contents ...

#endif // _WX_FOO_BAR_H_

Formally _WX prefix makes this prefix a reserved word, however this style is used in wxWidgets since 20+ years and hasn't created any problems so far, so we are going to continue using it.

Precompiled headers

A source file should always start by including the precompiled header file wx/wxprec.h which includes most of the commonly used wxWidgets headers if precompiled headers support is used. If it isn't, the individual headers actually used by the code need to be included inside WX_PRECOMP test. And headers not included by wx/wxprec.h need to be included in any case.

#include "wx/wxprec.h"

#ifdef __BORLANDC__
#pragma hdrstop
#endif

#ifndef WX_PRECOMP
#include "wx/msgdlg.h" // This header is normally included from wx/wx.h
#endif

#include "wx/richmsgdlg.h" // But this one never is

Use WXDLLIMPEXP_XXX with all public classes and functions

All public types and data must be declared with WXDLLIMPEXP_XXX in their declaration to be usable outside of the wxWidgets DLL. XXX here can be one of BASE, CORE, ADV and so on depending on the library the symbol is exported from, see include/wx/dlimpexp.h for the full list. For example:

bool WXDLLIMPEXP_BASE wxSomeUtilityFunction();
class WXDLLIMPEXP_CORE wxSomeGUIClass { ... };
WXDLLIMPEXP_DATA_CORE(extern wxApp*) wxTheApp;

Use only lower case filenames

To avoid conflicts between case-sensitive and case-insensitive (even if case-preserving) file systems, only use lower case ASCII letters (and numbers if necessary) in the file names.

Naming Conventions

The guidelines in this section describe the conventions used for naming identifiers in wxWidgets code, please follow them to avoid surprising people reading it.

Here is a quick example summarizing all the conventions described below:

extern void wxGlobalFunction();

enum wxFileOpenMode
{
    wxFILEOPEN_READ,
    wxFILEOPEN_WRITE,
    wxFILEOPEN_READWRITE
};

void wxCountXs()
{
    static int s_x;
    #ifdef wxSOMETHING_OR_OTHER
        s_x++;
    #endif
}

class wxSomeClass
{
public:
    int GetSomeX() const { return m_someX; }
    void SetSomeX(int x) { m_someX = x; }
private:
    int m_someX;
};

Use wx prefix for all public symbols

The prefix wx must be used for all public classes, functions, constants and macros, no exceptions.

Some old macros and a very few extremely old functions don't have this prefix, but all new public identifiers should use it. Of course, class members don't need to follow this rule as they don't have global visibility.

Use CamelCaseFunctions and camelCaseVariables

Use CamelCase for types (classes, structs, enums, unions), methods and functions and camelCase for the variables.

Use UPPER_CASE for constants

Use UPPER_CASE for constants, whether they are enum elements or preprocessor macros (the latter should be avoided if possible).

The same convention is also used for the macros, after taking the wx prefix into account.

Use m_ prefix for member variables

All member variables should have m_ prefix to distinguish them from local variables.

Possible exception: simple structs, not having any methods (except, possibly the constructor), may not use this prefix (rationale: there is no ambiguity as to what kind of variable this is if it's always used via struct object, pointer or reference).

Static variables should have s_ prefix to allow to easily grep for them.

Member static variables combine both rules above and use ms_ prefix (rationale for choosing this over sm_: grepping for s_ to check for possible multi-threading problems finds them with this naming convention).

Global variables shouldn't normally be used at all, but if they are, should have g_ prefix.

Exception: public global variables follow the rule above and so use wx prefix instead of g_ one. They also typically use The as part of their name, e.g. wxTheApp or wxTheClipboard.

Use Set/Get prefixes for accessors

There is a convention in wxWidgets to prefix the accessors (i.e. any simple, in general, inline function which does nothing else except changing or returning the value of a member variable) with either Set or Get.

On the contrary, do not use Get prefix for a function which is non-trivial, i.e. does something else than simply returning the internally stored value.

Coding Style

As any style, the guidelines here are at least partly subjective (although some of them do have an objective rationale), but please follow them even if they contradict your personal preferences to ensure consistent appearance of all the code in the library.

Indent your code with 4 spaces (no tabs!)

Configure your editor to use spaces, not TABs, for indentation and use 4 spaces per indentation level.

If you use Emacs, you can put the following in your .emacs file to help get indentation right:

;; CC-mode setup
(defun my-c-mode-common-hook ()
  ;; my customizations for all of c-mode, c++-mode, and objc-mode
  (c-set-offset 'substatement-open 0)
  (c-set-offset 'case-label 1)
  (c-set-offset 'defun-block-intro 4)
  (c-set-offset 'access-label -2)
  (setq c-recognize-knr-p nil)
  (setq c-basic-offset 4)
  (setq-default indent-tabs-mode nil)
;; Set the return key to autoindent
  (local-set-key "\C-m" 'reindent-then-newline-and-indent)
)
(add-hook 'c-mode-common-hook 'my-c-mode-common-hook)

Mattia Barbon suggests this:

(setq-default indent-tabs-mode nil)

(setq c-default-style
  '( ( c++-mode . "stroustrup" )
  ( c-mode . "stroustrup" ) )
)

Robin Dunn has this alternative:

(defun my-c-mode-common-hook ()
  ""
  (interactive)
  (c-set-style "bsd")
  ;;
  ;; configure vars for cc-mode based items
  ;;
  (setq c-auto-newline nil
        c-tab-always-indent nil
        c-basic-offset 4
        c-comment-only-line-offset 0
        c-hanging-comment-ender-p nil
        c-hanging-comment-starter-p nil
        c-comment-continuation-stars "** "
        c-cleanup-list (list 'empty-defun-braces
                                           'defun-close-semi
                                           'list-close-comma
                                           'scope-operator
                                           )
        compilation-ask-about-save nil
        compilation-read-command t
        compilation-scroll-output t
        fill-column 78
        comment-column 40
        tab-width 8
        indent-tabs-mode nil
        )

  (c-toggle-auto-hungry-state -1)
  (local-set-key "\C-m" 'newline-and-indent)
  (local-set-key "\C-cc" 'comment-region)
  (setq c-electric-pound-behavior '(alignleft))
  )

(add-hook 'c-mode-common-hook 'my-c-mode-common-hook)

If you use vi(m), you can add the following line to your ~/.vimrc or ~/.exrc:

au BufReadPost */wxWidgets/*/*.cpp,*/wxWidgets/*/*.h set sw=4 ts=4 et cin

(et and cin don't exist in plain vi)

Use 80 character lines

Please keep the line length under 80 characters.

Rationale: even with modern wide screen monitors, it is difficult to review side by side diffs for lines longer than that. Too long lines also often indicate some problem in the code, such as excessive nesting (in this case you should simplify the logic or refactor the function by extracting the inner parts in separate functions) or too complex expressions (in this case, simplify them by using named variables for the intermediate results).

Put opening curly bracket on its own line

Don't leave { on the preceding line, always put it on its own line, e.g.

if ( condition )
{
    if-statements;
}
else
{
    else-statements;
}

Use spaces with keywords

Use a space after a keyword such as if, for, while or switch and around the expression used by this keyword. Do not use spaces in these positions for the function calls but still use them to separate function arguments. For example:

for ( init; condition; loop )
{
    switch ( expression )
    {
        case 0:
            CallSomeFunction(with, some, arguments);
            break;
    }
}

Order of parts in a class declarations

By convention, please put the public parts of the class first, then protected, then private. The idea is to be able to quickly see the public API of a class without having to read through uninteresting implementation details in the public sections.

Also, please put all wxWidgets macros which must be used in the class declaration (such as wxDECLARE_EVENT_TABLE) at the end as, again, these are implementation details and shouldn't get in the way of the person who is reading the file.

Use // for most comments

Use C++ style // comments for most of them, not only single line ones. Reserve C style /* ... */ comments only for high level, very long explanatory comments. When using C style comments do not use any intermediate asterisks to make the comments easier to reformat.

Please always put a space after the beginning of a C++ comment, e.g. use // Whatever instead of //Whatever.

Notice that wxWidgets does not use Doxygen style comments (/** ... */ or ///) in its sources, these comments are only used in the special interface headers from which the API documentation is generated.

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.

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.

Use NULL rather than 0

We have chosen to use the standard NULL macro in the expressions involving the pointers instead of just the constant 0. Although both are perfectly equivalent in this context, we feel that using NULL provides better readability and stands out better.

A related advice is to use '\0' instead of 0 in the expressions involving char types.

Notice that nullptr can't be used in wxWidgets code as it must remain compatible with non-C++-11 compilers.

Use wxOVERRIDE for overridden virtual methods

When overriding a virtual method of the base class, use wxOVERRIDE in its declaration like this:

class Base
{
public:
        virtual void DoSomething();
};

class Derived : public Base
{
public:
        void DoSomething() wxOVERRIDE;
};

wxOVERRIDE macro will be expanded into override keyword if the compiler supports it (i.e. C++11) or nothing otherwise.

Notice that the virtual keyword is redundant as wxOVERRIDE already identifies the method as being virtual, so it can be omitted, even in most of the existing code both keywords are used together as wxOVERRIDE is a relatively recent addition and was just added to the existing method declarations.

Use wxFALLTHROUGH in switch statements

Use the special wxFALLTHROUGH macro in case of intentional fall through to the next case clause in a switch statement:

bool vertical = false,
     positive = true;
switch ( keycode )
{
    case WXK_UP:
        vertical = true;
        wxFALLTHROUGH;

    case WXK_LEFT:
        positive = false;
        break;

    ...
}

The use of this macro allows to enable warnings given by some compilers (notably Clang) about unintentional fall through, which is a common bug.

Restrictions on C++ features

Don't use exceptions

wxWidgets doesn't use exceptions, mostly for backwards compatibility reasons: the existing code doesn't expect wxWidgets functions to throw exceptions, as they had never been thrown before and so could be broken if they started now. It is also sometimes useful to compile the code without exceptions support, especially for embedded platforms where the resulting savings in the code size and speed can be noticeable.

In any case, as exceptions are not used for error reporting, the failure of the functions in wxWidgets API must be indicated by a special return value, e.g. false for functions returning bool or NULL for the functions returning pointers to objects. The special case of constructors, which have no return value, is typically handled by providing an IsOk() method indicating whether the object was constructed successfully and, for all wxWindow-derived classes, also by using two-step construction.

Don't use RTTI

wxWidgets doesn't use standard C++ RTTI, i.e. dynamic_cast<> and typeid shouldn't be used in its code. This is done to allow compiling code which disables RTTI support for performance reasons and also because wxWidgets has its own wxRTTI mechanism for wxObject-derived classes which is necessary anyhow, as, unlike the standard C++ RTTI, it also allows creation of C++ objects by name.

So wxDynamicCast() should be used instead of dynamic_cast<> (assuming it needs to be used at all) and wxObject::GetClassInfo() instead of typeid.

Notice that static_cast<>, const_cast<> and reinterpret_cast<> can be used as they don't require any run-time support.

Don't use many standard library classes

To allow creation of self-contained applications which don't depend on C++ standard library, wxWidgets still supports being built in a way which avoids all standard library dependencies.

This means that the standard container, string, stream and other classes should not be used inside the library itself. The most commonly used standard library class is probably std::vector<> and wxWidgets provides wxVector<> covering 99% of its features and which can be used instead -- and preferred to the old style macro-based container classes.

Don't use new logical operators keywords

While not widely known, C++ standard defines the following reserved words: or, and, not, xor, bitand, bitor, compl, and_eq, or_eq, not_eq, or_eq which can be used instead of the usual C operations &&, ||, ~, etc.

Support for these operators varies among compilers and some of them don't support them at all while others require special options to enable their use. For this reason, and also because there is absolutely no advantage in using these keywords compared to the traditional operators, as well as for the reasons of consistency, these keywords must not be used in wxWidgets sources.

Code Quality

The recommendations in this section are much less subjective and describe best practices when writing C++ code which should be followed in most, if not all, large scale C++ projects, and thus apply to wxWidgets.

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 often, however, all compiler warnings may be avoided (not suppressed!) with minimal changes to your code.

Avoid C style casts

Prefer C++ static_cast<> and const_cast<> to the C style casts if they can be used to replace them. Also use reinterpret_cast<> (while trying to avoid it as much as possible, of course), although C style casts are allowed for some low level code with a lot of unavoidable casts where using reinterpret_cast<> can make it even less readable.

Notice that dynamic_cast<> should not be used in wxWidgets code, see the RTTI section.

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 ).

Avoid default in switches over enums

Include cases for all enum elements and do not include the default case in switch statements over the values of an enum type. This ensures that the compiler will be able to warn if a newly added enum element is not handled by the switch.

For example, replace this code:

bool ConfirmOverwrite(wxFile::OpenMode mode)
{
    switch ( mode )
    {
        case wxFile::write:
        case wxFile::write_append:
        case wxFile::write_excl:
            // Ask the user about overwriting the file.
            break;

        default:
            return true;
    }

    return wxMessageBox("Overwrite?", ...) == wxID_YES;
}

with this, semantically equivalent, version:

bool ConfirmOverwrite(wxFile::OpenMode mode)
{
    switch ( mode )
    {
        case wxFile::write:
        case wxFile::write_append:
        case wxFile::write_excl:
            return wxMessageBox("Overwrite?", ...) == wxID_YES;

        case wxFile::read:
        case wxFile::read_write:
            return true;
    }

    wxFAIL_MSG("Unreachable");

    return true; // Still needed to pacify some compilers.
}

See debug macros section for more about the use of wxFAIL_MSG().

Avoid overloaded virtual functions

Avoid having overloaded virtual methods in a base class because if any of them is overridden in a derived class, then all others must be overridden as well or it would be impossible to call them on an object of derived class. For example, take 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");

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

The standard solution to this problem in wxWidgets (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 wxWidgets 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().

Don't use boolean function arguments

Using boolean parameters in the public function usually is a bad API design idea for two reasons which are explained below using the example of wxWidgets wxExecute function used to launch an external process but which are, in fact, quite general:

  1. Readability:

    When you see wxExecute(TRUE) or wxExecute(FALSE) you have absolutely no idea what does it mean. Even if you know (from reading the documentation) that the parameter specifies whether the program execution should be synchronous or not, you still don't know which one is which: TRUE and FALSE here are just the "bare numbers" which don't help the person reading the code to understand what do they stand for. The solution is the same for the other bare numbers: replace them with the symbolic constants. Indeed, when you see wxExecute(wxEXEC_ASYNC) you have no troubles understanding it on the first reading nor is there any danger of confusing the sync and async calls.

  2. Inextensible:

    Suppose now that you want to add another parameter to wxExecute(), for example whether a new console window should be opened, you have to add another parameter. And if it is bool as well you end up with such clear and expressible pearls as wxExecute(TRUE, FALSE).

    The solution is simple: replace bool with int and add some flags with symbolic names. This solves both problems at once: not only you can avoid adding a new parameter to the function (which is especially important if it already has two or three of them) but also a call such as wxExecute(wxEXEC_ASYNC | wxEXEC_NEW_CONSOLE) is as readable as it gets.

    The only situations in which bool parameters may be used are those where their meaning is immediately clear from the function purpose, for example it is not unreasonable to have a Show(bool show = TRUE) function which either shows or hides something. But in vast majority of the cases, the meaning is not immediately clear to the reader and using the symbolic flag constants is a better alternative.

Use forward declarations whenever possible

It's a really trivial piece of advice, but remember that using forward declarations instead of including the header of corresponding class is better because not only does it minimize the compile time, it also simplifies the dependencies between different source files.

On a related subject, in general, you should try not to include other headers from a header file.

Use debugging macros

wxWidgets provides the debugging macros wxASSERT, wxFAIL and wxCHECK_RET in file. Please use them as often as you can -- they will never do you any harm but can greatly simplify the bug tracking both for you and for others.

Also, please use wxFAIL_MSG("not implemented") instead of writing stubs for not (yet) implemented functions which silently return incorrect values -- otherwise, a person using a not implemented function has no idea that it is, in fact, not implemented.

As all debugging macros only are deactivated when NDEBUG is defined, build your code in debug version, i.e. without /DNDEBUG, when testing to give them a chance to help you.

Message and documentation conventions

These apply to messages in the wxWidgets source, as well as to wxWidgets documentation.

Use well-formed language for messages, not abbreviations or pseudo-English

Always starts a sentence with a capital letter

Always end a sentence with a period or other appropriate punctuation.

Never use "..." unless absolutely necessary.

Avoid "!" wherever possible

It's rarely necessary to emphasise a point this much (the same applies to documentation).

Use only one space after a period and before a new sentence.

Don't add a lone space after a period if it's not going to be followed by another sentence.

Don't use unnecessary parentheses in messages and documentation.

Instead, rearrange the sentence.