Coding-Conventions

Qt Coding Conventions

This is an overview of the high-level coding conventions we use when writing Qt code.
See Qt_Coding_Style for the lower-level conventions.

C++ features

• Don’t use exceptions
• Don’t use rtti (Run-Time Type Information; that is, the

  typeinfo
  

  struct, the

  dynamic_cast
  

  or the

  typeid
  

  operators, including throwing exceptions)
• Use templates wisely, not just because you can.

Hint: Use the

compile

autotest to see whether a C++ feature is supported by all compilers in the test farm.

Conventions in Qt source code

• All code is ascii only (7-bit characters only, run

  man ascii
  

  if unsure)
  • Rationale: We have too many locales inhouse and an unhealthy mix of UTF-8 and latin1 systems. Usually, characters > 127 can be broken without you even knowing by clicking SAVE in your favourite editor.
  • For strings: Use

    \nnn
    

    (where

    nnn
    

    is the octal representation of whatever character encoding you want your string in) or

    \xnn
    

    (where

    nn
    

    is hexadecimal). Example:

    QString s = QString::fromUtf8(”\213\005”);
    

  • For umlauts in documentation, or other non-ASCII characters, either use qdoc’s \unicode command or use the relevant macro; e.g. \uuml for ü
• Every QObject subclass must have a

  Q_OBJECT
  

  macro, even if it doesn’t have signals or slots, otherwise

  qobject_cast
  

  will fail.
• Normalize the arguments for signals + slots (see

  QMetaObject::normalizedSignature
  

  ) inside connect statements to get faster signal/slot lookups. You can use

  $QTDIR/util/normalize
  

  to normalize existing code.

Including headers

• In public header files, always use this form to include Qt headers:

  #include <QtCore/qwhatever.h>
  

  . The library prefix is neccessary for Mac OS X frameworks and is very convenient for non-qmake projects.
• In source files, include specialized headers first, then generic headers. Separate the categories with empty lines.

• If you need to include

  qplatformdefs.h
  

  , always include it as the first header file.
• If you need to include private headers, be careful. Then use the following syntax. It doesn’t matter in which module or directory whatever_p.h is in.

• If you need to include

  qt_x11_p.h
  

  , always include it as the last header file.

Casting

• Avoid C casts, prefer C++ casts (

  static_cast
  

  ,

  const_cast
  

  ,

  reinterpret_cast
  

  )
  • Rationale: Both reinterpret_cast and C-style casts are dangerous, but at least reinterpret_cast won’t remove the const modifier
• Don’t use

  dynamic_cast
  

  , use

  qobject_cast
  

  for QObjects or refactor your design, for example by introducing a type() method (see QListWidgetItem)
• Use the constructor to cast simple types:

  int(myFloat)
  

  instead of

  (int)myFloat
  

  • Rationale: When refactoring code, the compiler will instantly let you know if the cast would become dangerous.

Compiler/Platform specific issues

• Be extremely careful when using the questionmark operator. If the returned types aren’t identical, some compilers generate code that crashes at runtime (you won’t even get a compiler warning).

• Be extremely careful about alignment.
  • Whenever a pointer is cast such that the required alignment of the target is increased, the resulting code might crash at runtime on some architectures. For example, if a

    const char *
    

    is cast to an

    const int *
    

    , it’ll crash on machines where integers have to be aligned at a two- or four-byte boundaries.
  • Use a union to force the compiler to align variables correctly. In the example below, you can be sure that all instances of

    AlignHelper
    

    are aligned at integer-boundaries.

• Anything that has a constructor or needs to run code to be initialized cannot be used as global object in library code, since it is undefined when that constructor/code will be run (on first usage, on library load, before main() or not at all). Even if the execution time of the initializer is defined for shared libraries, you’ll get into trouble when moving that code in a plugin or if the library is compiled statically:

Things you can do:

Use

Q_GLOBAL_STATIC

to create static global objects instead:

Note: Static objects in a scope are no problem, the constructor will be run the first time the scope is entered. The code is not reentrant, though.

• A

  char
  

  is signed or unsigned dependent on the architecture. Use

  signed char
  

  or

  uchar
  

  if you explicitely want a signed/unsigned char. The following code will break on ppc, for example:

• Avoid 64-bit enum values.
  • The aapcs embedded ABI hard codes all enum values to a 32-bit integer.
  • Microsoft compilers don’t support 64-bit enum values. (confirmed with Microsoft ® C/C++ Optimizing Compiler Version 15.00.30729.01 for x64)

Aesthetics

• Prefer enums to define constants over

  static const int
  

  or defines.
  • enum values will be replaced by the compiler at compile time, resulting in faster code
  • defines are not namespace safe (and look ugly)
• Prefer verbose argument names in headers.
  • Most IDEs will show the argument names in their completion-box.
  • It will look better in the documentation
  • Bad style:

    doSomething(QRegion rgn, QPoint p)
    

    – use

    doSomething(QRegion clientRegion, QPoint gravitySource)
    

    instead
• When reimplementing a virtual method, do not put the `virtual` keyword in the header file.
  On Qt5, annotate them with the Q_DECL_OVERRIDE [qt.io] macro after the function declaration, just before the ‘;’ (or the ‘{’ ).

Things to avoid

• Do not inherit from template/tool classes
  • The destructors are not virtual, leading to potential memory leaks
  • The symbols are not exported (and mostly inline), leading to interesting symbol clashes.
  • Example: Library A has

    class Q_EXPORT X: public QList<QVariant> {};
    

    and library B has

    class Q_EXPORT Y: public QList<QVariant> {};
    

    . Suddenly, QList<QVariant>‘s symbols are exported from two libraries – /clash/.

• Don’t mix const and non-const iterators. This will silently crash on broken compilers.

• Q[Core]Application is a singleton class. There can only be one instance at a time. However, that instance can be destroyed and a new one can be created, which is likely in an <nop>ActiveQt or browser plugin. Code like this will easily break:

If the

QCoreApplication

application is destroyed,

obj

will be a dangling pointer. Use

Q_GLOBAL_STATIC

for static global objects or

qAddPostRoutine

to clean up.

• Avoid the use of anonymous namespaces in favor of the static keyword if possible. A name localized to the compilation unit with

  static
  

  is guaranteed to have internal linkage. For names declared in anonymous namespaces the C++ standard unfortunately mandates external linkage. (7.1.1/6, or see various discussions about this on the gcc mailing lists)

Binary and Source Compatibility

• Definitions:
  • Qt 4.0.0 is a major release, Qt 4.1.0 is a minor release, Qt 4.1.1 is a patch release
  • Backward binary compatibility: Code linked to an earlier version of the library keeps working
  • Forward binary compatibility: Code linked to a newer version of the library works with an older library
  • Source code compatibility: Code compiles without modification

• Keep backward binary compatibility + backward source code compatibility in minor releases

• Keep backward and forward binary compatibility + forward and backward source code compatibility in patch releases
  • Don’t add/remove any public API (e.g. global functions, public/protected/private methods)
  • Don’t reimplement methods (not even inlines, nor protected/private methods)
  • Check Binary Compatibility Workarounds for ways to keep b/c

• Info on binary compatibility: http://techbase.kde.org/Policies/Binary_Compatibility_Issues_With_C++

• When writing a QWidget subclass, always reimplement event(), even if it’s empty. This makes sure that the widget can be fixed without breaking binary compatibility.

• All exported functions from Qt must start with either ‘q’ or ‘Q’. Use the “symbols” autotest to find violations.

Namespacing

• Read Qt in Namespace and keep in mind that all of Qt except Tests and Webkit is “namespaced” code.

Operators

• The decision between member and non-member [stackoverflow.com]

A binary operator that treats both of its arguments equally should not be a member. Because, in addition to the reasons mentioned in the stack overflow answer, the arguments are not equal when the operator is a member.

Example with QLineF which unfortunately has its operator== as a member:

If the operator== was outside of the class, conversion rules would apply equally for both sides.

Conventions for public header files

Our public header files have to survive the strict settings of some of our users. All installed headers have to follow these rules:

• No C style casts (

  -Wold-style-cast
  

  )
  • Use static_cast, const_cast or reinterpret_cast
  • for basic types, use the constructor form: int(a) instead of (int)a
  • See chapter “Casting” for more info

• No float comparisons (

  -Wfloat-equal
  

  )
  • Use qFuzzyCompare to compare values with a delta
  • Use qIsNull to check whether a float is binary 0, instead of comparing it to 0.0.

• Don’t hide virtual methods in subclasses (

  -Woverloaded-virtual
  

  )
  • If the baseclass

    A
    

    has a

    virtual int val()
    

    and subclass

    B
    

    an overload with the same name,

    int val(int x)
    

    ,

    A
    

    ‘s

    val
    

    function is hidden. Use the

    using
    

    keyword to make it visible again, and add the following silly workaround for broken compilers:

• Don’t shadow variables (

  -Wshadow
  

  )
  • avoid things like

    this->x = x;
    

  • don’t give variables the same name as functions declared in your class

• Always check whether a preprocessor variable is defined before probing its value (

  -Wundef
  

  )

Categories:

• Developing_Qt
  • Guidelines