Qt Coding Style

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This is an overview of the low-level coding conventions we use when writing Qt code. See Coding Conventions for the higher-level conventions.

The data has been gathered by mining the Qt sources, discussion forums, email threads and through collaboration of the developers. Basic formatting (like indentation, use of spaces and newlines) can be fixed by running clang-format; see the section below.

Declaring variables

• Declare each variable on a separate line
• Avoid short or meaningless names (e.g. "a", "rbarr", "nughdeget")
• Single character variable names are only okay for counters and temporaries, where the purpose of the variable is obvious
• Wait when declaring a variable until it is needed

 // Wrong
 int a, b;
 char *c, *d;

 // Correct
 int height;
 int width;
 char *nameOfThis;
 char *nameOfThat;

• Variables and functions start with a lower-case letter. Each consecutive word in a variable's name starts with an upper-case letter
• Avoid abbreviations

 // Wrong
 short Cntr;
 char ITEM_DELIM = ' ';

 // Correct
 short counter;
 char itemDelimiter = ' ';

• Classes always start with an upper-case letter. Public classes start with a 'Q' (QRgb) followed by an upper case letter. Public functions most often start with a 'q' (qRgb).
• Acronyms are camel-cased (e.g. QXmlStreamReader, not QXMLStreamReader).

Braces

• Use attached braces: The opening brace goes on the same line as the start of the statement. If the closing brace is followed by another keyword, it goes into the same line as well:

 // Wrong
 if (codec)
 {
 }
 else
 {
 }

 // Correct
 if (codec) {
 } else {
 }

• Exception: Function implementations (but not lambdas) and class declarations always have the left brace on the start of a line:

 static void foo(int g)
 {
     qDebug("foo: %i", g);
 }

 class Moo
 {
 };

• Function implementations that are = default put the = default on a separate line, indented:

// Wrong
Class::~Class() = default;

// Correct
Class::~Class()
    = default;

• Rationale: Avoids having to touch unrelated lines when switching from user-defined to = default and back.

• Exception: In in-class definitions that are unlikely to ever be anything but = default, place everything on the same line:

class MoveOnly {
   Q_DISABLE_COPY(MoveOnly);
public:
   ~~~
   MoveOnly(MoveOnly &&) = default;          // OK
   MoveOnly &operator=(MoveOnly) = default;  // OK
   ~~~
};

• Use curly braces only when the body of a conditional statement contains more than one line:

 // Wrong
 if (address.isEmpty()) {
     return false;
 }

 for (int i = 0; i < 10; ++i) {
     qDebug("%i", i);
 }

 // Correct
 if (address.isEmpty())
     return false;

 for (int i = 0; i < 10; ++i)
     qDebug("%i", i);

• Exception 1: Use braces also if the parent statement covers several lines / wraps:

 // Correct
 if (address.isEmpty() || !isValid()
     || !codec) {
     return false;
 }

• Exception 2: Brace symmetry: Use braces also in if-then-else blocks where either the if-code or the else-code covers several lines:

 // Wrong
 if (address.isEmpty())
     qDebug("empty!");
 else {
     qDebug("%s", qPrintable(address));
     it;
 }

 // Correct
 if (address.isEmpty()) {
     qDebug("empty!");
 } else {
     qDebug("%s", qPrintable(address));
     it;
 }

 // Wrong
 if (a)
     …
 else
     if (b)
         …

 // Correct
 if (a) {
     …
 } else {
     if (b)
         …
 }

• Use curly braces when the body of a conditional statement is empty

 // Wrong
 while (a);

 // Correct
 while (a) {}

Parentheses

• Use parentheses to group expressions:

 // Wrong
 if (a && b || c)

 // Correct
 if ((a && b) || c)

 // Wrong
 a + b & c

 // Correct
 (a + b) & c

Templates

• Qt has traditionally used a space before the opening < of a template-initializer. But since the addition of the _clang-format file to qt5.git, which incorrectly configured clang-format to drop the space, we have a wild mishmash of styles in Qt. So pick whatever is most prevalent in the file/module you're working on. In QtBase, that would be with space.

// traditional Qt
template <typename T>
void foo();

// new-fangled
template<typename T>
void foo();

Switch statements

• The case labels are in the same column as the switch
• Every case must have a break (or return) statement at the end or use Q_FALLTHROUGH() to indicate that there's intentionally no break, except where there is no code between two case labels.

 switch (myEnum) {
 case Value1:
   doSomething();
   break;
 case Value2:
 case Value3:
   doSomethingElse();
   Q_FALLTHROUGH();
 default:
   defaultHandling();
   break;
 }

Jump statements (break, continue, return, and goto)

• Do not put 'else' after jump statements:

 // Wrong
 if (thisOrThat)
     return;
 else
     somethingElse();

 // Correct
 if (thisOrThat)
     return;
 somethingElse();

• Exception: If the code is inherently symmetrical, use of 'else' is allowed, to make that symmetry more visible.

Spacing

Judicious use of space can make code easier to read.

Indentation

• 4 spaces are used for indentation
• Spaces, not tabs!

Line breaks

• Keep lines shorter than 100 characters; wrap if necessary
  • Comment/apidoc lines should be kept below 80 columns of actual text. Adjust to the surroundings, and try to flow the text in a way that avoids "jagged" paragraphs.
• Commas go at the end of wrapped lines; operators start at the beginning of the new lines. An operator at the end of the line is easy to miss if the editor is too narrow.

 // Wrong
 if (longExpression +
     otherLongExpression +
     otherOtherLongExpression) {
 }

 // Correct
 if (longExpression
     + otherLongExpression
     + otherOtherLongExpression) {
 }

• Use blank lines to group statements together where suited
• Always use only one blank line
• Do not put multiple statements on one line
• By extension, use a new line for the body of a control flow statement:

 // Wrong
 if (foo) bar();

 // Correct
 if (foo)
     bar();

Spaces within code

• Always use a single space after a flow-control keyword and before a curly brace:

 // Wrong
 if(foo){
 }

 // Correct
 if (foo) {
 }

• For pointers or references, always use a single space between the type and '*' or '&', but no space between the '*' or '&' and the variable name:

 char *x;
 const QString &myString;
 const char * const y = "hello";

• Surround binary operators with spaces
• Leave a space after each comma
• No space after a cast (and avoid C-style casts)

 // Wrong
 char* blockOfMemory = (char* ) malloc(data.size());

 // Correct
 char *blockOfMemory = reinterpret_cast<char *>(malloc(data.size()));

General exceptions

• When strictly following a rule makes your code look bad, feel free to break it.
• If there is a dispute in any given module, the Maintainer has the final say on the accepted style (as per The Qt Governance Model).

Tools support

Various tools can help with checking and enforcing this style.

clang-format

You can use clang-format and git-clang-format to reformat your code. The qt5.git repository features a _clang-format file that attempts to encode the rules for Qt code. Copy this to your root directory to let clang-format pick it up.

NB: clang-format cannot be configured to correctly reproduce the Qt style, so unless a module was developed with clang-format from the get-go, it's best to disable git-clang-format. This is true, in particular, for QtBase.

There is also a commit-hook provided by the qtrepotools project, which will be set up by init-repository by default when checking out the qt5</kdb> top-level module.

Artistic Style

The following snippet can be used by artistic style for reformatting your code.

--style=kr 
--indent=spaces=4 
--align-pointer=name 
--align-reference=name 
--convert-tabs 
--attach-namespaces
--max-code-length=100 
--max-instatement-indent=120 
--pad-header
--pad-oper

Note that "unlimited" --max-instatement-indent is used only because astyle is not smart enough to wrap the first argument if subsequent lines would need indentation limitation. You are encouraged to manually limit in-statement-indent to roughly 50 colums:

    int foo = some_really_long_function_name(and_another_one_to_drive_the_point_home(
            first_argument, second_argument, third_arugment));