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. The qt5.git repository features a _clang-format file that encodes the rules for Qt code.

Indentation

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

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).

Whitespace

• Use blank lines to group statements together where suited
• Always use only one blank line
• Always use a single space after a 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
• Avoid C-style casts when possible

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

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

• 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();

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
 {
 };

• 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

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, unless another case follows immediately.

 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 visualize that symmetry

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) {
 }

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).

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));