Qt Coding Style

This is a overview of the coding convention we use when writing Qt code.
The data has been gathered by mining the Qt sources, discussion forums,
email threads and through collaboration of the developers.

Indentation

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

Declaring variables

• Declare each variable on a separate line
  
• Avoid short (e.g. “a”, “rbarr”, “nughdeget”) names whenever possible
  
• Single character variable names are only okay for counters and temporaries, where the purpose of the variable is obvious
  

• Wait with 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 small letter. Each consecive word in a variable’s
  name starts with a capital letter

• Avoid abbreviations

  // Wrong
  short Cntr;  
  char ITEM_DELIM = '\t';  
  
  // Correct  
  short counter;  
  char itemDelimiter = '\t';  
  

• Classes always start with a big letter. Public classes start with a ‘Q’ (QRgb). Public functions most often start with a ‘q’ (qRgb).

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";
  

• No space after a cast.
  

• Avoid C-style casts when possible.

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

Braces

• As a base rule, the left curly brace goes on the same line as the start of the statement:

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

• Exception: Function implementations 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 when the body of a conditional statement contains more than one line,
  and also if a single line statement is somewhat complex.

  // 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: Use braces also in if-then-else blocks where either the if-code or the else-code covers several lines

  // Wrong
  if (address.isEmpty())  
      return false;  
  else {  
      qDebug("%s", qPrintable(address));  
      ++it;  
  }  
  
  // Correct  
  if (address.isEmpty()) {  
      return false;  
  } else {  
      qDebug("%s", qPrintable(address));  
      ++it;  
  }  
  
  // Wrong  
  if (a)  
      if (b)  
          ...  
      else  
          ...  
  
  // Correct  
  if (a) {  
      if (b)  
          ...  
      else  
          ...  
  }  
  

• 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 on the same column as the switch
  

• Every case must have a break (or return) statement at the end or a comment to indicate that there’s intentionally no break

  switch (myEnum) {
  case Value1:  
      doSomething();  
      break;  
  case Value2:  
      doSomethingElse();  
      // fall through  
  default:  
      defaultHandling();  
      break;  
  }  
  

Line breaks

• Keep lines shorter than 100 characters; insert
  line breaks if necessary.

• Commas go at the end of a broken line; operators start at the beginning of the new line. The operator is at the end of the line to avoid having to scroll if your editor is too narrow.

  // Correct
  if (longExpression
  

  + otherLongExpression
  + otherOtherLongExpression) {  
  

  }
  

  // Wrong
  if (longExpression +
  

  otherLongExpression +
  otherOtherLongExpression) {  
  

  }
  

General exception

• Feel free to break a rule if it makes your code look bad.