Tag Archives: pretty print

Pretty printing C++ with Clang-Format

I use some of the code generation and refactoring tools in QtCreator. These save a lot of time, but they don’t format C++ code how I like it. For example they produce C++ code like this:

void MyClass::foo(int *x)

But I like my code formatted like this:

void MyClass::foo( int* x )

The differences may seem minor, but they are a source of significant irritation to me. I like my code how I like it, goddammit! And consistent formatting enhances readability. However re-formatting it by hand is time-consuming and tedious.

What I need is a tool that can enforce consistent formatting in the style that I like, or something close. I have tried to use automatic C++ formatting (pretty printing) tools in the past, but I couldn’t get them to produce a format that was close enough to what I wanted. But I have finally found the tool for the job. Clang-Format.

Clang-Format is part of the LLVM family of tools. It is a free, command-line tool that reformats C++, Objective-C or C according to the settings in a config file. As with many free tools, it isn’t terribly well documented. Some of the documentation on the web is out of date and some of it is incomplete. But I have managed to find out enough to configure it how I like it.

To run it you just need to place your options in a .clang-format file, make sure the clang-format executable is in the path and then run it:

clang-format.exe -i -style=file <C++ file>

Here are the settings I am currently using in my .clang-format file:

Language: Cpp
AccessModifierOffset: -4
AlignAfterOpenBracket: false
AlignConsecutiveAssignments: false
AlignConsecutiveDeclarations: false
AlignEscapedNewlinesLeft: false
AlignOperands: true
AlignTrailingComments: false
AllowAllParametersOfDeclarationOnNextLine: false
AllowShortBlocksOnASingleLine: false
AllowShortCaseLabelsOnASingleLine: false
AllowShortFunctionsOnASingleLine: Inline
AllowShortIfStatementsOnASingleLine: false
AllowShortLoopsOnASingleLine: false
AlwaysBreakAfterDefinitionReturnType: None
AlwaysBreakAfterReturnType: None
AlwaysBreakBeforeMultilineStrings: false
AlwaysBreakTemplateDeclarations: false
BinPackArguments: true
BinPackParameters: true
BraceWrapping:
  AfterClass:      true
  AfterControlStatement: true
  AfterEnum:       true
  AfterFunction:   true
  AfterNamespace:  true
  AfterObjCDeclaration: true
  AfterStruct:     true
  AfterUnion:      false
  BeforeCatch:     true
  BeforeElse:      true
  IndentBraces:    false
BreakBeforeBinaryOperators: None
BreakBeforeBraces: Allman
BreakBeforeTernaryOperators: true
BreakConstructorInitializersBeforeComma: false
ColumnLimit: 0
CommentPragmas: '^ IWYU pragma:'
ConstructorInitializerAllOnOneLineOrOnePerLine: false
ConstructorInitializerIndentWidth: 0
ContinuationIndentWidth: 4
Cpp11BracedListStyle: true
DerivePointerAlignment: false
DisableFormat: false
ExperimentalAutoDetectBinPacking: false
ForEachMacros: [ foreach, Q_FOREACH, BOOST_FOREACH ]
IndentCaseLabels: true
IndentWidth: 4
IndentWrappedFunctionNames: false
KeepEmptyLinesAtTheStartOfBlocks: true
MacroBlockBegin: ''
MacroBlockEnd: ''
MaxEmptyLinesToKeep: 2
NamespaceIndentation: None
PenaltyBreakBeforeFirstCallParameter: 100
PenaltyBreakComment: 300
PenaltyBreakFirstLessLess: 120
PenaltyBreakString: 1000
PenaltyExcessCharacter: 10000
PointerAlignment: Left
ReflowComments: true
SortIncludes: false
SpaceAfterCStyleCast: false
SpaceBeforeAssignmentOperators: true
SpaceBeforeParens: ControlStatements
SpaceInEmptyParentheses: false
SpacesBeforeTrailingComments: 1
SpacesInAngles: true
SpacesInContainerLiterals: true
SpacesInCStyleCastParentheses: true
SpacesInParentheses: true
SpacesInSquareBrackets: true
Standard: Cpp11
TabWidth: 4
UseTab: Never

It took me a few hours of fiddling with the settings to find the best combination. It would be really useful if someone could write a tool that would analyze your C++ code and create a .clang-format file for you. You would probably only want to do this once though, so I don’t think it has much potential as a commercial product.

There are only two things I couldn’t get quite right in the formatting:

  1. I couldn’t get it to add a blank line after public, protected and private declarations. I fixed this with a quick Perl hack (see below).
  2. I couldn’t get it to indent continuation lines how I would like (ideally indented 1 or 2 spaces from the first line). It is a small price to pay and I am just putting up with it for now.

Perhaps there are options to do these and I just didn’t find them.

Here is the Windows .bat script I used to format all the C++ files in a folder.

for %%f in (*.h *.cpp *.inl) do (
clang-format.exe -i -style=file %%f
)

for %%f in (*.h) do (
clang-format.exe -i -style=file %%f
perl -p -i.bak -e "s/public:/public:\n/g" %%f
perl -p -i.bak -e "s/protected:/protected:\n/g" %%f
perl -p -i.bak -e "s/private:/private:\n/g" %%f
perl -p -i.bak -e "s/    Q_OBJECT/Q_OBJECT/g" %%f
)

del *.bak
del *.tmp

No doubt there is a more elegant way to do the Perl, but it works.

I now just run this batch periodically to keep my code beautiful and consistent.