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<!-- --- title: The OpenFOAM C++ Coding Style Guide -->
[![home](/icons/home.svg "wiki home")](/home)
This sets out some guidelines for the coding style in the OpenFOAM
code base. As usual, _there are no rules without exceptions_, but
following these guidelines does help people navigating the code and
making changes. It also greatly eases the task of integrating community
code contributions and enhances long-term maintenance.
[[_TOC_]]
## Code
### General
- 80 character lines max
- The normal indentation is 4 __spaces__ per logical level.
- **No** tab characters - only use spaces for indentation.
- No trailing whitespace. It creates spurious file changes that make it
difficult to notice *real* changes (it also looks ugly).
- The body of control statements (eg, `if`, `else`, `while`, _etc_.). are
always delineated with braces. A possible exception can be
made in conjunction with `break` or `continue` as part of a control
structure.
- The body of `case` statements is usually delineated with braces.
- For stream output, the `<<` operator is always four characters after
the start of the stream to ensure that the `<<` symbols align.
For example,
```
Info<< ...
os << ...
```
and
```
WarningInFunction
<< "Warning message"
```
_NOT_ as
```
WarningInFunction
<< "Warning message"
```
- Remove unnecessary class section headers. For example, remove
```
// * * * * * * * * * * * * Private Member Functions * * * * * * * * * * //
// Check
// Edit
// Write
```
if they contain nothing, even if planned for 'future use'
- Class titles should be centred
```
/*-----------------------------------------------------------------------*\
Class exampleClass Declaration
\*-----------------------------------------------------------------------*/
```
_NOT_
```
/*-----------------------------------------------------------------------*\
Class exampleClass Declaration
\*-----------------------------------------------------------------------*/
```
### The _.H_ Header Files
- Inline functions
- Use inline functions where appropriate in a separate _classNameI.H_
file. Avoid cluttering the header file with function bodies.
- Lines spacing
- Leave two empty lines between sections
(as per functions in the _.C_ file etc.)
- Use C++ `// ...` comments in header file to add descriptions to class
data and functions do be included in the Doxygen documentation.
- Note that the initial `//-` tag is special to OpenFOAM for
introducing doxygen comments in a compact form.
- Destructor
- The destructor is documented with `//-` like a normal method:
```
//- Destructor
~className();
```
#### Doxygen documentation in the _.H_ Header Files
When generating documentation with Doxygen, the comments are filtered:
- Text on the line starting with `//-` becomes the Doxygen `\brief`
description.
- Subsequent lines that are also start with `//-` are included
as part of the Doxygen `\brief` description.
- Following comment lines (with a plain `// ` ) become the Doxygen
detailed description.
Examples:
```
//- A function that returns a thing
// This is a detailed description of the function
// that processes stuff and returns other stuff
// depending on various things.
thing function(stuff1, stuff2);
```
Or
```
//- A function that returns a thing,
//- when a particular flow condition has been satisfied.
// This is a detailed description of the function
// that processes stuff and returns other stuff
// depending on various things.
thing function(stuff1, stuff2);
```
- List entries start with `-` or `-#` for numbered lists but cannot start
on the line immediately below the brief description.
An additional line it thus required. For example,
```
//- Compare triFaces
// Returns:
// - 0: different
// - +1: identical
// - -1: same face, but different orientation
static int compare(const triFace&, const triFace&);
```
_OR_
```
//- Compare triFaces returning 0, +1 or -1
//
// - 0: different
// - +1: identical
// - -1: same face, but different orientation
static int compare(const triFace&, const triFace&);
```
_NOT_
```
//- Compare triFaces returning 0, +1 or -1
// - 0: different
// - +1: identical
// - -1: same face, but different orientation
static int compare(const triFace&, const triFace&);
```
- List can be nested as well. For example,
```
//- Search for \em name
// in the following hierarchy:
// -# user settings
// - ~/.OpenFOAM/\<API\>/
// - ~/.OpenFOAM/
// -# group settings
// - $WM_PROJECT_DIR/site/\<API\>
// - $WM_PROJECT_DIR/site/
// -# other (shipped) settings
// - $WM_PROJECT_DIR/etc/
//
// \return The full path name of the first file found, or an empty fileName.
fileName findEtcFile(const fileName& name, bool mandatory=false);
```
For many more details, see the
[Doxygen manual](http://doxygen.nl/manual/index.html)
### The _.C_ Source Files
- Use two empty lines between functions
- Do not open/close namespaces in a _.C_ file
- Fully scope the function name, i.e.
```
Foam::returnType Foam::className::functionName()
```
_NOT_
```
namespace Foam
{
...
returnType className::functionName()
...
}
```
_Exception_
When there are multiple levels of namespace, they may be used in the
_.C_ file to avoid excessive clutter, i.e.
```
namespace Foam
{
namespace compressible
{
namespace RASModels
{
...
} // End namespace RASModels
} // End namespace compressible
} // End namespace Foam
```
#### Coding Practice
- Passing data as arguments or return values
- Pass bool, label, scalar and other primitive types as copy,
anything larger by reference.
- Use `const` everywhere it is applicable.
- Initialise _references_ using `=`
```
const className& variableName = otherClass.data();
```
_NOT_
```
const className& variableName(otherClass.data());
```
- Virtual functions
- If a class is virtual, make all derived classes virtual.
- Default constructors
- If the class is not copy constructible or assignable,
remove the default generated constructors by marking them
as `delete`.
```
//- No copy construct
className(const className&) = delete;
//- No copy assignment
void operator=(const& className) = delete;
```
It is immaterial if these are marked as delete within `private`,
`protected` or `public`, but they are mostly not marked within
`public` for slightly more tidiness.
- Default destructor
- If the class easily permits it - all member data types are known
within the header, and no special cleanup is required -
it is clearer to declare accordingly in the header.
```
//- Destructor
~className() = default;
```
- Conditional Statements
```
if (condition)
{
code;
}
```
OR
```
if
(
long condition
)
{
code;
}
```
_NOT_ (no space between `if` and `(` used)
```
if(condition)
{
code;
}
```
- `for` and `while` Loops
```
for (i = 0; i < maxI; ++i)
{
code;
}
```
OR
```
for
(
i = 0;
i < maxI;
++i
)
{
code;
}
```
_NOT_ this (no space between `for` and `(` used)
```
for(i = 0; i < maxI; ++i)
{
code;
}
```
Range-based for should have a space surrounding the ':'
```
for (auto i : range)
{
code;
}
```
Note that when indexing through iterators, it is often more efficient
to use the pre-increment form. That is, `++iter` instead of `iter++`
- Macro loops : `forAll`, `forAllIters`, `forAllConstIters`, _etc._
are like `for` loops, but without a space. Eg,
```
forAll(
```
_NOT_
```
forAll (
```
In many cases, the new `forAllIters` and `forAllConstIters` macros
provide a good means of cycling through iterators (when a range-based
for doesn't apply). These use the C++11 `decltype` and thus work without
explicitly specifying the container class:
```
forAllIters(myEdgeHash, iter)
```
Using the older `forAllIter` and `forAllConstIter` macros may
still be seen (but are generally disappearing).
However, since they are macros, they will fail if
the iterated object contains any commas.
For example, the following will FAIL!:
```
forAllIter(HashTable<labelPair, edge, Hash<edge>>, myEdgeHash, iter)
```
These convenience macros are also generally avoided in other
container classes and OpenFOAM primitive classes.
In certain cases, the range-based for can also be used.
- Splitting Over Multiple Lines
- Splitting return type and function name
- Split initially after the function return type and left align
- Do not put `const` onto its own line - use a split to keep it with
the function name and arguments.
```
const Foam::longReturnTypeName&
Foam::longClassName::longFunctionName const
```
_NOT_
```
const Foam::longReturnTypeName&
Foam::longClassName::longFunctionName const
```
_nor_
```
const Foam::longReturnTypeName& Foam::longClassName::longFunctionName
const
```
_nor_
```
const Foam::longReturnTypeName& Foam::longClassName::
longFunctionName const
```
- If it needs to be split again, split at the function name (leaving
behind the preceding scoping `::`s), and again, left align, i.e.
```
const Foam::longReturnTypeName&
Foam::veryveryveryverylongClassName::
veryveryveryverylongFunctionName const
```
- Splitting long lines at an `=` sign.
Indent after split
```
variableName =
longClassName.longFunctionName(longArgument);
```
_OR_ (where necessary)
```
variableName =
longClassName.longFunctionName
(
longArgument1,
longArgument2
);
```
_NOT_
```
variableName =
longClassName.longFunctionName(longArgument);
```
_NOR_
```
variableName = longClassName.longFunctionName
(
longArgument1,
longArgument2
);
```
- Maths and Logic
- Operator spacing
```
a + b, a - b
a*b, a/b
a & b, a ^ b
a = b, a != b
a < b, a > b, a >= b, a <= b
a || b, a && b
```
- Splitting formulae over several lines:
Split and indent as per "splitting long lines at an ="
with the operator on the lower line. Align operator so that first
variable, function or bracket on the next line is 4 spaces indented i.e.
```
variableName =
a*(a + b)
*exp(c/d)
*(k + t);
```
This is sometimes more legible when surrounded by extra parentheses:
```
variableName =
(
a*(a + b)
*exp(c/d)
*(k + t)
);
```
- Splitting logical tests over several lines:
outdent the operator so that the next variable to test is aligned with
the four space indentation, i.e.
```
if
(
a == true
&& b == c
)
```
## Documentation
### General
- For readability in the comment blocks, certain tags are used that are
translated by pre-filtering the file before sending it to Doxygen.
- The tags start in column 1, the contents follow on the next lines and
indented by 4 spaces. The filter removes the leading 4 spaces from the
following lines until the next tag that starts in column 1.
- The 'Class' and 'Description' tags are the most important ones.
- The first paragraph following the 'Description' will be used for the
brief description, the remaining paragraphs become the detailed
description. For example,
```
Class
Foam::myClass
Description
A class for specifying the documentation style.
The class is implemented as a set of recommendations that may
sometimes be useful.
```
- The class name must be qualified by its namespace, otherwise Doxygen
will think you are documenting some other class.
- If you don't have anything to say about the class (at the moment), use
the namespace-qualified class name for the description. This aids with
finding these under-documented classes later.
```
Class
Foam::myUnderDocumentedClass
Description
Foam::myUnderDocumentedClass
```
- Use 'Class' and 'Namespace' tags in the header files.
The Description block then applies to documenting the class.
- Use 'InClass' and 'InNamespace' in the source files.
The Description block then applies to documenting the file itself.
```
InClass
Foam::myClass
Description
Implements the read and writing of files.
```
#### Doxygen Special Commands
Doxygen has a large number of special commands with a `\` prefix.
Since the filtering removes the leading spaces within the blocks, the
Doxygen commands can be inserted within the block without problems.
```
InClass
Foam::myClass
Description
Implements the read and writing of files.
An example input file:
\verbatim
patchName
{
type patchType;
refValue 100;
value uniform 1;
}
\endverbatim
Within the implementation, a loop over all patches is done:
\code
forAll(patches, patchi)
{
... // some operation
}
\endcode
```
#### HTML Special Commands
Since Doxygen also handles HTML tags to a certain extent, the angle
brackets need quoting in the documentation blocks. Non-HTML tags cause
Doxygen to complain, but seem to work anyhow. For example,
- The template with type `<HR>` is a bad example.
- The template with type `\<HR\>` is a better example.
- The template with type `<Type>` causes Doxygen to complain about an
unknown html type, but it seems to work okay anyhow.
#### Documenting Namespaces
- If namespaces are explicitly declared with the `Namespace()` macro,
they should be documented there.
- If the namespaces is used to hold sub-models, the namespace can be
documented in the same file as the class with the model selector.
For example,
```
documented namespace 'Foam::functionEntries' within the
class 'Foam::functionEntry'
```
- If nothing else helps, find some sensible header.
```
namespace 'Foam' is documented in the foamVersion.H file
```
#### Documenting Applications
Any number of classes might be defined by a particular application, but
these classes will not, however, be available to other parts of
OpenFOAM. At the moment, the sole purpose for running Doxygen on the
applications is to extract program usage information for the `-doc`
option.
The documentation for a particular application is normally contained
within the first comment block in a _.C_ source file. The solution is this
to invoke a special filter for the "/applications/{solver,utilities}/"
directories that only allows the initial comment block for the _.C_ files
through.
The layout of the application documentation has not yet been finalized,
but foamToVTK shows an initial attempt.
### Orthography
Given the origins of OpenFOAM, the British spellings (eg, neighbour and
not neighbor) are generally favoured.
Both '-ize' and the '-ise' variant are found in the code comments. If
used as a variable or class method name, it is probably better to use
'-ize', which is considered the main form by the Oxford University
Press. Although this particular view may not actually be followed.
```
myClass.initialize()
```
#### References
References provided in the `Description` section of the class header files
should be formatted in the [APA (American
Psychological Association)](http://www.apastyle.org) style.
Example from `kEpsilon.H`:
```
Description
Standard k-epsilon turbulence model for incompressible and compressible
flows including rapid distortion theory (RDT) based compression term.
Reference:
\verbatim
Standard model:
Launder, B. E., & Spalding, D. B. (1972).
Lectures in mathematical models of turbulence.
Launder, B. E., & Spalding, D. B. (1974).
The numerical computation of turbulent flows.
Computer methods in applied mechanics and engineering,
3(2), 269-289.
DOI:10.1016/B978-0-08-030937-8.50016-7
For the RDT-based compression term:
El Tahry, S. H. (1983).
k-epsilon equation for compressible reciprocating engine flows.
Journal of Energy, 7(4), 345-353.
DOI:10.2514/3.48086
\endverbatim
```
The APA style is a commonly used standard and references are available in
this format from many sources including
[Citation Machine](http://www.citationmachine.net/apa/cite-a-book) and
[Google Scholar]([http://scholar.google.com).