OpenDDS Development Guidelines¶

This document organizes our current thoughts around development guidelines in a place that’s readable and editable by the overall user and maintainer community. It’s expected to evolve as different maintainers get a chance to review and contribute to it.

Although ideally all code in the repository would already follow these guidelines, in reality the code has evolved over many years by a diverse group of developers. At one point an automated re-formatter was run on the codebase, migrating from the GNU C style to the current, more conventional style, but automated tools can only cover a subset of the guidelines.

Table of Contents

Repository¶

The repository is hosted on Github at objectcomputing/OpenDDS and is open for pull requests.

Automated Build Systems¶

Pull requests will be tested automatically and full CI builds of the master branch can be found at http://scoreboard.ociweb.com/oci-dds.html.

All tests listed in DDS_ROOT/bin/dcps_tests.lst are part of the automated test suite.

Doxygen¶

Doxygen is run on OpenDDS regularly. There are two hosted versions of this:

Latest Release
- Based on the current release of OpenDDS.
Master
- Based on the master branch in the repository. To access it, go to the scoreboard and click the green “Doxygen” link near the top.
- Depending on the activity in the repository this might be unstable because of the time it takes to get the updated Doxygen on to the web sever. Prefer latest release unless working with newer code.

See Documenting Code for Doxygen to see how to take advantage of Doxygen when writing code in OpenDDS.

Dependencies¶

MPC is the build system, used to configure the build and generate platform specific build files (Makefiles, VS solution files, etc.).
ACE is a library used for cross-platform compatibility, especially networking and event loops. It is used both directly and through TAO.
TAO is a C++ CORBA implementation built on ACE used extensively in the traditional OpenDDS operating mode which uses the DCPSInfoRepo. TAO types are also used in the End User DDS API. The TAO IDL compiler is used internally and by the end user to allow OpenDDS to use user defined IDL types as topic data.
Perl is an interpreted language used in the configure script, the tests, and any other scripting in OpenDDS codebase.
Google Test is required for OpenDDS tests. By default, CMake will be used to build a specific version of Google Test that we have as a submodule. An appropriate prebuilt or system Google Test can also be used.

See dependencies.md for all dependencies and details on how these are used in OpenDDS.

Text File Formatting¶

All text files in the source code repository follow a few basic rules. These apply to C++ source code, Perl scripts, MPC files, and any other plaintext file.

A text file is a sequence of lines, each ending in the “end-of-line” character (AKA Unix line endings).
Based on this rule, all files end with the end-of-line character.
The character before end-of-line is a non-whitespace character (no trailing whitespace).
Tabs are not used.
- One exception, MPC files may contain literal text that’s inserted into Makefiles which could require tabs.
- In place of a tab, use a set number of spaces (depending on what type of file it is, C++ uses 2 spaces).
Keep line length reasonable. I don’t think it makes sense to strictly enforce an 80-column limit, but overly long lines are harder to read. Try to keep lines to roughly 80 characters.

C++ Standard¶

The C++ standard used in OpenDDS is C++03. There are some caveats to this but the OpenDDS must be able to be compiled with C++ 2003 compilers.

Use the C++ standard library as much as possible. The standard library should be preferred over ACE, which in turn should be preferred over system specific libraries. The C++ standard library includes the C standard library by reference, making those identifiers available in namespace std. Not all supported platforms have standard library support for wide characters (wchar_t) but this is rarely needed. Preprocessor macro DDS_HAS_WCHAR can be used to detect those platforms.

C++ Coding Style¶

C++ code in OpenDDS must compile under the compilers listed in the README.md file.
Commit code in the proper style from the start, so follow-on commits to adjust style don’t clutter history.
C++ source code is a plaintext file, so the guidelines in Text file formatting apply.
A modified Stroustrup style is used (see tools/scripts/style).
- Warning: not everything in tools/scripts/style represents the current guidelines.
Sometimes the punctuation characters are given different names, this document will use:
- Parentheses ( )
- Braces { }
- Brackets [ ]

Example¶

template<typename T>
class MyClass : public Base1, public Base2 {
public:
  bool method(const OtherClass& parameter, int idx = 0) const;
};

template<typename T>
bool MyClass<T>::method(const OtherClass& parameter, int) const
{
  if (parameter.foo() > 42) {
    return member_data_;
  } else {
    for (int i = 0; i < some_member_; ++i) {
      other_method(i);
    }
    return false;
  }
}

Punctuation¶

The punctuation placement rules can be summarized as:

Open brace appears as the first non-whitespace character on the line to start function definitions.
Otherwise the open brace shares the line with the preceding text.
Parentheses used for control-flow keywords (if, while, for, switch) are separated from the keyword by a single space.
Otherwise parentheses and brackets are not preceded by spaces.

Whitespace¶

Each “tab stop” is two spaces.
Namespace scopes that span most or all of a file do not cause indentation of their contents.
Otherwise lines ending in { indicate that subsequent lines should be indented one more level until }.
Continuation lines (when a statement spans more than one line) can either be indented one more level, or indented to nest “under” an ( or similar punctuation.
Add space around binary operators and after commas: a + b
Do not add space around parentheses for function calls, a properly formatted function call looks like func(arg1, arg2, arg3);
Do not add space around brackets for indexing, instead it should look like: mymap[key]
In general, do not add space :) Do not add extra spaces to make syntax elements (that span lines/statements) line up. This only causes unnecessary changes in adjacent lines as the code evolves.

Language Usage¶

Add braces following control-flow keywords even when they are optional.
this-> is not used unless required for disambiguation or to access members of a template-dependent base class.
Declare local variables at the latest point possible.
const is a powerful tool that enables the compiler to help the programmer find bugs. Use const everywhere possible, including local variables.
Modifiers like const appear left of the types they modify, like: const char* cstring = .... char const* is equivalent but not conventional.
For function arguments that are not modified by the callee, pass by value for small objects (8 bytes?) and pass by const-reference for everything else.
Arguments unused by the implementation have no names (in the definition that is, the declarations still have names), or a /*commented-out*/ name.
Use explicit constructors unless implicit conversions are intended and desirable.
Use the constructor initializer list and make sure its order matches the declaration order.
Prefer pre-increment/decrement (++x) to post-increment/decrement (x++) for both objects and non-objects.
All currently supported compilers use the template inclusion mechanism. Thus function/method template definitions may not be placed in normal *.cpp files, instead they can go in _T.cpp (which are #included and not separately compiled), or directly in the *.h. In this case, *_T.cpp takes the place of *.inl (except it is always inlined). See ACE for a description of *.inl files.

Pointers and References¶

Pointers and references go along with the type, not the identifier. For example:

int* intPtr = &someInt;

Watch out for multiple declarations in one statement. int* c, b; does not declare two pointers! It’s best just to break these into separate statements:

int* c;
int* b;

In code targeting C++03, 0 should be used as the null pointer. For C++11 and later, nullptr should be used instead. NULL should never be used.

Naming¶

(For library code that the user may link to)

Preprocessor macros visible to user code must begin with OPENDDS_
C++ identifiers are either in top-level namespace DDS (OMG spec defined) or OpenDDS (otherwise)
Within the OpenDDS namespace there are some nested namespaces:
- DCPS: anything relating to the implementation of the DCPS portion of the DDS spec
- RTPS: types directly tied to the RTPS spec
- Federator: DCPSInfoRepo federation
- FileSystemStorage: reusable component for persistent storage
Naming conventions
- ClassesAreCamelCaseWithInitialCapital
- methodsAreCamelCaseWithInitialLower OR methods_are_lower_case_with_underscores
- member_data_use_underscores_and_end_with_an_underscore_
- ThisIsANamespaceScopedOrStaticClassMemberConstant

Comments¶

Add comments only when they will provide MORE information to the reader.
Describing the code verbatim in comments doesn’t add any additional information.
If you start out implementation with comments describing what the code will do (or pseudocode), review all comments after implementation is done to make sure they are not redundant.
Do not add a comment before the constructor that says // Constructor. We know it’s a constructor. The same note applies to any redundant comment.

Documenting Code for Doxygen¶

Doxygen is run on the codebase with each change in master and each release. This is a simple guide showing the way of documenting in OpenDDS.

Doxygen supports multiple styles of documenting comments but this style should be used in non-trivial situations:

/**
 * This sentence is the brief description.
 *
 * Everything else is the details.
 */
class DoesStuff {
// ...
};

For simple things, a single line documenting comment can be made like:

/// Number of bugs in the code
unsigned bug_count = -1; // Woops

The extra * on the multiline comment and / on the single line comment are important. They inform Doxygen that comment is the documentation for the following declaration.

If referring to something that happens to be a namespace or other global object (like DDS, OpenDDS, or RTPS), you should precede it with a %. If not it will turn into a link to that object.

For more information, see the Doxygen manual.

Preprocessor¶

If possible, use other language features things like inlining and constants instead of the preprocessor.
Prefer #ifdef and #ifndef to #if defined and #if !defined when testing if a single macro is defined.
Leave parentheses off preprocessor operators. For example, use #if defined X && defined Y instead of #if defined(X) && defined(Y).
As stated before, preprocessor macros visible to user code must begin with OPENDDS_.
Ignoring the header guard if there is one, preprocessor statements should be indented using two spaces starting at the pound symbol, like so:

#if defined X && defined Y
#  if X > Y
#    define Z 1
#  else
#    define Z 0
#  endif
#else
#  define Z -1
#endif

Includes¶

Order¶

As a safeguard against headers being dependant on a particular order, includes should be ordered based on a hierarchy going from local headers to system headers, with spaces between groups of includes. This order can be generalized as the following:

The corresponding header to the source file (Foo.h if we were in Foo.cpp).
Headers from the local project.
Headers from external OpenDDS-based libraries.
Headers from dds/DCPS.
dds/*C.h Headers
Headers from external TAO-based libraries.
Headers from TAO.
Headers from external ACE-based libraries.
Headers from ACE.
Headers from external non-ACE-based libraries.
Headers from system and C++ standard libraries.

Path¶

Headers should only use local includes (#include "foo/Foo.h") if the header is relative to the file. Otherwise system includes (#include <foo/Foo.h>) should be used to make it clear that the header is on the system include path.

In addition to this, includes for a file that will always be relative to the including file should have a relative include path. For example, a dds/DCPS/bar.cpp should include dds/DCPS/bar.h using #include "bar.h", not #include <dds/DCPS/bar.h> and especially not #include "dds/DCPS/bar.h".

Time¶

Measurements of time can be broken down into two basic classes: A specific point in time (Ex: 00:00 January 1, 1970) and a length or duration of time without context (Ex: 134 Seconds). In addition, a computer can change its clock while a program is running, which could mess up any time lapses being measured. To solve this problem, operating systems provide what’s called a monotonic clock that runs independently of the normal system clock.

ACE can provide monotonic clock time and has a class for handling time measurements, ACE_Time_Value, but it doesn’t differentiate between specific points in time and durations of time. It can differentiate between the system clock and the monotonic clock, but it does so poorly. OpenDDS provides three classes that wrap ACE_Time_Value to fill these roles: TimeDuration, MonotonicTimePoint, and SystemTimePoint. All three can be included using dds/DCPS/TimeTypes.h. Using ACE_Time_Value is discouraged unless directly dealing with ACE code which requires it and using ACE_OS::gettimeofday() or ACE_Time_Value().now() in C++ code in dds/DCPS treated as an error by the lint.pl linter script.

MonotonicTimePoint should be used when tracking time elapsed internally and when dealing with ACE_Time_Values being given by the ACE_Reactor in OpenDDS. ACE_Conditions, like all ACE code, will default to using system time. They must modified to use monotonic time using by passing ConditionAttributesMonotonic() as the second argument in the constructor. An example of this can be seen wait_messages_pending() in dds/DCPS/MessageTracker.cpp.

More information on using monotonic time with ACE can be found here.

SystemTimePoint should be used when dealing with the DDS API and timestamps on incoming and outgoing messages.