coding guidelines

This page documents some coding guidelines for Kea project.

see Stork coding guilelines for UI related guidelines. Also refer to the BIND 9 coding guidelines.

Common

Testing/Documentation addresses and prefixes

Use 192.0.2.0/24 and 2001:db8::/32 for purposes like addresses used in test cases or examples in documentation. Likewise, use reserved example domain names such as example.com, .test, .example, etc for domain names used in these cases. They are reserved by specifications and should be the safest in terms of collision avoidance.

TODO Comments

We sprinkle comments in code with keywords to indicate pending work.

In Kea, @todo is preferred. It should be prepended with triple ///, so it will show up on a nicely auto-generated Doxygen todo list. If there is a corresponding ticket, feel free to specify its number in the comment. Unless other wise specified, issue #1234 means a ticket in the trac, available at http://kea.isc.org.

Dead code

Dead code is bad; it suffers from code rot, and it looks unclean. There are some circumstances where there is a reason to keep a bit of unused code around for a while, but these should be the exception rather than the rule, and it should be very clear why it is there, and on what conditions and when it will be re-enabled or removed completely.

Any dead code (both files that are unused and blocks of commented-out code) should in principle be removed. If there is a very good reason to keep it around for a while, it must be accompanied by a comment explaining why it is still there, and when it will be removed or enabled again. This comment should point to a ticket so that we do not forget about it.

Python Style

We don't use python code anymore in the core code. If you found any leftovers, feel free to remove them.

(note this can change with the RBAC reverse proxy prototype and anyway documentation is built using Sphinx so we have a lot of python code for not core critical code)

C++ Style

File Name Conventions

Use .cc for C++ source files. This is basically a mere preference and to ensure consistency throughout the package.

Use .h for C++ header files. This is because we may want to provide a C-callable wrapper for some APIs, and some C++ header files are to be included in a C source file. In that case C-compatible file names will look more natural.

Use all all-lowercase characters for file names. This is consistent with the current recommendation for python, and so it will make the file name convention consistent throughout the BIND10 source tree. Not mixing lower/upper cases will also help avoid name conflicts in a case insensitive file system. Note that this policy may not compatible with C++ class name convention (see below) if the file name is based on the class name (e.g., name "myclass.cc" for the definition of the "Myclass" classs). We explicitly accept the conflict, but note that this means it will effectively prohibit mixing cases in class names ("Myclass" and "!MyClass" may not coexist).

Note header files should not include config.h.

Ordering Include Files

We include our own project headers first, then library, and finally system headers, whenever possible. Each header is expected to have any necessary #include statements it needs, and this helps insure that.

#include <dns/message.h> 
#include <boost/shared_ptr.hpp>
#include <string>

There can be several exceptions. Some specific libraries require a particular header file must be included first. A notable example is Python.h. The top level config.h generated by autoconf should also be included before other generic header files.

Include Style

We use the pointy brackets version of #include. Also, we use a full path to the include (relative to some -I switch, not to the current directory).

Correct:

#include <util/threads/thread.h>
#include <dns/name.h>

Incorrect:

#include <thread.h>
#include "name.h"

Line length

Source code not exceeding 80 columns is preferred. This is derived from the [wiki:BIND9CodingGuidelines BIND 9 coding guidelines], mainly for style consistency. However, C++ names (especially with namespaces) are significantly longer, thus strictly adhering to this causes overly wrapped code that is both ugly and difficult to read. Therefore it is permitted to use lines up to 100 columns long.

Rationale: It is 2015 and all ISC employees have equipment that is at most 3 years old. You should have FullHD (or better) display available to you. You should be able to display two files side by side comfortably.

Tabs & Indentation

Do not use hard tabs.

Indentation at each level is 4 spaces for C++, other languages should use what is "usual and expected."

In C++ we use the BSD style (also from BIND 9), where continuing lines are aligned with the corresponding opening parenthesis, like this:

if (JS_DefineProperty(cx, o, "data",
                      STRING_TO_JSVAL(JS_NewStringCopyN(cx, data, res)),
                      NULL, NULL, JSPROP_ENUMERATE) != 0) {

Curly Braces

Always add braces even for a single-line block:

if (something_holds) {
    perform something;
} else if (nonorthogonal_condition) {
    perform otherthing;
} else { // optionally comment to clarify the fully orthogonal case
    perform finalthing;
}

Opening Curly Braces for Functions

The opening curly brace should occur on the same line as the argument list, unless the argument list is more than one line long.

void
f(int i) {
    // whatever
}

int
g(int i, /* other args here */
  int last_argument)
{
    return (i * i);
}

This was derived from the BIND 9 coding guideline. It's known this style may look awkward (and even may look inconsistent) for some, but for the reason stated at the beginning we follow this style.

Curly Braces for Catch

A catch statement should have braces on a single line, like this:

     .
     . 
     .
   } catch (const SomeException& ex) { 
     .
     .
     .

Note if the ex parameter is not used it should be omitted. There are only two forms of catch clauses which make sense: a const reference and the ... catch all.

Parentheses

Do put a space after 'return', and also parenthesize the return value.

    return 1;   // BAD
    return (1); // Good

This was derived from the BIND 9 coding guideline.

Operators

Use operator methods in a readable way. In particular, use the following style with operator==:

    if (x == 10) {  // Good
        // do something that has to be done when x is equal to 10
    }

instead of this:

    if (10 == x) {  // BAD
        // do something that has to be done when x is equal to 10
    }

because the former style is much more readable and intuitive for humans. While the latter style might help detect bugs like dropping one = in the expression, modern compilers with proper warning levels can do the same job more comprehensively. This is especially so for cleanly written C++ code (compared to plain old C).

See also developers' discussions at: https://lists.isc.org/pipermail/bind10-dev/2012-March/003266.html

Increment and Decrement operators (++/--)

Use the prefix form of increment/decrement operators by default:

    // for (int i = 0; i < 10; i++) { // No good
    for (int i = 0; i < 10; ++i) { // Good
        // do something for i = 0..10
    }

Preferring the prefix form of these operators is a well known practice for non trivial types due to performance reasons. And, for consistency, we use the same style for basic types like int even if it's mostly a preference matter for such types. By being consistent, it will be easier to notice when we use the less efficient style when it really matters.

Sometimes the context requires the use of postfix form, in which case it's okay to use that form. But if the intent is not obvious from the context, leave a comment about why the different form is used (since it's subjective whether it's "obvious", it's generally a good idea to leave the comment in this case).

Operator Overloading

Operator overloading is allowed when it's considered intuitive and helpful for improving code readability. But care should be taken, because often it could be only intuitive for that developer who introduced it. If it doesn't look intuitive for the reviewer, the developer has responsibility to convince the reviewer; if it fails the default action is to use non operator method/function for that purpose.

It's recommended to define operator<<(std::ostream& os, const TheClass& obj) if TheClass has operator==() and toText() methods. This allows the class can be used in EXPECT_EQ (and its variants) in googletests.

The following rule was deprecated. It doesn't seem to be followed anyway, and no one remembered why it had been introduced.

~~When a class supports operator overloading, then there should also be non-overloaded methods:~~

class Foo {
public:
    // This rule was deprecated.
    //bool equals(const Foo& other) const;
    bool operator==(const Foo& other) const { return (equals(other)); }
}

Explicit Constructors

By default C++ constructors with one argument are conversion functions. When they are not (which is the general case) they should be explicit as in:

class Foo {
public:
   // Constructor with one argument
   explicit Foo(std::string name);
}

Class Attributes

Accessors for class attributes should be called '''getXxx()'''.

Mutators for class attributes should be called '''setXxx()'''.

(where xxx is the attribute)

Non-copyable Classes

If you want a class to be non-copyable (neither copy constructor nor assignment), inherit from boost::noncopyable rather than implementing this yourself.

http://www.boost.org/doc/libs/1_47_0/libs/utility/utility.htm#Class_noncopyable

Naming

Don't start things with underscores. According to Stroustrup's C++ book: Names starting with an underscore are reserved for special facilities in the implementation and the run-time environment, so such names should not be used in application programs.

Class names are '''LikeThis''', methods are '''likeThis()''', variables are '''like_this''', and constants are '''LIKE_THIS'''. Data class members are '''like_this_'''.

Enumerations are written as

enum EnumName {
  FOO,
  BAR
} enum_instance;

Note that unless you have a specific reason to set specific values, leave specific values off. These can be written if needed:

enum ExamplePortNumbers {
  DNS = 53,
  DHCP = 68
};

== Where to Put Reference and Pointer Operators ==

In C++, it seems to be more common to not insert a space between the type and the operator:

int* int_var;
int& int_ref;

Sizeof Bool

The C++ standard does not require that sizeof(bool) is one: it is compiler dependent! So it should not be used and sizeof(uint8_t) is recommended instead.

Comments

Multiline comments can be written in C++ or C style (that is, with // or /* */ marks).

/*
 * This is a comment.  It is important probably.
 */

//
// This is a comment.  It is important probably.
//

/* This is also ok. */

// As is this.

Comments at the end of lines should usually be C++ style:

class Foo {
    int bar_length;  // The length of the bar in millimeters.
};

Doxygen Comment Style

When writing a Doxygen special comment block there are several possible styles:

http://www.stack.nl/~dimitri/doxygen/docblocks.html

Doxygen keywords should be prepended with @, not with a backslash. The reason to prefer @ is that backslash may confuse scripts that would go over the code. There is some inconsistency in this regard. There are large parts of older code that still use backslash. It is a matter of personal taste to keep consistency with what is in the file vs. strictly sticking with this principle.

We use the C++ style of 3-slashes:

/// A lot of examples are called foo().
///
/// @param baz foo() usually takes an argument
void
foo(Bar baz) {
    ...
}

Make sure inserting a blank line between two function/method declarations or definitions:

class Bad {
   /// @brief Short description for bad1
   void bad1();
   /// @brief Short description for bad2, which may also look for bad1().
   void bad2();
};

class Good {
   /// @brief Short description for good1
   void good1();

   /// @brief Short description for good2, which should be much clearer.
   void good2();
};

Explicit @brief for Doxygen

If you don't use @brief as the first thing in your doxygen comment, then doxygen will turn the first paragraph into a @brief description anyway. However, we include it anyway so that everybody understands that this is the @brief description.

Methods and Functions

Opening Curly Braces

For methods where the arguments all fit on one line with the curly brace, it should be written on one line:

int
methodName(int argument_one, std::string message) {
    ...
}

Where this is not possible the curly brace should go on a line by itself:

int
methodName(int argument_one, std::string message,
           int another_argument)
{
    ...
}

Virtual Methods

Explicitly add virtual to method declarations in derived classes:


class Base {
    // this 'virtual' is absolutely necessary
    virtual void toBeVirtual();
};

class Derived : public Base {
    // this 'virtual' is not necessarily needed, but add it per this guideline
    virtual void toBeVirtual();
}

This way it's easier to recognize Derived::toBeVirtual() is (more likely to be) defined as a virtual method in the Base class without referring to the base class definition. It could also mean that toBeVirtual is not defined in the base class and is intended to work as a virtual methods for classes derived further from Derived, but in practice that's a very rare case; in most cases we use these classes through the (top) base class interfaces.

Const references

With anything but primitive types (like int or bare pointer), it is better to pass them as const reference when possible, to avoid overhead of calling the copy constructor and copying a lot of data.

This includes smart pointers, some of them can be relatively expensive to copy.

void
function(const boost::shared_ptr<DataType>& param) {
	...
}

Exception-safe getters and string-production methods

Unless there's a compelling reason to do so neither member value getter methods nor and string-production methods, such as toString(), should throw exceptions. Normally a class member is prevented from ever having an invalid value so there is arguably a getter never has a reason to throw, and string-production methods should always be safe to invoke once a class has been instantiated. Both types of methods are commonly used as log statement arguments where one should not have to worry about catching exceptions.

Namespaces

Namespaces will be lower case: '''isc::dns''', or '''isc::cc'''.

'''using namespace''' should never be used in a header file. They may be used in .cc source files.

Consts

Use const as much as possible. Make class methods const member functions whenever possible.

If a function has a parameter with a "top-level const", make sure the const appears not only in the definition but also in the declaration. For example, if you define the following function in a .cc file:

int
foo(const int param) {
...
}

then make sure the const appears in the corresponding declaration in a header file (.h):

int foo(const int param);  // const cannot be omitted

Technically, the latter const is redundant. But !SunStudio C++ compilers have a bug in name mangling that requires the "consistency": http://developers.sun.com/sunstudio/documentation/ss12/mr/READMEs/c++.html#cNameMangling

Unfortunately, we want to be as portable as possible, and so need to work around this by being redundant.

NULL pointer check

Use the "implicit" form when testing to see whether a pointer is 0 (NULL):

    SomeType* ptr;
    if (ptr) { // instead of 'ptr != NULL'
        // do something if the value of ptr is not 0
    }
    if (!ptr) { // instead of 'ptr == NULL'
        // do something if the value of ptr is 0
    }

This is different from what is suggested in the [wiki:BIND9CodingGuidelines BIND 9 coding guidelines]. We changed the guideline for BIND 10 for the following reasons:

The implicit form is more compatible with smart pointers. For example, if we need to write a "non-null check" for a boost::shared_ptr value without its type conversion (to bool) operator, it would look quite ugly:

    boost::shared_ptr<SomeType> ptr;
    if (ptr.get() != NULL) { // ugly
       // do something if the underlying pointer value of ptr is not 0
    }
    if (ptr) { // same effect, more concise
       // ditto
    }

So for smart pointers we definitely want to use the latter, and it's better to use the consistent form for both smart pointers and bare pointers. (And, in any case, it's generally better to minimize direct use of bare pointers).

For some developers the "implicit" form was more intuitive, while others had no strong preference.
This has been one of the most-broken guidelines (perhaps partly because of the incompatibility with the smart pointer usage). If a guideline is so difficult to meet, it probably means we should revisit the guideline rather than pursuing how to enforce it.
The implicit form doesn't weaken code safety.
NULL is usually defined as a macro. In general, it's better to avoid macros in C++ code.

Note that there are still many NULL checks in the "explicit" form in the source tree as of this writing (April 2013) as we began with the BIND 9 style. New code should follow this guideline, and it's encouraged to update existing code gradually while we update the code for other reasons (bug fix, enhancement, etc). But during the transition period (which will be long) we'll see both forms.

See also a mailing list discussion starting from: https://lists.isc.org/pipermail/bind10-dev/2012-December/004129.html and related discussion in a conference call http://bind10.isc.org/wiki/WeeklyMinutes20130115 when we decided to make the change.

Log Statement Safety

It is extremely important to examine all arguments passed into a log statement to ensure they will produce safe values at runtime:

Can the argument (or any part of it) be NULL? If so is this taken into account?
If the argument invokes any fuctions, are they exception safe?
If it involves indirection, does this always resolve into a usable value?
If it raises an exception, is the exception caught? This includes double errors, i.e., log statements in an exception handler.
Does it have the intended data type? For example, it is a common oversight that 8-bit integers are displayed as their ASCII character counterpart. To solve this, prepending a unary plus is the recommended solution (cppreference.com: 1. unary plus (promotion). [...] Integral promotion is performed on the operand if it has integral or unscoped enumeration type and determines the type of the result.).

Log statements are less than helpful if they cause the program to segfault or throw.

Google Test Style

Try to use EXPECT_() rather than EXPECT_TRUE():

EXPECT_TRUE(run_time > 1);   // BAD

EXPECT_GT(run_time, 1);      // Good

because when the test fails the latter will provide more detailed information (the values of "actual" and "expected").

Note that this is not always possible, especially when checking an object that has no operator<<() (or more obviously no operator()), which is used to output on failure.

Test naming

Names of tests should be similar to classes and method names for that language. For example:

for gtest C++ tests, you'd use CamelCaseTest as the fixture class name, and testName() as the test name (as it is a C++ method).
for Python unittests, you'd use CamelCaseTest as the fixture class name, and test_something() as the test name (as it is a Python method).

Makefile.am Guidelines

Make portability

Makefiles must never depend on a particular make, for instance the $(<function> ...) construct of GNU make for shell, foreach, wildcard, etc, are forbidden.

Subdirectories

If there is are one or more subdirectories, the first line in the file should be the SUBDIRS entry, e.g.:

SUBDIRS = . tests

If relevant (as is usually the case), the current directory should be included in the list at the appropriate point in the list.

Preprocessor flags / includes

Next should be the setting of the AM_CPPFLAGS, which applies to every module built with C++ in the Makefile.am, e.g.:

AM_CPPFLAGS = -I$(top_srcdir)/src/lib -I$(top_builddir)/src/lib
AM_CPPFLAGS += $(BOOST_INCLUDES)

This can be overriden on a per-module basis in the file by specifying the variable module_CPPFLAGS. (Note that if the CPPFLAGS environment variable is set, its flags are appended to the command line after AM_CPPFLAGS.)

Directory variables should be relative (i.e., no abs_top_builddir) for efficiency. Third (or more) levels should not be used, e.g.:

NO += -I$(top_srcdir)/src/lib/dns -I$(top_builddir)/src/lib/dns

AM_CXXFLAGS should be initialized to KEA_CXXFLAGS, e.g.:

AM_CXXFLAGS = $(KEA_CXXFLAGS)

Linker flags

When executables are build, AM_LDFLAGS must be conditionally set to static, e.g.:

if USE_STATIC_LINK
AM_LDFLAGS = -static
endif

Library dependencies

With the exception of archives, (dynamic) libraries and executables should be linked with all dependencies in the opposite order of src/lib/Makefile.am for shared libraries (so finishing by libkea-exceptions.la) followed by external dependencies. The idea is to not rely on libtool which has shown some problems handling dependencies.

Dynamic Shared Objects built as modules to be dynamically loaded must have no dependency when static linking is enforced: their undefined symbols should be resolved using executables they are loaded into.

If the libcfgrpt is used for embedding config_report it must be linked first, in particular if it is linked after libprocess some linkers can believe the config_report symbol is resolved by libprocess and does not include its static definition.

User Interface (UI) Guidelines

Kea is a server process, so does not have much that would be considered a user interface. This section discusses what we do have. See Stork coding guidelines for UI guidelines for Stork.

IP address and port formatting

Whenever an IP address and port is output, IETF RFC 2396 and RFC 2732 should be followed.

For IPv4 addresses, this looks like this:

192.0.2.1:53

For IPv6 addresses, this looks like this:

Imported Code

If you import code from another project, try to continue the style of the imported project if changes need to be made. This is for two reasons, one is to make merging future versions easier. The other is the encouragement of submitting changes upstream.

Deprecating options

Sometimes we need to introduce changes that are not backward compatible. If possible we try to support the old way while making the new way possible. When introducing new alternative ("new way") that eventually going to replace an existing option ("old way"), there should be at least one stable series which supports both old and new way. If old syntax is used, a warning should be printed urging people to update their syntax.

For example: The old reservation-mode parameter has been replaced with reservations-global, reservations-in-subnet and reservations-out-of-pool in 1.9.2. A warning should be printed if reservation-mode is used. Such a warning should be coded as soon as realistically possible to give people as much advance warning as possible. As such, this will be implemented in 1.9.2, so early adopters will see it right way. It will be included in 2.0, where users preferring stable will see it. The old way can be removed somewhere in 2.1.x, so 2.2 will not accept it anymore. This way people who use only stable version will have time to migrate. They'll see the warning when moving from 1.8 to 2.0 and will have time until 2.2.

Guidelines Adopted by Other Projects

Other projects have their own coding guidelines. Here're some examples of such guidelines. These are reference purposes only; unless explicitly stated we also adopt some part of other guidelines, they are not part of the BIND 10's coding guidelines.