6. Coding style¶
When writing code to be contributed to the ns-3 open source project, we ask that you follow the coding standards, guidelines, and recommendations found below.
6.1. Clang-format¶
The ns-3 project uses clang-format to define and enforce the C++ coding style. Clang-format can be easily integrated with modern IDEs or run manually on the command-line.
Besides clang-format, ns-3 adopts other coding-style guidelines that are not covered
by clang-format, which are explained in this document.
Read the check-style-clang-format.py
section below for information on how to
use this Python script to check and fix all formatting guidelines followed by ns-3.
6.1.1. Clang-format installation¶
Clang-format can be installed using your OS’s package manager. Please note that you should install one of the supported versions of clang-format, which are listed in the following section.
6.1.2. Supported versions of clang-format¶
Since each new major version of clang-format can add or modify properties, newer versions of clang-format might produce different outputs compared to previous versions.
The following list contains the set of clang-format versions that are verified to produce consistent output among themselves.
clang-format-17
clang-format-16
clang-format-15
clang-format-14
6.1.3. Integration with IDEs¶
Clang-format can be integrated with modern IDEs (e.g., VS Code) that are able to
read the .clang-format
file and automatically format the code on save or on typing.
Please refer to the documentation of your IDE for more information. Some examples of IDE integration are provided in clang-format documentation
As an example, VS Code can be configured to automatically format code on save, on paste and on type by enabling the following settings:
{
"editor.formatOnSave": true,
"editor.formatOnPaste": true,
"editor.formatOnType": true,
}
6.1.4. Clang-format usage in the terminal¶
In addition to IDE support, clang-format can be manually run on the terminal.
To format a file in-place, run the following command:
clang-format -i $FILE
To check that a file is well formatted, run the following command on the terminal. If the file is not well formatted, clang-format indicates the lines that need to be formatted.
clang-format --dry-run $FILE
6.1.5. Clang-format Git integration¶
Clang-format integrates with Git to format Git commits or changes not yet committed, such as pending merge requests on the GitLab repository. The full documentation is available on clang-format Git integration
To fix the formatting of files with Git, run the following commands in the ns-3 main directory. These commands do not change past commits. Instead, the reformatted files are left in the workspace. These changes should be squashed to the corresponding commits, in order to fix them.
# Fix all commits of the current branch, relative to the master branch
git clang-format master
# Fix all staged changes (i.e., changes that have been `git add`ed):
git clang-format
# Fix all changes staged and unstaged:
git clang-format -f
# Fix specific files:
git clang-format path_to_file
# Check what formatting changes are needed (if no files provided, check all staged files):
git clang-format --diff
Note that this only fixes formatting issues related to clang-format.
For other ns-3 coding style guidelines, read the check-style-clang-format.py
section below.
In addition to Git patches,
clang-format-diff
can also be used to reformat existing patches produced with the diff
tool.
6.1.6. Disable formatting in specific files or lines¶
To disable formatting in specific lines, surround them with the following C++ comments:
// clang-format off
...
// clang-format on
To exclude an entire file from being formatted, surround the whole file with the special comments.
6.2. check-style-clang-format.py¶
To facilitate checking and fixing source code files according to the ns-3 coding style,
ns-3 maintains the check-style-clang-format.py
Python script (located in utils/
).
This script is a wrapper to clang-format and provides useful options to check and fix
source code files. Additionally, it performs other manual checks and fixes in text files
(described below).
We recommend running this script over your newly introduced C++ files prior to submission as a Merge Request.
The script performs multiple style checks. By default, the script runs the following checks:
Check code formatting using clang-format. Respects clang-format guards.
Check if local
#include
headers do not use the “ns3/” prefix. Respects clang-format guards.Check if there are no trailing whitespace. Always checked.
Check if there are no tabs. Respects clang-format guards.
Check if source code files use SPDX licenses rather than GPL license text. Respects clang-format guards.
The process returns a zero exit code if all files adhere to these rules. If there are files that do not comply with the rules, the process returns a non-zero exit code and lists the respective files. This mode is useful for developers editing their code and for the GitLab CI/CD pipeline to check if the codebase is well formatted. All checks are enabled by default. Users can disable specific checks using the corresponding flags:
--no-formatting
--no-include-prefixes
--no-whitespace
--no-tabs
Additional information about the formatting issues detected by the script can be enabled
by adding the -v, --verbose
flag.
In addition to checking the files, the script can automatically fix detected issues in-place.
This mode is enabled by adding the --fix
flag.
The formatting and tabs checks respect clang-format guards, which mark code blocks that should not be checked. Trailing whitespace is always checked regardless of clang-format guards.
The complete API of the check-style-clang-format.py
script can be obtained with the
following command:
./utils/check-style-clang-format.py --help
For quick-reference, the most used commands are listed below:
# Entire codebase (using paths relative to the ns-3 main directory)
./utils/check-style-clang-format.py --fix .
# Entire codebase (using absolute paths)
/path/to/utils/check-style-clang-format.py --fix /path/to/ns3
# Specific directory or file
/path/to/utils/check-style-clang-format.py --fix absolute_or_relative/path/to/directory_or_file
# Files modified by the current branch, relative to the master branch
git diff --name-only master | xargs ./utils/check-style-clang-format.py --fix
6.3. Clang-tidy¶
The ns-3 project uses clang-tidy to statically analyze (lint) C++ code and help developers write better code. Clang-tidy can be easily integrated with modern IDEs or run manually on the command-line.
The list of clang-tidy checks currently enabled in the ns-3 project is saved on the
.clang-tidy
file. The full list of clang-tidy checks and their detailed explanations
can be seen in Clang-tidy checks.
6.3.1. Clang-tidy installation¶
Clang-format can be installed using your OS’s package manager. Please note that you should install one of the supported versions of clang-format, which are listed in the following section.
6.3.2. Minimum clang-tidy version¶
Since clang-tidy is a linter that analyzes code and outputs errors found during the analysis, developers can use different versions of clang-tidy on the workflow. Newer versions of clang-tidy might produce better results than older versions. Therefore, it is recommended to use the latest version available.
To ensure consistency among developers, ns-3 defines a minimum version of clang-tidy, whose warnings must not be ignored. Therefore, developers should, at least, scan their code with the minimum version of clang-tidy.
The minimum version is clang-tidy-14.
6.3.3. Integration with IDEs¶
Clang-tidy automatically integrates with modern IDEs (e.g., VS Code) that read the
.clang-tidy
file and automatically checks the code of the currently open file.
Please refer to the documentation of your IDE for more information. Some examples of IDE integration are provided in clang-tidy documentation
6.3.4. Clang-tidy usage¶
In order to use clang-tidy, ns-3 must first be configured with the flag
--enable-clang-tidy
. To configure ns-3 with tests, examples and clang-tidy enabled,
run the following command:
./ns3 configure --enable-examples --enable-tests --enable-clang-tidy
Due to the integration of clang-tidy with CMake, clang-tidy can be run while building ns-3. In this way, clang-tidy errors will be shown alongside build errors on the terminal. To build ns-3 and run clang-tidy, run the the following command:
./ns3 build
To run clang-tidy without building ns-3, use the following commands on the ns-3 main directory:
# Analyze (and fix) a single file with clang-tidy
clang-tidy -p cmake-cache/ [--fix] [--format-style=file] [--quiet] $FILE
# Analyze (and fix) multiple files in parallel
run-clang-tidy -p cmake-cache/ [-fix] [-format] [-quiet] $FILE1 $FILE2 ...
# Analyze (and fix) the entire ns-3 codebase in parallel
run-clang-tidy -p cmake-cache/ [-fix] [-format] [-quiet]
When running clang-tidy, please note that:
Clang-tidy only analyzes implementation files (i.e.,
*.cc
files). Header files are analyzed when they are included by implementation files, with the#include "..."
directive.Not all clang-tidy checks provide automatic fixes. For those cases, a manual fix must be made by the developer.
Enabling clang-tidy will add time to the build process (in the order of minutes).
6.3.5. Disable clang-tidy analysis in specific lines¶
To disable clang-tidy analysis of a particular rule in a specific function, specific clang-tidy comments have to be added to the corresponding function. Please refer to the official clang-tidy documentation for more information.
To disable modernize-use-override
checking on func()
only, use one of the
following two “special comment” syntaxes:
//
// Syntax 1: Comment above the function
//
// NOLINTNEXTLINE(modernize-use-override)
void func();
//
// Syntax 2: Trailing comment
//
void func(); // NOLINT(modernize-use-override)
To disable a specific clang-tidy check on a block of code, for instance the
modernize-use-override
check, surround the code block with the following
“special comments”:
// NOLINTBEGIN(modernize-use-override)
void func1();
void func2();
// NOLINTEND(modernize-use-override)
To disable all clang-tidy checks on a block of code, surround it with the following “special comments”:
// NOLINTBEGIN
void func1();
void func2();
// NOLINTEND
To exclude an entire file from being checked, surround the whole file with the “special comments”.
6.4. Source code formatting¶
The ns-3 coding style was changed between the ns-3.36 and ns-3.37 release.
Prior to ns-3.37, ns-3 used a base GNU coding style. Since ns-3.37, ns-3 changed the
base coding style to what is known in the industry as Allman-style braces,
with four-space indentation. In clang-format, this is configured by selecting the
Microsoft
base style. The following examples illustrate the style.
6.4.1. Indentation¶
Indent code with 4 spaces. When breaking statements into multiple lines, indent the following lines with 4 spaces.
void
Func()
{
int x = 1;
}
Indent constructor’s initialization list with 4 spaces.
MyClass::MyClass(int x, int y)
: m_x(x),
m_y(y)
{
}
Do not use tabs in source code. Always use spaces for indentation and alignment.
6.4.2. Line endings¶
Files use LF (\n
) line endings.
6.4.3. Column limit¶
Code lines should not extend past 100 characters. This allows reading code in wide-screen monitors without having to scroll horizontally, while also allowing editing two files side-by-side.
6.4.4. Braces style¶
Braces should be formatted according to the Allman style. Braces are always on a new line and aligned with the start of the corresponding block. The main body is indented with 4 spaces.
Always surround conditional or loop blocks (e.g., if
, for
, while
)
with braces, and always add a space before the condition’s opening parentheses.
void Foo()
{
if (condition)
{
// do stuff here
}
else
{
// do other stuff here
}
for (int i = 0; i < 100; i++)
{
// do loop
}
while (condition)
{
// do while
}
do
{
// do stuff
} while (condition);
}
6.4.5. Spacing¶
To increase readability, functions, classes and namespaces are separated by one new line. This spacing is optional when declaring variables or functions. Declare one variable per line. Do not mix multiple statements on the same line.
Do not add a space between the function name and the opening parentheses. This rule applies to both function (and method) declarations and invocations.
void Func(const T&); // OK
void Func (const T&); // Not OK
6.4.6. Trailing whitespace¶
Source code and text files must not have trailing whitespace.
6.4.7. Code alignment¶
To improve code readability, trailing comments should be aligned.
int varOne; // Variable one
double varTwo; // Variable two
The trailing \
character of macros should be aligned to the far right
(equal to the column limit). This increases the readability of the macro’s body,
without forcing unnecessary whitespace diffs on surrounding lines when only one
line is changed.
#define MY_MACRO(msg) \
do \
{ \
std::cout << msg << std::endl; \
} while (false);
6.4.8. Class members¶
Definition blocks within a class should be organized in descending order of
public exposure, that is: static
> public
> protected
> private
.
Separate each block with a new line.
class MyClass
{
public:
static int m_counter = 0;
MyClass(int x, int y);
private:
int x;
int y;
};
6.4.9. Function arguments bin packing¶
Function arguments should be declared in the same line as the function declaration. If the arguments list does not fit the maximum column width, declare each one on a separate line and align them vertically.
void ShortFunction(int x, int y);
void VeryLongFunctionWithLongArgumentList(int x,
int y,
int z);
The constructor initializers are always declared one per line, with a trailing comma:
void
MyClass::MyClass(int x, int y)
: m_x(x),
m_y(y)
{
}
6.4.10. Function return types¶
In function declarations, return types are declared on the same line. In function implementations, return types are declared on a separate line.
// Function declaration
void Func(int x, int y);
// Function implementation
void
Func(int x, int y)
{
// (...)
}
6.4.11. Templates¶
Template definitions are always declared in a separate line from the main function declaration:
template <class T>
void Func(T t);
6.4.12. Naming¶
6.4.12.1. Name encoding¶
Function, method, and type names should follow the
CamelCase convention: words are
joined without spaces and are capitalized. For example, “my computer” is
transformed into MyComputer
. Do not use all capital letters such as
MAC
or PHY
, but choose instead Mac
or Phy
. Do not use all
capital letters, even for acronyms such as EDCA
; use Edca
instead.
This applies also to two-letter acronyms, such as IP
(which becomes
Ip
). The goal of the CamelCase convention is to ensure that the words
which make up a name can be separated by the eye: the initial Caps
fills that role. Use PascalCasing (CamelCase with first letter capitalized)
for function, property, event, and class names.
Variable names should follow a slight variation on the base CamelCase
convention: camelBack. For example, the variable user name
would be named
userName
. This variation on the basic naming pattern is used to allow a
reader to distinguish a variable name from its type. For example,
UserName userName
would be used to declare a variable named
userName
of type UserName
.
Global variables should be prefixed with a g_
and member variables
(including static member variables) should be prefixed with a m_
. The goal
of that prefix is to give a reader a sense of where a variable of a given
name is declared to allow the reader to locate the variable declaration and
infer the variable type from that declaration. Defined types will start
with an upper case letter, consist of upper and lower case letters, and may
optionally end with a _t
. Defined constants (such as static const class
members, or enum constants) will be all uppercase letters or numeric digits,
with an underscore character separating words. Otherwise, the underscore
character should not be used in a variable name. For example, you could
declare in your class header my-class.h
:
typedef int NewTypeOfInt_t;
constexpr uint8_t PORT_NUMBER = 17;
class MyClass
{
void MyMethod(int aVar);
int m_aVar;
static int m_anotherVar;
};
and implement in your class file my-class.cc
:
int MyClass::m_anotherVar = 10;
static int g_aStaticVar = 100;
int g_aGlobalVar = 1000;
void
MyClass::MyMethod(int aVar)
{
m_aVar = aVar;
}
As an exception to the above, the members of structures do not need to be
prefixed with an m_
.
Finally, do not use Hungarian notation, and do not prefix enums, classes, or delegates with any letter.
6.4.12.2. Choosing names¶
Variable, function, method, and type names should be based on the English
language, American spelling. Furthermore, always try to choose descriptive
names for them. Types are often english names such as: Packet, Buffer, Mac,
or Phy. Functions and methods are often named based on verbs and adjectives:
GetX
, DoDispose
, ClearArray
, etc.
A long descriptive name which requires a lot of typing is always better than
a short name which is hard to decipher. Do not use abbreviations in names
unless the abbreviation is really unambiguous and obvious to everyone (e.g.,
use size
over sz
). Do not use short inappropriate names such as foo,
bar, or baz. The name of an item should always match its purpose. As such,
names such as tmp
to identify a temporary variable, or such as i
to
identify a loop index are OK.
If you use predicates (that is, functions, variables or methods which return a single boolean value), prefix the name with “is” or “has”.
6.4.13. File layout and code organization¶
A class named MyClass
should be declared in a header named my-class.h
and implemented in a source file named my-class.cc
. The goal of this
naming pattern is to allow a reader to quickly navigate through the ns-3
codebase to locate the source file relevant to a specific type.
Each my-class.h
header should start with the following comment to ensure
that your code is licensed under the GPL, that the copyright holders are properly
identified (typically, you or your employer), and that the actual author
of the code is identified. The latter is purely informational and we use it
to try to track the most appropriate person to review a patch or fix a bug.
Please do not add the “All Rights Reserved” phrase after the copyright
statement.
/*
* Copyright (c) YEAR COPYRIGHTHOLDER
*
* SPDX-License-Identifier: GPL-2.0-only
*
* Author: MyName <myemail@example.com>
*/
Below these C-style comments, always include the following which defines a
set of header guards (MY_CLASS_H
) used to avoid multiple header includes,
which ensures that your code is included in the ns-3 namespace and which
provides a set of Doxygen comments for the public part of your class API.
Detailed information on the set of tags available for doxygen documentation
is described in the Doxygen website.
#ifndef MY_CLASS_H
#define MY_CLASS_H
namespace ns3
{
/**
* \brief short one-line description of the purpose of your class
*
* A longer description of the purpose of your class after a blank
* empty line.
*/
class MyClass
{
public:
MyClass();
/**
* A detailed description of the purpose of the method.
*
* \param firstParam a short description of the purpose of this parameter
* \return a short description of what is returned from this function.
*/
int DoSomething(int firstParam);
private:
/**
* Private method doxygen is also recommended
*/
void MyPrivateMethod();
int m_myPrivateMemberVariable; ///< Brief description of member variable
};
} // namespace ns3
#endif // MY_CLASS_H
The my-class.cc
file is structured similarly:
/*
* Copyright (c) YEAR COPYRIGHTHOLDER
*
* SPDX-License-Identifier: GPL-2.0-only
*
* Author: MyName <myemail@foo.com>
*/
#include "my-class.h"
namespace ns3
{
MyClass::MyClass()
{
}
...
} // namespace ns3
6.4.14. Header file includes¶
Included header files should be organized by source location. The sorting order is as follows:
// Header class (applicable for *.cc files)
#include "my-class.h"
// Includes from the same module
#include "header-from-same-module.h"
// Includes from other modules
#include "ns3/header-from-different-module.h"
// External headers (e.g., STL libraries)
#include <iostream>
Groups should be separated by a new line. Within each group, headers should be sorted alphabetically.
For standard headers, use the C++ style of inclusion:
#include <cstring> // OK
#include <string.h> // Avoid
inside .h files, always use
#include <ns3/header.h>
inside .cc files, use
#include "header.h"
if file is in same directory, otherwise use
#include <ns3/header.h>
6.4.15. Variables and constants¶
Each variable declaration is on a separate line. Variables should be declared at the point in the code where they are needed, and should be assigned an initial value at the time of declaration.
// Do not declare multiple variables per line
int x, y;
// Declare one variable per line and assign an initial value
int x = 0;
int y = 0;
Named constants defined in classes should be declared as static constexpr
instead of
macros, const, or enums. Use of static constexpr
allows a single instance to be
evaluated at compile-time. Declaring the constant in the class enables it to share the scope
of the class.
If the constant is only used in one file, consider declaring the constant in the implementation
file (*.cc
).
// Avoid declaring constants as enum
class LteRlcAmHeader : public Header
{
enum ControlPduType_t
{
STATUS_PDU = 000,
};
};
// Prefer to declare them as static constexpr (in class)
class LteRlcAmHeader : public Header
{
static constexpr uint8_t STATUS_PDU{0};
};
// Or as constexpr (in implementation files)
constexpr uint8_t STATUS_PDU{0};
When declaring variables that are easily deducible from context, prefer to declare them
with auto
instead of repeating the type name. Not only does this improve code readability,
by making lines shorter, but it also facilitates future code refactoring.
// Avoid repeating the type name when declaring iterators or pointers, and casting variables
std::map<uint32_t, std::string>::const_iterator it = myMap.find(key);
int* ptr = new int[10];
uint8_t m = static_cast<uint8_t>(97 + (i % 26));
// Prefer to declare them with auto
auto it = myMap.find(key);
auto* ptr = new int[10];
auto m = static_cast<uint8_t>(97 + (i % 26));
6.4.16. Initialization¶
When declaring variables, prefer to use direct-initialization, to avoid repeating the type name.
// Avoid splitting the declaration and initialization of variables
Ipv4Address ipv4Address = Ipv4Address("192.168.0.1")
// Prefer to use direct-initialization
Ipv4Address ipv4Address("192.168.0.1")
Variables with no default constructor or of primitive types should be initialized when declared.
Variables with default constructors do not need to be explicitly initialized, since the compiler
already does that. An example of this is the ns3::Time
class, which will initialize to zero.
Member variables of structs and classes should be initialized unless the member has a default constructor that guarantees initialization. Preferably, variables should be initialized together with the declaration (in the header file). Alternatively, they can be initialized in the default constructor (in the implementation file), and you may see instances of this in the codebase, but direct initialization upon declaration is preferred going forward.
If all member variables of a class / struct are directly initialized (see above), they do not
require explicit default initialization. But if not all variables are initialized, those
non-initialized variables will contain garbage. Therefore, initializing the class object with
{}
allows all member variables to always be initialized – either with the provided default
initialization or with the primitive type’s default value (typically 0).
C++ supports two syntax choices for direct initialization, either ()
or {}
. There are
various tradeoffs in the choices for more complicated types (consult the C++ literature on
brace vs. parentheses initialization), but for the fundamental types like double
, either is
acceptable (please use consistently within files).
Regarding ns3::Time
, do not initialize to non-zero integer values as follows, assuming
that it will be converted to nanoseconds:
Time t{1000000}; // This is disallowed
The value will be interpreted according to the current resolution, which is ambiguous. A user’s program may have already changed the resolution from the default of nanoseconds to something else by the time of this initialization, and it will be instead interpreted according to 10^6 * the new resolution unit.
Time initialization to raw floating-point values is additionally fraught, because of rounding. Doing so with small values has led to bugs in practice such as timer timeout values of zero time.
When declaring or manipulating Time
objects with known values, prefer to use integer-based representations and
arguments over floating-point fractions, where possible, because integer-based is faster.
This means preferring the use of NanoSeconds
, MicroSeconds
, and MilliSeconds
over
Seconds
. For example, to represent a tenth of a second, prefer MilliSeconds(100)
to Seconds(0.1)
.
To summarize Time declaration and initialization, consider the following examples and comments:
Time t; // OK, will be value-initialized to integer zero
Time t{MilliSeconds(100)}; // OK, fastest, no floating point involved
Time t{"100ms"}; // OK, will perform a string conversion; integer would be faster
Time t{Seconds(0.1)}; // OK, will invoke Seconds(double); integer would be faster
Time t{100000000}; // NOT OK, is interpreted differently when ``Time::SetResolution()`` called
Time t{0.1}; // NOT OK, will round to zero; see above and also merge request !2007
6.4.17. Comments¶
The project uses Doxygen to document
the interfaces, and uses comments for improving the clarity of the code
internally. All classes, methods, and members should have Doxygen comments.
Doxygen comments should use the C-style comment (also known as Javadoc) style.
For comments that are intended to not be exposed publicly in the Doxygen output,
use the @internal
and @endinternal
tags.
Please use the @see
tag for cross-referencing.
All parameters and return values should be documented. The ns-3 codebase uses
both the @
or \
characters for tag identification; please make sure
that usage is consistent within a file.
/**
* MyClass description.
*/
class MyClass
{
/**
* Constructor.
*
* \param n Number of elements.
*/
MyClass(int n);
};
All the functions and variables must be documented, with the exception of member functions inherited from parent classes (the documentation is copied automatically from the parent class), and default constructor/destructor.
It is strongly suggested to use grouping to bind together logically related classes (e.g., all the classes in a module). E.g.;
/**
* \defgroup mynewmodule This is a new module
*/
/**
* \ingroup mynewmodule
*
* MyClassOne description.
*/
class MyClassOne
{
};
/**
* \ingroup mynewmodule
*
* MyClassTwo description.
*/
class MyClassTwo
{
};
In the tests for the module, it is suggested to add an ancillary group:
/**
* \defgroup mynewmodule-test Tests for new module
* \ingroup mynewmodule
* \ingroup tests
*/
/**
* \ingroup mynewmodule-tests
* \brief MyNewModule Test
*/
class MyNewModuleTest : public TestCase
{
};
/**
* \ingroup mynewmodule-tests
* \brief MyNewModule TestSuite
*/
class MyNewModuleTestSuite : public TestSuite
{
public:
MyNewModuleTestSuite();
};
/**
* \ingroup mynewmodule-tests
* Static variable for test initialization
*/
static MyNewModuleTestSuite g_myNewModuleTestSuite;
As for comments within the code, comments should be used to describe intention
or algorithmic overview where is it not immediately obvious from reading the
code alone. There are no minimum comment requirements and small routines
probably need no commenting at all, but it is hoped that many larger
routines will have commenting to aid future maintainers. Please write
complete English sentences and capitalize the first word unless a lower-case
identifier begins the sentence. Two spaces after each sentence helps to make
emacs sentence commands work. Sometimes NS_LOG_DEBUG
statements can
be also used in place of comments.
Short one-line comments and long comments can use the C++ comment style;
that is, //
, but longer comments may use C-style comments.
Use one space after //
or /*
.
/*
* A longer comment,
* with multiple lines.
*/
Variable declaration should have a short, one or two line comment describing the purpose of the variable, unless it is a local variable whose use is obvious from the context. The short comment should be on the same line as the variable declaration, unless it is too long, in which case it should be on the preceding lines.
int nNodes = 3; // Number of nodes
/// Node container with the Wi-Fi stations
NodeContainer wifiStations(3);
6.4.18. Casts¶
Where casts are necessary, use the Google C++ guidance: “Use C++-style casts
like static_cast<float>(double_value)
, or brace initialization for
conversion of arithmetic types like int64 y = int64{1} << 42
.”
Do not use C-style casts, since they can be unsafe.
Try to avoid (and remove current instances of) casting of uint8_t
type to larger integers in our logging output by overriding these types
within the logging system itself. Also, the unary +
operator can be used
to print the numeric value of any variable, such as:
uint8_t flags = 5;
std::cout << "Flags numeric value: " << +flags << std::endl;
Avoid unnecessary casts if minor changes to variable declarations can solve
the issue. In the following example, x
can be declared as float
instead of
int
to avoid the cast, or write numbers in decimal format:
// Do not declare x as int, to avoid casting it to float
int x = 3;
float y = 1 / static_cast<float>(x);
// Prefer to declare x as float
float x = 3.0;
float y = 1 / x;
// Or use 1.0 instead of just 1
int x = 3;
float y = 1.0 / x;
6.4.19. Namespaces¶
Code should always be included in a given namespace, namely ns3
.
In order to avoid exposing internal symbols, consider placing the code in an
anonymous namespace, which can only be accessed by functions in the same file.
Code within namespaces should not be indented. To more easily identify the end of a namespace, add a trailing comment to its closing brace.
namespace ns3
{
// (...)
} // namespace ns3
Namespace names should follow the snake_case convention.
6.4.20. Unused variables¶
Compilers will typically issue warnings on unused entities (e.g., variables,
function parameters). Use the [[maybe_unused]]
attribute to suppress
such warnings when the entity may be unused depending on how the code
is compiled (e.g., if the entity is only used in a logging statement or
an assert statement).
The general guidelines are as follows:
If a function’s or a method’s parameter is definitely unused, prefer to leave it unnamed. In the following example, the second parameter is unnamed.
void UanMacAloha::RxPacketGood(Ptr<Packet> pkt, double, UanTxMode txMode) { UanHeaderCommon header; pkt->RemoveHeader(header); ... }
In this case, the parameter is also not referenced by Doxygen; e.g.,:
/** * Receive packet from lower layer (passed to PHY as callback). * * \param pkt Packet being received. * \param txMode Mode of received packet. */ void RxPacketGood(Ptr<Packet> pkt, double, UanTxMode txMode);
The omission is preferred to commenting out unused parameters, such as:
void UanMacAloha::RxPacketGood(Ptr<Packet> pkt, double /*sinr*/, UanTxMode txMode) { UanHeaderCommon header; pkt->RemoveHeader(header); ... }
If a function’s parameter is only used in certain cases (e.g., logging), or it is part of the function’s Doxygen, mark it as
[[maybe_unused]]
.void TcpSocketBase::CompleteFork(Ptr<Packet> p [[maybe_unused]], const TcpHeader& h, const Address& fromAddress, const Address& toAddress) { NS_LOG_FUNCTION(this << p << h << fromAddress << toAddress); // Remaining code that definitely uses 'h', 'fromAddress' and 'toAddress' ... }
If a local variable saves the result of a function that must always run, but whose value may not be used, declare it
[[maybe_unused]]
.void MyFunction() { int result [[maybe_unused]] = MandatoryFunction(); NS_LOG_DEBUG("result = " << result); }
If a local variable saves the result of a function that is only run in certain cases, prefer to not declare the variable and use the function’s return value directly where needed. This avoids unnecessarily calling the function if its result is not used.
void MyFunction() { // Prefer to call GetDebugInfo() directly on the log statement NS_LOG_DEBUG("Debug information: " << GetDebugInfo()); // Avoid declaring a local variable with the result of GetDebugInfo() int debugInfo [[maybe_unused]] = GetDebugInfo(); NS_LOG_DEBUG("Debug information: " << debugInfo); }
If the calculation of the maybe unused variable is complex, consider wrapping the calculation of its value in a conditional block that is only run if the variable is used.
if (g_log.IsEnabled(ns3::LOG_DEBUG)) { auto debugInfo = GetDebugInfo(); auto value = DoComplexCalculation(debugInfo); NS_LOG_DEBUG("The value is " << value); }
6.4.21. Unnecessary else after return¶
In order to increase readability and avoid deep code nests, consider not adding
an else
block if the if
block breaks the control flow (i.e., when using
return
, break
, continue
, etc.).
For instance, the following code:
if (n < 0)
{
return false;
}
else
{
n += 3;
return n;
}
can be rewritten as:
if (n < 0)
{
return false;
}
n += 3;
return n;
6.4.22. Boolean Simplifications¶
In order to increase readability and performance, avoid unnecessarily complex boolean expressions in if statements and variable declarations.
For instance, the following code:
bool IsPositive(int n)
{
if (n > 0)
{
return true;
}
else
{
return false;
}
}
void ProcessNumber(int n)
{
if (IsPositive(n) == true)
{
...
}
}
can be rewritten as:
bool IsPositive(int n)
{
return n > 0;
}
void ProcessNumber(int n)
{
if (IsPositive(n))
{
...
}
}
6.4.23. Smart pointer boolean comparisons¶
As explained in this issue,
the ns-3 smart pointer class Ptr
should be used in boolean comparisons as follows:
for Ptr<> p, do not use: use instead:
======================== =================================
if (p != nullptr) {...} if (p) {...}
if (p != NULL) {...}
if (p != 0) {...} if (p) {...}
if (p == nullptr) {...} if (!p) {...}
if (p == NULL) {...}
if (p == 0) {...}
NS_ASSERT...(p != nullptr, ...) NS_ASSERT...(p, ...)
NS_ABORT... (p != nullptr, ...) NS_ABORT... (p, ...)
NS_ASSERT...(p == nullptr, ...) NS_ASSERT...(!p, ...)
NS_ABORT... (p == nullptr, ...) NS_ABORT... (!p, ...)
NS_TEST... (p, nullptr, ...) NS_TEST... (p, nullptr, ...)
6.4.24. Code performance tips¶
While developing code, consider the following tips to improve the code’s performance. Some tips are general recommendations, but are not strictly enforced. Other tips are enforced by clang-tidy. Please refer to the clang-tidy section below for more details.
Prefer to use
.emplace_back()
over.push_back()
to optimize performance.When initializing STL containers (e.g.,
std::vector
) with known size, reserve memory to store all items, before pushing them in a loop.constexpr int N_ITEMS = 5; std::vector<int> myVector; myVector.reserve(N_ITEMS); // Reserve memory to store all items for (int i = 0; i < N_ITEMS; i++) { myVector.emplace_back(i); }
Prefer to initialize STL containers (e.g.,
std::vector
,std::map
, etc.) directly through the constructor or with a braced-init-list, instead of pushing elements one-by-one.// Prefer to initialize containers directly std::vector<int> myVector1{1, 2, 3}; std::vector<int> myVector2(myVector1.begin(), myVector1.end()); std::vector<bool> myVector3(myVector2.size(), true); // Avoid pushing elements one-by-one std::vector<int> myVector1; myVector1.reserve(3); myVector1.emplace_back(1); myVector1.emplace_back(2); myVector1.emplace_back(3); std::vector<int> myVector2; myVector2.reserve(myVector1.size()); for (const auto& v : myVector1) { myVector2.emplace_back(v); } std::vector<bool> myVector3; myVector3.reserve(myVector1.size()); for (std::size_t i = 0; i < myVector1.size(); i++) { myVector3.emplace_back(true); }
When looping through containers, prefer to use const-ref syntax over copying elements.
std::vector<int> myVector{1, 2, 3}; for (const auto& v : myVector) { ... } // OK for (auto v : myVector) { ... } // Avoid
Prefer to use the
empty()
function of STL containers (e.g.,std::vector
), instead of the conditionsize() > 0
, to avoid unnecessarily calculating the size of the container.Avoid unnecessary calls to the functions
.c_str()
and.data()
ofstd::string
.Avoid unnecessarily dereferencing std smart pointers (
std::shared_ptr
,std::unique_ptr
) with calls to their member function.get()
. Prefer to use the std smart pointer directly where needed.auto ptr = std::make_shared<Node>(); // OK if (ptr) { ... } // Avoid if (ptr.get()) { ... }
Consider caching frequently-used results (especially expensive calculations, such as mathematical functions) in a temporary variable, instead of calculating them in every loop.
// Prefer to cache intermediate results const double sinTheta = std::sin(theta); const double cosTheta = std::cos(theta); for (uint8_t i = 0; i < NUM_VALUES; i++) { double power = std::pow(2, i); array1[i] = (power * sinTheta) + cosTheta; array2[i] = (power * cosTheta) + sinTheta; } // Avoid repeating calculations for (uint8_t i = 0; i < NUM_VALUES; i++) { array1[i] = (std::pow(2, i) * std::sin(theta)) + std::cos(theta); array2[i] = (std::pow(2, i) * std::cos(theta)) + std::sin(theta); }
Do not include inline implementations in header files; put all implementation in a
.cc
file (unless implementation in the header file brings demonstrable and significant performance improvement).Avoid declaring trivial destructors, to optimize performance.
6.4.25. C++ standard¶
As of ns-3.36, ns-3 permits the use of C++-17 (or earlier) features in the implementation files.
If a developer would like to propose to raise this bar to include more features than this, please email the developers list. We will move this language support forward as our minimally supported compiler moves forward.
6.4.26. Guidelines for using maps¶
Maps (associative containers) are used heavily in ns-3 models to store key/value pairs. The C++ standard, over time, has added various methods to insert elements to maps, and the ns-3 codebase has made use of most or all of these constructs. For the sake of uniformity and readability, the following guidelines are recommended for any new code.
Prefer the use of std::map
to std::unordered_map
unless there is
a measurable performance advantage. Use std::unordered_map
only for
use cases in which the map does not need to be iterated or the iteration
order does not affect the results of the operation (because different
implementations of the hash function may lead to different iteration orders
on different systems).
Keep in mind that C++ now allows several methods to insert values into
maps, and the behavior can be different when a value already exists for
a key. If the intended behavior is that the insertion should not overwrite
an existing value for the key, try_emplace()
can be a good choice. If
the intention is to allow the overwriting of a key/value pair,
insert_or_assign()
can be a good choice. Both of the above methods
provide return values that can be checked– in the case of try_emplace()
,
whether the insertion succeeded or did not occur, and in the case of
insert_or_assign()
, whether an insertion or assignment occurred.
6.4.27. Miscellaneous items¶
NS_LOG_COMPONENT_DEFINE("log-component-name");
statements should be placed withinnamespace ns3
(for module code) and after theusing namespace ns3;
. In examples,NS_OBJECT_ENSURE_REGISTERED()
should also be placed withinnamespace ns3
.Pointers and references are left-aligned:
int x = 1; int* ptr = &x; int& ref = x;
Use a trailing comma in braced-init-lists, so that each item is positioned in a new line.
const std::vector<std::string> myVector{ "string-1", "string-2", "string-3", }; const std::map<int, std::string> myMap{ {1, "string-1"}, {2, "string-2"}, {3, "string-3"}, };
Const reference syntax:
void MySub(const T& t); // OK void MySub(T const& t); // Not OK
Do not use
NULL
,nil
or0
constants; usenullptr
(improves portability)Consider whether you want the default constructor, copy constructor, or assignment operator in your class. If not, explicitly mark them as deleted and make the declaration public:
class MyClass { public: // Allowed constructors MyClass(int i); // Deleted constructors. // Explain why they are not supported. MyClass() = delete; MyClass(const MyClass&) = delete; MyClass& operator=(const MyClass&) = delete; };
Avoid returning a reference to an internal or local member of an object:
MyType& foo(); // Avoid. Prefer to return a pointer or an object. const MyType& foo(); // Same as above.
This guidance does not apply to the use of references to implement operators.
Expose class members through access functions, rather than direct access to a public object. The access functions are typically named
Get
andSet
followed by the member’s name. For example, a memberm_delayTime
might have accessor functionsGetDelayTime()
andSetDelayTime()
.Do not bring the C++ standard library namespace into ns-3 source files by using the
using namespace std;
directive.Do not use the C++
goto
statement.Do not add the
enum
orstruct
specifiers when declaring the variable’s type.Do not unnecessarily add
typedef
tostruct
orenum
.// Avoid typedef struct { ... } MyStruct; // Prefer struct MyStruct { ... };
When checking whether a Time value is zero, use
Time::IsZero()
rather than comparing it to a zero-valued time object withoperator==
, to avoid construction of a temporary. Similar guidance applies to the related functionsTime::IsPositive()
,Time::IsNegative()
,Time::IsStrictlyPositive
, andTime::IsStrictlyNegative()
.Time t = ...; // prefer the below: if (t.IsStrictlyPositive()) {...} // to this alternative: if (t > Seconds(0)) {...}
6.4.28. Clang-tidy rules¶
Please refer to the .clang-tidy
file in the ns-3 main directory for the full list
of rules that should be observed while developing code.
Some rules are explained in the corresponding sections above. The remaining rules are explained here.
Explicitly mark inherited functions with the
override
specifier.When creating STL smart pointers, prefer to use
std::make_shared
orstd::make_unique
, instead of creating the smart pointer withnew
.auto node = std::make_shared<Node>(); // OK auto node = std::shared_ptr<Node>(new Node()); // Avoid
When looping through containers, prefer to use range-based for loops rather than index-based loops.
std::vector<int> myVector{1, 2, 3}; for (const auto& v : myVector) { ... } // Prefer for (int i = 0; i < myVector.size(); i++) { ... } // Avoid
Avoid accessing class static functions and members through objects. Instead, prefer to access them through the class.
// OK MyClass::StaticFunction(); // Avoid MyClass myClass; MyClass.StaticFunction();
Prefer using type traits in short form
traits_t<...>
andtraits_v<...>
, instead of the long formtraits<...>::type
andtraits<...>::value
, respectively.// Prefer using the shorter version of type traits std::is_same_v<int, float> std::is_integral_v<T> std::enable_if_t<std::is_integral_v<T>, Time> // Avoid the longer form of type traits std::is_same<int, float>::value std::is_integral<T>::value std::enable_if<std::is_integral<T>::value, Time>::type
Avoid using integer values (
1
or0
) to represent boolean variables (true
orfalse
), to improve code readability and avoid implicit conversions.Prefer to use
static_assert()
overNS_ASSERT()
when conditions can be evaluated at compile-time.Prefer using transparent functors to non-transparent ones, to avoid repeating the type name. This improves readability and avoids errors when refactoring code.
// Prefer using transparent functors std::map<MyClass, int, std::less<>> myMap; // Avoid repeating the type name "MyClass" in std::less<> std::map<MyClass, int, std::less<MyClass>> myMap;
In conditional control blocks (i.e., if-else and switch-case), avoid declaring multiple branch conditions with the same content to avoid duplicating code.
In if-else blocks, prefer grouping the identical bodies in a single if condition with a disjunction of the multiple conditions.
if (condition1) { Foo(); } else if (condition2) { // Same body as condition 1 Foo(); } else { Bar(); } // Prefer grouping the two conditions if (condition1 || condition2) { Foo(); } else { Bar(); }
In switch-case blocks, prefer grouping identical
case
labels by removing the duplicate bodies of the formercase
labels.switch (condition) { case 1: Foo(); break; case 2: // case 2 has the same body as case 1 Foo(); break; case 3: Bar(); break; } switch (condition) { // Group identical cases by removing the content of case 1 and letting it fallthrough to case 2 case 1: case 2: Foo(); break; case 3: Bar(); break; }
6.5. CMake file formatting¶
The CMakeLists.txt
and other *.cmake
files follow the formatting rules defined in
build-support/cmake-format.yaml
and build-support/cmake-format-modules.yaml
.
The first set of rules applies to CMake files in all directories that are not modules, while the second one applies to files within modules.
Those rules are enforced via the cmake-format tool, that can be installed via Pip.
pip install cmake-format pyyaml
After installing cmake-format, it can be called to fix the formatting of a CMake file with the following command:
cmake-format -c ./build-support/cmake-format.yaml CMakeLists.txt
To check the formatting, add the –check option to the command, before specifying the list of CMake files.
Instead of calling this command for every single CMake file, it is recommended to use
the ns3
script to run the custom targets that do that automatically.
# Check CMake formatting
./ns3 build cmake-format-check
# Check and fix CMake formatting
./ns3 build cmake-format
Custom functions and macros need to be explicitly configured in the cmake-format.yaml
files,
otherwise their formatting will be broken.
6.6. Python file formatting¶
Python format style and rule enforcement is based on the default settings for the Black formatter tool and Isort import sorter tool. Black default format is detailed in Black current style.
The custom settings for both tools are set in the pyproject.toml
file.
These tools that can be installed via Pip, using the following command:
pip install black isort
To check the formatting, add the –check option to the command:
black --check .
isort --check .
To check and fix the formatting, run the commands as follows:
black .
isort .
For VS Code users, MS Black formatter and MS Isort extensions, which repackage
Black and Isort for VS Code, can be installed to apply fixes regularly.
To configure VS Code to automatically format code when saving, editing or pasting code,
add the following configuration to .vscode/settings.json
:
{
"editor.formatOnPaste": true,
"editor.formatOnSave": true,
"editor.formatOnType": true,
"[python]": {
"editor.defaultFormatter": "ms-python.black-formatter",
"editor.codeActionsOnSave": {
"source.organizeImports": "explicit",
},
},
"black-formatter.args": [
"--config",
"pyproject.toml",
],
"isort.check": true,
"isort.args": [
"--sp",
"pyproject.toml",
],
}