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if (showOneValue) { ShowHideByName(showOneValue[2]); } RefreshVisibilityFromHashParam(); } </SCRIPT> </HEAD> <BODY> <H1>ALICE O² C++ Coding Guidelines</H1> <address> Vasco Barroso<br> Alina GrigoraȘ<br> Ivana Hřivnáčová<br> Matthias Kretz<br> Adriana Telesca<br> Barthélémy von Haller<br> </address> <p align="right"> This document is based on the work of <br> B. Weinberger, C. Silverstein, <br> G. Eitzmann, M. Mentovai <br> and T.Landray <br> at <a href="http://google-styleguide.googlecode.com"> http://google-styleguide.googlecode.com</a>, <br> C++ Google Style guide, Revision 3.274 <br> under the <a rel="license" href="http://creativecommons.org/licenses/by/3.0/deed.en_US">CC-By 3.0 License</a> </p> <DIV style="margin-left: 50%; font-size: 75%;"> <P> Each style point has a summary for which additional information is available by toggling the accompanying arrow button that looks this way: <SPAN class="showhide_button" style="margin-left: 0; float: none">▶</SPAN>. You may toggle all summaries with the big arrow button: </P> <DIV style=" font-size: larger; margin-left: +2em;"> <SPAN class="showhide_button" style="font-size: 180%; float: none" onclick="javascript:ShowHideAll()" name="show_hide_all_button" id="show_hide_all_button">▶</SPAN> Toggle all summaries </DIV> <DIV style=" font-size: larger; margin-left: +2em;"> <SPAN class="showhide_extrabutton" style="font-size: 180%; float: none" onclick="javascript:ShowHideAllExtra()" name="show_hide_all_extra_button" id="show_hide_all_extra_button">▶</SPAN> Toggle all extra details </DIV> </DIV> <DIV class="toc"> <DIV class="toc_title">Table of Contents</DIV> <TABLE> <TR valign="top" class=""> <TD><DIV class="toc_category"><A href="#Header_Files">Header Files</A></DIV></TD> <TD><DIV class="toc_stylepoint"> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#The__define_Guard">The #define Guard</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Forward_Declarations">Forward Declarations</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Inline_Functions">Inline Functions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Names_and_Order_of_Includes">Names and Order of Includes</A></SPAN> </DIV></TD> </TR> <TR valign="top" class=""> <TD><DIV class="toc_category"><A href="#Namespaces">Namespaces</A></DIV></TD> <TD><DIV class="toc_stylepoint"> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#General_guideline">General guideline</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Using_declarations_and_directives">Using declarations and directives</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Unnamed_namespaces">Unnamed namespaces</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Namespace_aliases">Namespace aliases</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#std_namespace">std namespace</A></SPAN> </DIV></TD> </TR> <TR valign="top" class=""> <TD><DIV class="toc_category"><A href="#Scoping">Scoping</A></DIV></TD> <TD><DIV class="toc_stylepoint"> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Nonmember_and_Global_Functions">Nonmember and Global Functions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Local_Variables">Local Variables</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Variables_Initialization">Variables Initialization</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Brace_Initialization">Brace Initialization</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Global_Variables">Global Variables</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Global_variables_initialization">Global variables initialization</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Static_Variables_in_functions">Static Variables in functions</A></SPAN> </DIV></TD> </TR> <TR valign="top" class=""> <TD><DIV class="toc_category"><A href="#Classes">Classes</A></DIV></TD> <TD><DIV class="toc_stylepoint"> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Initialization">Initialization</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Virtual_functions_in_constructors_and_destructors">Virtual functions in constructors and destructors</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Explicit_Constructors">Explicit Constructors</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Copy__and_Move_">Copy (and Move)</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Delegating_and_inheriting_constructors">Delegating and inheriting constructors</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Structs_vs._Classes">Structs vs. Classes</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Destructors">Destructors</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Inheritance">Inheritance</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Multiple_Inheritance">Multiple Inheritance</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Interfaces">Interfaces</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Operator_Overloading">Operator Overloading</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Access_Control">Access Control</A></SPAN> </DIV></TD> </TR> <TR valign="top" class=""> <TD><DIV class="toc_category"><A href="#Others">Others</A></DIV></TD> <TD><DIV class="toc_stylepoint"> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Exceptions">Exceptions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Use_of_const">Use of const</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Use_of_constexpr">Use of constexpr</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Smart_Pointers">Smart Pointers</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Magic_numbers">Magic numbers</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Preprocessor_Macros">Preprocessor Macros</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Write_Short_Functions">Write Short Functions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Run-Time_Type_Information__RTTI_">Run-Time Type Information (RTTI)</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Casting">Casting</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Variable-Length_Arrays_and_alloca__">Variable-Length Arrays and alloca()</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Increment_and_Decrement_operators">Increment and Decrement operators</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Loops_and_Switch_Statements">Loops and Switch Statements</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Integer_Types">Integer Types</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Portability">Portability</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#0_and_nullptr">0 and nullptr</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#sizeof">sizeof</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#auto">auto</A></SPAN> </DIV></TD> </TR> <TR valign="top" class=""> <TD><DIV class="toc_category"><A href="#Exceptions_to_the_Rules">Exceptions to the Rules</A></DIV></TD> <TD><DIV class="toc_stylepoint"> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Existing_Non-conformant_Code">Existing Non-conformant Code</A></SPAN> </DIV></TD> </TR> </TABLE> </DIV> <DIV class=""> <H2 name="Important_Note" id="Important_Note">Important Note</H2> <DIV class=""> <H3><A name="Displaying_Hidden_Details_in_this_Guide" id="Displaying_Hidden_Details_in_this_Guide">Displaying Hidden Details in this Guide</A></H3> <SPAN class="link_button" id="link-Displaying_Hidden_Details_in_this_Guide__button" name="link-Displaying_Hidden_Details_in_this_Guide__button"><A href="?showone=Displaying_Hidden_Details_in_this_Guide#Displaying_Hidden_Details_in_this_Guide"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Displaying_Hidden_Details_in_this_Guide')" name="Displaying_Hidden_Details_in_this_Guide__button" id="Displaying_Hidden_Details_in_this_Guide__button">▶</SPAN> <DIV style="display:inline;" class=""> This style guide contains many details that are initially hidden from view. They are marked by the triangle icon, which you see here on your left. The first level of hidden information is the subsection <i>Summary</i> in each rule and the second level of hidden information is the optional subsection <i>Extra details and exceptions to the rule</i>. Click the arrow on the left now, you should see "Hooray" appear below. </DIV> <DIV class=""><DIV class="stylepoint_body" name="Displaying_Hidden_Details_in_this_Guide__body" id="Displaying_Hidden_Details_in_this_Guide__body" style="display: none"> <p> Hooray! Now you know you can expand points to get more details. Alternatively, there are an "expand all summaries" and an "expand all summaries and extra details" at the top of this document. </p> </DIV></DIV> </DIV> </DIV> <DIV class=""> <H2 name="Background" id="Background">Background</H2> <p> As every C++ programmer knows, the language has many powerful features, but this power brings with it complexity, which in turn can make code more bug-prone and harder to read and maintain. </p> <p> The goal of this guide is to manage this complexity by describing in detail the dos and don'ts of writing C++ code. These rules exist to keep the code base manageable while still allowing coders to use C++ language features productively. </p> <p> Another issue this guide addresses is that of C++ feature bloat. C++ is a huge language with many advanced features. In some cases we constrain, or even ban, use of certain features. We do this to keep code simple and to avoid the various common errors and problems that these features can cause. This guide lists these features and explains why their use is restricted. </p> <p> Note that this guide is not a C++ tutorial: we assume that the reader is familiar with the language. </p> </DIV> <DIV class=""> <H2 name="Header_Files" id="Header_Files">Header Files</H2> <p> In general, every <code>.cxx</code> file should have an associated <code>.h</code> file. There are some common exceptions, such as unittests and small <code>.cxx</code> files containing just a <code>main()</code> function. </p> <p> Correct use of header files can make a huge difference to the readability, size and performance of your code. </p> <p> The following rules will guide you through the various pitfalls of using header files. </p> <DIV class=""> <H3><A name="The__define_Guard" id="The__define_Guard">The #define Guard</A></H3> <SPAN class="link_button" id="link-The__define_Guard__button" name="link-The__define_Guard__button"><A href="?showone=The__define_Guard#The__define_Guard"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('The__define_Guard')" name="The__define_Guard__button" id="The__define_Guard__button">▶</SPAN> <DIV style="display:inline;" class=""> All header files should have <code>#define</code> guards to prevent multiple inclusion. The format of the symbol name should be <code><i><PROJECT></i>_<i><PATH></i>_<i><FILE></i>_H_</code>. Avoid using <code>#pragma once</code>. </DIV> <DIV class=""><DIV class="stylepoint_body" name="The__define_Guard__body" id="The__define_Guard__body" style="display: none"> <p> To guarantee uniqueness, they should be based on the full path in a project's source tree. For example, the file <code>foo/src/bar/myFile.h</code> in project <code>foo</code> should have the following guard: </p> <DIV class=""><PRE>#ifndef FOO_BAR_MYFILE_H_ #define FOO_BAR_MYFILE_H_ ... #endif // FOO_BAR_MYFILE_H_</PRE></DIV> </DIV></DIV> </DIV> <DIV class=""> <H3><A name="Forward_Declarations" id="Forward_Declarations">Forward Declarations</A></H3> <SPAN class="link_button" id="link-Forward_Declarations__button" name="link-Forward_Declarations__button"><A href="?showone=Forward_Declarations#Forward_Declarations"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Forward_Declarations')" name="Forward_Declarations__button" id="Forward_Declarations__button">▶</SPAN> <DIV style="display:inline;" class=""> You may forward declare ordinary classes in order to avoid unnecessary <code>#include</code>s. </DIV> <DIV class=""><DIV class="stylepoint_body" name="Forward_Declarations__body" id="Forward_Declarations__body" style="display: none"> <P class=""> <SPAN class="stylepoint_section">Definition: </SPAN> A "forward declaration" is a declaration of a class, function, or template without an associated definition. <code>#include</code> lines can often be replaced with forward declarations of whatever symbols are actually used by the client code. </P> <P class=""> <SPAN class="stylepoint_section">Pros: </SPAN> <ul> <li>Unnecessary <code>#include</code>s force the compiler to open more files and process more input.</li> <li>They can also force your code to be recompiled more often, due to changes in the header.</li> </ul> </P> <P class=""> <SPAN class="stylepoint_section">Cons: </SPAN> <ul> <li>It can be difficult to determine the correct form of a forward declaration in the presence of features like templates, typedefs, default parameters, and using declarations.</li> <li>Forward declaring multiple symbols from a header can be more verbose than simply including the header.</li> <li>Forward declarations of functions and templates can prevent the header owners from making otherwise-compatible changes to their APIs; for example, widening a parameter type, or adding a template parameter with a default value.</li> <li>Forward declaring symbols from namespace <code>std::</code> usually yields undefined behavior.</li> </ul> </P> <P class=""> <SPAN class="stylepoint_section">Decision: </SPAN> <ul> <li>When using a function declared in a header file, always <code>#include</code> that header.</li> <li>When using a class template, prefer to <code>#include</code> its header file.</li> <li>When using an ordinary class, relying on a forward declaration is OK, but be wary of situations where a forward declaration may be insufficient or incorrect; when in doubt, just <code>#include</code> the appropriate header.</li> <li>Do not replace data members with pointers just to avoid an <code>#include</code>.</li> </ul> Always <code>#include</code> the file that actually provides the declarations/definitions you need; do not rely on the symbol being brought in transitively via headers not directly included. One exception is that <code>myFile.cxx</code> may rely on <code>#include</code>s and forward declarations from its corresponding header file <code>myFile.h</code>. </P> </DIV></DIV> </DIV> <DIV class=""> <H3><A name="Inline_Functions" id="Inline_Functions">Inline Functions</A></H3> <SPAN class="link_button" id="link-Inline_Functions__button" name="link-Inline_Functions__button"><A href="?showone=Inline_Functions#Inline_Functions"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Inline_Functions')" name="Inline_Functions__button" id="Inline_Functions__button">▶</SPAN> <DIV style="display:inline;" class=""> As a general rule, put function definitions into the <code>.cxx</code> file and let the compiler decide what gets inlined (it can decide anyway, regardless of the inline keyword). Use inline when you require the implementation of a function in multiple translation units (e.g. template classes/functions). </DIV> <DIV class=""><DIV class="stylepoint_body" name="Inline_Functions__body" id="Inline_Functions__body" style="display: none"> <P class=""> <SPAN class="stylepoint_section">Definition: </SPAN> <p> The inline keyword indicates that inline substitution of the function body at the point of call is to be preferred to the usual function call mechanism. But a compiler is not required to perform this inline substitution at the point of call. </p> <p> Functions that are defined within a class definition are implicitly inline. </p> <p> An inline function must be defined in every translation unit from where it is called. It is undefined behavior if the definition of the inline function is not the same for all translation units. Note that this implies that the function is defined in a header file. This can have an impact on compile time and lead to longer (= less efficient) development cycles. </p> <p> Note that the inline keyword has no effect on the linkage of a function. Linkage can be changed via unnamed namespaces or the static keyword. </p> </P> <P class=""> <SPAN class="stylepoint_section">Decision: </SPAN> <p> If you add a new function, put it into the <code>.cxx</code> file per default. Small functions, like accessors and mutators may be placed into the <code>.h</code> file instead (<code>inline</code>). Also, most template function implementations need to go into the <code>.h</code> file. If you later determine that a function should be moved from the <code>.cxx</code> file into the <code>.h</code> file, please make sure that it helps the compiler in optimizing the code. Otherwise you're just increasing compile time. </p> </P> </DIV></DIV> </DIV> <DIV class=""> <H3><A name="Names_and_Order_of_Includes" id="Names_and_Order_of_Includes">Names and Order of Includes</A></H3> <SPAN class="link_button" id="link-Names_and_Order_of_Includes__button" name="link-Names_and_Order_of_Includes__button"><A href="?showone=Names_and_Order_of_Includes#Names_and_Order_of_Includes"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Names_and_Order_of_Includes')" name="Names_and_Order_of_Includes__button" id="Names_and_Order_of_Includes__button">▶</SPAN> <DIV style="display:inline;" class=""> Include headers from external libraries using angle brackets. Include headers from your own project/libraries using double quotes.<br> Do not rely on implicit includes. Make header files self-sufficient. <br> </DIV> <DIV class=""><DIV class="stylepoint_body" name="Names_and_Order_of_Includes__body" id="Names_and_Order_of_Includes__body" style="display: none"> <p> There are two types of #include statements: <code>#include <myFile.h></code> and <code>#include “myFile.h”</code>. </p> <ul> <li> Include headers from external libraries using angle brackets. <DIV class=""><PRE>#include <iostream> #include <QtCore/QDate> #include <zlib.h></PRE></DIV> </li> <li> Include headers from your own project using double quotes. <DIV class=""><PRE>#include "MyClass.h"</PRE></DIV> </li> </ul> <p> The header files of external libraries are obviously not in the same directory as your source files. So you need to use angle brackets. </p> <p> Headers of your own application have a defined relative location to the source files of your application. Using double quotes, you have to specify the correct relative path to the include file. </p> <i>Include order </i> <p> Another important aspect of include management is the include order. Typically, you have a class named Foo, a file Foo.h and a file Foo.cxx . The rule is : In your file Foo.cxx, you should include Foo.h as the first include, before the system includes. </p> <p> The rationale behind that is to make your header standalone. </p> <p> Let's imagine that your Foo.h looks like this: <DIV class=""><PRE class="badcode">class Foo { public: Bar getBar(); }</PRE></DIV> </p> <p> And your Foo.cpp looks like this: <DIV class=""><PRE class="badcode">#include "Bar.h" #include "Foo.h"</PRE></DIV> </p> <p> Your Foo.cxx file will compile, but it will not compile for other people using Foo.h without including Bar.h. Including Foo.h first makes sure that your Foo.h header works for others. <DIV class=""><PRE>// Foo.h #include "Bar.h" class Foo { public: Bar getBar(); } // Foo.cxx #include "Foo.h"</PRE></DIV> </p> <p> For more details: <a href="https://community.kde.org/Policies/Library_Code_Policy#Getting_.23includes_right">Getting #includes right</a>. </p> </DIV></DIV> </DIV> </DIV> <DIV class=""> <H2 name="Namespaces" id="Namespaces">Namespaces</H2> <p> Namespaces subdivide the global scope into distinct, named scopes, and thus are useful for logically grouping related types and functions and preventing name collisions. </p> <DIV class=""> <H3><A name="General_guideline" id="General_guideline">General guideline</A></H3> <SPAN class="link_button" id="link-General_guideline__button" name="link-General_guideline__button"><A href="?showone=General_guideline#General_guideline"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('General_guideline')" name="General_guideline__button" id="General_guideline__button">▶</SPAN> <DIV style="display:inline;" class=""> Use a namespace for the project, a second nested namespace for code inside a sub-project. Use one or two more nested namespaces to logically group types and functions together. </DIV> <DIV class=""><DIV class="stylepoint_body" name="General_guideline__body" id="General_guideline__body" style="display: none"> <p> Example of a typical class definition within a project and sub-project namespace: </p> <DIV class=""><PRE>namespace project::subproject { class MyClass { ... }; } // namespace subproject::project</PRE></DIV> <p>Alternatively, the C++98/11/14 style for nested namespaces can be used:</p> <DIV class=""><PRE>namespace project { namespace subproject { class MyClass { ... }; } // namespace subproject } // namespace project</PRE></DIV> <p> A nonmember function that is logically tied to a specific type should be in the same namespace as that type. </p> </DIV></DIV> </DIV> <DIV class=""> <H3><A name="Using_declarations_and_directives" id="Using_declarations_and_directives">Using declarations and directives</A></H3> <SPAN class="link_button" id="link-Using_declarations_and_directives__button" name="link-Using_declarations_and_directives__button"><A href="?showone=Using_declarations_and_directives#Using_declarations_and_directives"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Using_declarations_and_directives')" name="Using_declarations_and_directives__button" id="Using_declarations_and_directives__button">▶</SPAN> <DIV style="display:inline;" class=""> Don't write namespace using declarations or using directives in a header file or before an #include. </DIV> <DIV class=""><DIV class="stylepoint_body" name="Using_declarations_and_directives__body" id="Using_declarations_and_directives__body" style="display: none"> <p> <DIV class=""><PRE># include "Bar.h" // OK in .cxx after include statements using namespace foo; // sometimes a using declaration is preferable, to be precise // about the symbols that get imported using Foo::Type;</PRE></DIV> </p> <p> <DIV class=""><PRE class="badcode">// Forbidden in .h -- This pollutes the namespace. using namespace foo;</PRE></DIV> </p> <SPAN class="showhide_extrabutton" onclick="javascript:ToggleExtraByName('Using_declarations_and_directives')" name="Using_declarations_and_directives__extra_button" id="Using_declarations_and_directives__extra_button">▶</SPAN><P> <SPAN class="stylepoint_section">Extra details and exceptions to the rules: </SPAN><div style="display:none;" class="stylepoint_extra" name="Using_declarations_and_directives__extra_body"> The using directive can sometimes be useful in header files to import one namespace into another one. This can effectively hide a namespace from the public interface, similar to what an inline namespace does. </div> </P> </DIV></DIV> </DIV> <DIV class=""> <H3><A name="Unnamed_namespaces" id="Unnamed_namespaces">Unnamed namespaces</A></H3> <SPAN class="link_button" id="link-Unnamed_namespaces__button" name="link-Unnamed_namespaces__button"><A href="?showone=Unnamed_namespaces#Unnamed_namespaces"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Unnamed_namespaces')" name="Unnamed_namespaces__button" id="Unnamed_namespaces__button">▶</SPAN> <DIV style="display:inline;" class=""> Unnamed namespaces are allowed and even encouraged in <code>.cxx</code> files, they are not allowed in <code>.h</code> files. </DIV> <DIV class=""><DIV class="stylepoint_body" name="Unnamed_namespaces__body" id="Unnamed_namespaces__body" style="display: none"> <P class=""> <SPAN class="stylepoint_section">Definition: </SPAN> All symbols inside an unnamed namespace (sometimes also called anonymous namespace) will have internal linkage. Thus, it is impossible to access these objects from other translation units (.cxx files). </P> <P class=""> <SPAN class="stylepoint_section">Pros: </SPAN> Because of the internal linkage it is easier to reason about the code. This can lead to better optimization from compilers and clearer encapsulation. </P> <P class=""> <SPAN class="stylepoint_section">Decision: </SPAN> Use unnamed namespaces in <code>.cxx</code> files whenever possible. </P> <p> <DIV class=""><PRE>namespace project { namespace // This is in a .cxx file. { int someFunction(int value) { return value + 1; } } // unnamed namespace void someOtherFunction() { int value = 0; value = someFunction(value); ... } }</PRE></DIV> For the above code the compiler can see that <code>someFunction</code> is called from only one place. It can thus inline the code and drop <code>someFunction</code>. </p> <SPAN class="showhide_extrabutton" onclick="javascript:ToggleExtraByName('Unnamed_namespaces')" name="Unnamed_namespaces__extra_button" id="Unnamed_namespaces__extra_button">▶</SPAN><P> <SPAN class="stylepoint_section">Extra details and exceptions to the rules: </SPAN><div style="display:none;" class="stylepoint_extra" name="Unnamed_namespaces__extra_body"> In some special cases an unnamed namespace can be useful in a header file. Leave such decisions to architecture designers. </div> </P> </DIV></DIV> </DIV> <DIV class=""> <H3><A name="Namespace_aliases" id="Namespace_aliases">Namespace aliases</A></H3> <SPAN class="link_button" id="link-Namespace_aliases__button" name="link-Namespace_aliases__button"><A href="?showone=Namespace_aliases#Namespace_aliases"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Namespace_aliases')" name="Namespace_aliases__button" id="Namespace_aliases__button">▶</SPAN> <DIV style="display:inline;" class=""> Namespace aliases are allowed anywhere in a <code>.cxx</code> file, inside the named namespace that wraps an entire <code>.h</code> file and in functions and methods. </DIV> <DIV class=""><DIV class="stylepoint_body" name="Namespace_aliases__body" id="Namespace_aliases__body" style="display: none"> <p> <DIV class=""><PRE>// Shorten access to some commonly used names in .cxx files. namespace fbz = ::foo::bar::baz; // Shorten access to some commonly used names (in a .h file). namespace librarian { // The following alias is available to all files including // this header (in namespace librarian): // alias names should therefore be chosen consistently // within a project. namespace pds = ::pipeline_diagnostics::Sidetable; inline void myInlineFunction() { // namespace alias local to a function (or method). namespace fbz = ::foo::bar::baz; ... } } // namespace librarian</PRE></DIV> </p> <p> Note that an alias in a .h file is visible to everyone including that file, so public headers (those available outside a project) and headers transitively included by them, should avoid defining aliases, as part of the general goal of keeping public APIs as small as possible. </p> </DIV></DIV> </DIV> <DIV class=""> <H3><A name="std_namespace" id="std_namespace">std namespace</A></H3> <SPAN class="link_button" id="link-std_namespace__button" name="link-std_namespace__button"><A href="?showone=std_namespace#std_namespace"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('std_namespace')" name="std_namespace__button" id="std_namespace__button">▶</SPAN> <DIV style="display:inline;" class=""> Do not declare anything in namespace <code>std</code>, not even forward declarations of standard library classes. </DIV> <DIV class=""><DIV class="stylepoint_body" name="std_namespace__body" id="std_namespace__body" style="display: none"> <p> Declaring entities in namespace <code>std</code> represents undefined behavior, i.e., not portable. To declare entities from the standard library, include the appropriate header file. </p> </DIV></DIV> </DIV> </DIV> <DIV class=""> <H2 name="Scoping" id="Scoping">Scoping</H2> <DIV class=""> <H3><A name="Nonmember_and_Global_Functions" id="Nonmember_and_Global_Functions">Nonmember and Global Functions</A></H3> <SPAN class="link_button" id="link-Nonmember_and_Global_Functions__button" name="link-Nonmember_and_Global_Functions__button"><A href="?showone=Nonmember_and_Global_Functions#Nonmember_and_Global_Functions"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Nonmember_and_Global_Functions')" name="Nonmember_and_Global_Functions__button" id="Nonmember_and_Global_Functions__button">▶</SPAN> <DIV style="display:inline;" class=""> Nonmember functions (also known as global functions) should be within a namespace. </DIV> <DIV class=""><DIV class="stylepoint_body" name="Nonmember_and_Global_Functions__body" id="Nonmember_and_Global_Functions__body" style="display: none"> <p>Putting nonmember functions in a namespace avoids polluting the global namespace. Static member functions are an alternative as long as it makes sense to include the function within the class.</p> <DIV class=""><PRE>namespace mynamespace { void doGlobalFoo(); // Good -- doGlobalFoo is within a namespace. class MyClass { public: ... // Good -- doGlobalBar is a static member of class MyClass and // has a reason to be part of this class (not shown here). static Bar* doGlobalBar(); }</PRE></DIV> </DIV></DIV> </DIV> <DIV class=""> <H3><A name="Local_Variables" id="Local_Variables">Local Variables</A></H3> <SPAN class="link_button" id="link-Local_Variables__button" name="link-Local_Variables__button"><A href="?showone=Local_Variables#Local_Variables"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Local_Variables')" name="Local_Variables__button" id="Local_Variables__button">▶</SPAN> <DIV style="display:inline;" class=""> Declare variables as locally as possible. </DIV> <DIV class=""><DIV class="stylepoint_body" name="Local_Variables__body" id="Local_Variables__body" style="display: none"> <p> Variables whose lifetime are longer than necessary have several drawbacks: <ul> <li>They make the code harder to understand and maintain.</li> <li>They can't be always sensibly initialized.</li> </ul> </p> <SPAN class="showhide_extrabutton" onclick="javascript:ToggleExtraByName('Local_Variables')" name="Local_Variables__extra_button" id="Local_Variables__extra_button">▶</SPAN><P> <SPAN class="stylepoint_section">Extra details and exceptions to the rules: </SPAN><div style="display:none;" class="stylepoint_extra" name="Local_Variables__extra_body"> <ul> <li>It can be sometimes more efficient to declare a variable (usually of an object type) outside a loop. <p> If the variable is an object, its constructor is invoked every time it enters scope and is created, and its destructor is invoked every time it goes out of scope. </p> <DIV class=""><PRE class="badcode">// Inefficient implementation: for (int i = 0; i < bigNumber; ++i) { Foo foo; // My ctor and dtor get called bigNumber times each. foo.doSomething(i); }</PRE></DIV> <p> It may be more efficient to declare such a variable used in a loop outside that loop: </p> <DIV class=""><PRE>Foo foo; // My ctor and dtor get called once each. for (int i = 0; i < bigNumber; ++i) { foo.doSomething(i); }</PRE></DIV> </li> <li> This item does not apply to constants, because constants don't add a state. </li> </ul> </div> </P> </DIV></DIV> </DIV> <DIV class=""> <H3><A name="Variables_Initialization" id="Variables_Initialization">Variables Initialization</A></H3> <SPAN class="link_button" id="link-Variables_Initialization__button" name="link-Variables_Initialization__button"><A href="?showone=Variables_Initialization#Variables_Initialization"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Variables_Initialization')" name="Variables_Initialization__button" id="Variables_Initialization__button">▶</SPAN> <DIV style="display:inline;" class=""> Always initialize variables.<br> </DIV> <DIV class=""><DIV class="stylepoint_body" name="Variables_Initialization__body" id="Variables_Initialization__body" style="display: none"> <p> Do not separate initialization from declaration, e.g. </p> <DIV class=""><PRE class="badcode">int value; value = function(); // Bad -- initialization separate from declaration.</PRE></DIV> <DIV class=""><PRE>int value = function(); // Good -- declaration has initialization.</PRE></DIV> <p> Use a default initial value or ? to reduce mixing data flow with control flow. </p> <DIV class=""><PRE class="badcode">int speedupFactor; // Bad: does not initialize variable if (condition) { speedupFactor = NoFactor; } else { speedupFactor = DoubleFactor; }</PRE></DIV> <DIV class=""><PRE>int speedupFactor = DoubleFactor; // Good: initializes variable if (condition) { speedupFactor = NoFactor; }</PRE></DIV> <DIV class=""><PRE>int speedupFactor = condition ? NoFactor : DoubleFactor; // Good: initializes variable</PRE></DIV> <p> Prefer declaration of loop variables inside a loop, e.g. </p> <DIV class=""><PRE class="badcode">int i; // Bad: does not initialize variable for (i = 0; i < number; ++i) { doSomething(i); }</PRE></DIV> <DIV class=""><PRE>for (int i = 0; i < number; ++i) { doSomething(i); // Good }</PRE></DIV> </DIV></DIV> </DIV> <DIV class=""> <SPAN class="cpp11_marker">since C++11</SPAN><H3><A name="Brace_Initialization" id="Brace_Initialization">Brace Initialization</A></H3> <SPAN class="link_button" id="link-Brace_Initialization__button" name="link-Brace_Initialization__button"><A href="?showone=Brace_Initialization#Brace_Initialization"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Brace_Initialization')" name="Brace_Initialization__button" id="Brace_Initialization__button">▶</SPAN> <DIV style="display:inline;" class=""> Prefer initialization with braces except for single-argument assignment. </DIV> <DIV class=""><DIV class="stylepoint_body" name="Brace_Initialization__body" id="Brace_Initialization__body" style="display: none"> <p>In C++11, the brace initialization syntax for builtin arrays and POD structures has been extended for use with all other datatypes. <p> Example of brace initialization: </p> <DIV class=""><PRE>std::vector<std::string> myVector{"alpha", "beta", "gamma"};</PRE></DIV> </p> <p> Example of single-argument assignments: </p> <DIV class=""><PRE>int value = 3; // preferred style std::string name = "Some Name"; int value { 3 }; // also possible std::string name{ "Some Name" }; std::string name = { "Some Name" };</PRE></DIV> <p>User data types can also define constructors that take <code>initializer_list</code>, which is automatically created from <i>braced-init-list</i>: <DIV class=""><PRE>#include <initializer_list> class MyType { public: // initializer_list is a reference to the underlying init list, // so it can be passed by value. MyType(std::initializer_list<int> initList) { for (int element : initList) { .. } } }; MyType myObject{2, 3, 5, 7};</PRE></DIV></p> <p>Finally, brace initialization can also call ordinary constructors of data types that do not have <code>initializer_list</code> constructors. <DIV class=""><PRE>// Calls ordinary constructor as long as MyOtherType has no // initializer_list constructor. class MyOtherType { public: explicit MyOtherType(std::string name); MyOtherType(int value, std::string name); }; MyOtherType object1 = {1, "b"}; // If the constructor is explicit, you can't use the "= {}" form. MyOtherType object2{"b"};</PRE></DIV></p> <p>Never assign a <i>braced-init-list</i> to an auto local variable. In the single element case, what this means can be confusing. <DIV class=""><PRE class="badcode">auto value = {1.23}; // value is an initializer_list<double></PRE></DIV> <DIV class=""><PRE>auto value = double{1.23}; // Good -- value is a double, not an initializer_list.</PRE></DIV> </p> <p> For clarity of the examples above we use directly explicit values, however following the rule about <a href="#Magic_Numbers">magic numbers</a> requires to define all such numbers as named constants or <code>constexpr</code> first. </p> </DIV></DIV> </DIV> <DIV class=""> <H3><A name="Global_Variables" id="Global_Variables">Global Variables</A></H3> <SPAN class="link_button" id="link-Global_Variables__button" name="link-Global_Variables__button"><A href="?showone=Global_Variables#Global_Variables"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Global_Variables')" name="Global_Variables__button" id="Global_Variables__button">▶</SPAN> <DIV style="display:inline;" class=""> Variables declared in the global scope are not allowed. Other global variables, including static class variables and variables in namespace scope, should be avoided where other means of communication are possible. </DIV> <DIV class=""><DIV class="stylepoint_body" name="Global_Variables__body" id="Global_Variables__body" style="display: none"> <P class=""> <SPAN class="stylepoint_section">Definition: </SPAN> A global variable is a variable that can be accessed (theoretically) from everywhere in the program. The adjective "global" should rather be understood as concerning its linkage, not whether it is in the global scope. <DIV class=""><PRE class="badcode">int gBar; // this is obviously global class Something { private: static int sId; // but this one too (details at the end of the rule) }; namespace notglobalscope { Foo fooObject; // and finally this one as well }</PRE></DIV> </P> <P class=""> <SPAN class="stylepoint_section">Pros: </SPAN> Global variables are a simple solution to sharing of data. </P> <P class=""> <SPAN class="stylepoint_section">Cons: </SPAN> Global variables make it harder to reason about the code (for humans and compilers): the smaller the number of variables a given region of code reads and writes, the easier. Global variables can be read and written from anywhere. Therefore, global variables pose a challenge to the optimizer. </P> <P class=""> <SPAN class="stylepoint_section">Decision: </SPAN> We want to reduce the shared data in our software to the unavoidable minimum. Therefore, global variables should be avoided where other means of communication are possible. </P> <SPAN class="showhide_extrabutton" onclick="javascript:ToggleExtraByName('Global_Variables')" name="Global_Variables__extra_button" id="Global_Variables__extra_button">▶</SPAN><P> <SPAN class="stylepoint_section">Extra details and exceptions to the rules: </SPAN><div style="display:none;" class="stylepoint_extra" name="Global_Variables__extra_body"> Note that a private static class variable sId is global. For example two threads having each an instance of the class could access sId via these instances. </div> </P> </DIV></DIV> </DIV> <DIV class=""> <H3><A name="Global_variables_initialization" id="Global_variables_initialization">Global variables initialization</A></H3> <SPAN class="link_button" id="link-Global_variables_initialization__button" name="link-Global_variables_initialization__button"><A href="?showone=Global_variables_initialization#Global_variables_initialization"> link </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Global_variables_initialization')" name="Global_variables_initialization__button" id="Global_variables_initialization__button">▶</SPAN> <DIV style="display:inline;" class=""> In the rare and justified cases where you use global variables, including file-static variables, static member variables and variables in namespace scope, initialize them statically. </DIV> <DIV class=""><DIV class="stylepoint_body" name="Global_variables_initialization__body" id="Global_variables_initialization__body" style="display: none"> <P class=""> <SPAN class="stylepoint_section">Definition: </SPAN> <ul> <li>This rule additionally applies to file-static variables.</li> |
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