This document lists C++ coding recommendations common in the C++ development community.
The recommendations are based on established standards collected from a number of sources.
There are several reasons for introducing a new guideline rather than just referring to the ones above. The main reason is that these guides are far too general in their scope and that more specific rules (especially naming rules) need to be established. Also, the present guide has an annotated form that makes it far easier to use during project code reviews than most other existing guidelines. In addition, programming recommendations generally tend to mix style issues with language technical issues in a somewhat confusing manner. The present document does not contain any C++ technical recommendations at all, but focuses mainly on programming style.
While a given development environment (IDE) can improve the readability of code by access visibility, color coding, automatic formatting and so on, the programmer should never rely on such features. Source code should always be considered larger than the IDE it is developed within and should be written in a way that maximise its readability independent of any IDE.
1.1 Layout of the Recommendations.
The recommendations are grouped by topic and each recommendation is numbered to make it easier to refer to during reviews.
Layout of the recommendations is as follows:
n. Guideline short description
Example code if applicable
Motivation, background and additional information.
The motivation section is important. Coding standards and guidelines tend to start "religious wars", and it is important to state the background for the recommendation.
1.2 Recommendation Importance
In the guideline sections the terms must, should and can have special meaning. A must requirement must be followed, a should is a strong recommendation, and a can is a general guideline.
2 General Recommendations
1. Any violation to the guide is allowed if it enhances readability.
The main goal of the recommendation is to improve readability and thereby the understanding and the maintainability and general quality of the code. It is impossible to cover all the specific cases in a general guide and the programmer should be flexible.
2. The rules can be violated if there are strong personal objections against them.
The attempt is to make a guideline, not to force a particular coding style onto individuals. Experienced programmers normally want to adopt a style like this anyway, but having one, and at least requiring everyone to get familiar with it, usually makes people start thinking about programming style and evaluate their own habits in this area.
On the other hand, new and inexperienced programmers normally use a style guide as a convenience of getting into the programming jargon more easily.
3 Naming Conventions
3.1 General Naming Conventions
3. Names representing types must be in mixed case starting with upper case.
Line, SavingsAccount
Common practice in the C++ development community.
4. Variable names must be in mixed case starting with lower case.
line, savingsAccount
Common practice in the C++ development community. Makes variables easy to distinguish from types, and effectively resolves potential naming collision as in the declaration Line line;
5. Named constants (including enumeration values) must be all uppercase using underscore to separate words.
MAX_ITERATIONS, COLOR_RED, PI
Common practice in the C++ development community. In general, the use of such constants should be minimized. In many cases implementing the value as a method is a better choice:
Code: Select all
int getMaxIterations() // NOT: MAX_ITERATIONS = 25
{
return 25;
}6. Names representing methods or functions must be verbs and written in mixed case starting with lower case.
getName(), computeTotalWidth()
Common practice in the C++ development community. This is identical to variable names, but functions in C++ are already distingushable from variables by their specific form.
7. Names representing namespaces should be all lowercase.
model::analyzer, io::iomanager, common::math::geometry
Common practice in the C++ development community.
8. Names representing template types should be a single uppercase letter.
template<class T> ...
template<class C, class D> ...
Common practice in the C++ development community. This makes template names stand out relative to all other names used.
9. Abbreviations and acronyms must not be uppercase when used as name [4].
exportHtmlSource(); // NOT: exportHTMLSource();
openDvdPlayer(); // NOT: openDVDPlayer();
Using all uppercase for the base name will give conflicts with the naming conventions given above. A variable of this type whould have to be named dVD, hTML etc. which obviously is not very readable. Another problem is illustrated in the examples above; When the name is connected to another, the readbility is seriously reduced; the word following the abbreviation does not stand out as it should.
10. Global variables should always be referred to using the :: operator.
::mainWindow.open(), ::applicationContext.getName()
In general, the use of global variables should be avoided. Consider using singleton objects instead.
11. Private class variables should have underscore suffix.
Code: Select all
class SomeClass {
private:
int length_;
}A side effect of the underscore naming convention is that it nicely resolves the problem of finding reasonable variable names for setter methods and constructors:
Code: Select all
void setDepth (int depth)
{
depth_ = depth;
}It should be noted that scope identification in variables has been a controversial issue for quite some time. It seems, though, that this practice now is gaining acceptance and that it is becoming more and more common as a convention in the professional development community.
12. Generic variables should have the same name as their type.
void setTopic(Topic* topic) // NOT: void setTopic(Topic* value)
// NOT: void setTopic(Topic* aTopic)
// NOT: void setTopic(Topic* t)
void connect(Database* database) // NOT: void connect(Database* db)
// NOT: void connect (Database* oracleDB)
Reduce complexity by reducing the number of terms and names used. Also makes it easy to deduce the type given a variable name only.
If for some reason this convention doesn't seem to fit it is a strong indication that the type name is badly chosen.
Non-generic variables have a role. These variables can often be named by combining role and type:
Code: Select all
Point startingPoint, centerPoint;
Name loginName;13. All names should be written in English.
fileName; // NOT: filNavn
English is the preferred language for international development.
14. Variables with a large scope should have long names, variables with a small scope can have short names [1].
Scratch variables used for temporary storage or indices are best kept short. A programmer reading such variables should be able to assume that its value is not used outside of a few lines of code. Common scratch variables for integers are i, j, k, m, n and for characters c and d.
15. The name of the object is implicit, and should be avoided in a method name.
line.getLength(); // NOT: line.getLineLength();
The latter seems natural in the class declaration, but proves superfluous in use, as shown in the example.
3.2 Specific Naming Conventions
17. The terms get/set must be used where an attribute is accessed directly.
employee.getName();
employee.setName(name);
matrix.getElement(2, 4);
matrix.setElement(2, 4, value);
Common practice in the C++ development community. In Java this convention has become more or less standard.
18. The term compute can be used in methods where something is computed.
valueSet->computeAverage();
matrix->computeInverse()
Give the reader the immediate clue that this is a potentially time-consuming operation, and if used repeatedly, he might consider caching the result. Consistent use of the term enhances readability.
19. The term find can be used in methods where something is looked up.
vertex.findNearestVertex();
matrix.findMinElement();
Give the reader the immediate clue that this is a simple look up method with a minimum of computations involved. Consistent use of the term enhances readability.
20. The term initialize can be used where an object or a concept is established.
printer.initializeFontSet();
The american initialize should be preferred over the English initialise. Abbreviation init should be avoided.
21. Variables representing GUI components should be suffixed by the component type name.
mainWindow, propertiesDialog, widthScale, loginText,
leftScrollbar, mainForm, fileMenu, minLabel, exitButton, yesToggle etc.
Enhances readability since the name gives the user an immediate clue of the type of the variable and thereby the objects resources.
22. Plural form should be used on names representing a collection of objects.
vector<Point> points;
int values[];
Enhances readability since the name gives the user an immediate clue of the type of the variable and the operations that can be performed on its elements.
23. The prefix n should be used for variables representing a number of objects.
nPoints, nLines
The notation is taken from mathematics where it is an established convention for indicating a number of objects.
24.The suffix No should be used for variables representing an entity number.
tableNo, employeeNo
The notation is taken from mathematics where it is an established convention for indicating an entity number.
An elegant alternative is to prefix such variables with an i: iTable, iEmployee. This effectively makes them named iterators.
25. Iterator variables should be called i, j, k etc.
Code: Select all
for (int i = 0; i < nTables); i++) {
:
}
for (vector<MyClass>::iterator i = list.begin(); i != list.end(); i++) {
Element element = *i;
...
}Variables named j, k etc. should be used for nested loops only.
26. The prefix is should be used for boolean variables and methods.
isSet, isVisible, isFinished, isFound, isOpen
Common practice in the C++ development community and partially enforced in Java.
Using the is prefix solves a common problem of choosing bad boolean names like status or flag. isStatus or isFlag simply doesn't fit, and the programmer is forced to choose more meaningful names.
There are a few alternatives to the is prefix that fit better in some situations. These are the has, can and should prefixes:
Code: Select all
bool hasLicense();
bool canEvaluate();
bool shouldSort();27. Complement names must be used for complement operations [1].
get/set, add/remove, create/destroy, start/stop, insert/delete,
increment/decrement, old/new, begin/end, first/last, up/down, min/max,
next/previous, old/new, open/close, show/hide, suspend/resume, etc.
Reduce complexity by symmetry.
28. Abbreviations in names should be avoided.
computeAverage(); // NOT: compAvg();
There are two types of words to consider. First are the common words listed in a language dictionary. These must never be abbreviated. Never write:
cmd instead of command
cp instead of copy
pt instead of point
comp instead of compute
init instead of initialize
etc.
Then there are domain specific phrases that are more naturally known through their abbreviations/acronym. These phrases should be kept abbreviated. Never write:
HypertextMarkupLanguage instead of html
CentralProcessingUnit instead of cpu
PriceEarningRatio instead of pe
etc.
29. Naming pointers specifically should be avoided.
Line* line; // NOT: Line* pLine;
// NOT: LIne* linePtr;
Many variables in a C/C++ environment are pointers, so a convention like this is almost impossible to follow. Also objects in C++ are often oblique types where the specific implementation should be ignored by the programmer. Only when the actual type of an object is of special significance, the name should emphasize the type.
30. Negated boolean variable names must be avoided.
bool isError; // NOT: isNoError
bool isFound; // NOT: isNotFound
The problem arises when such a name is used in conjunction with the logical negation operator as this results in a double negative. It is not immediately apparent what !isNotFound means.
31. Enumeration constants can be prefixed by a common type name.
Code: Select all
enum Color {
COLOR_RED,
COLOR_GREEN,
COLOR_BLUE
};An alternative approach is to always refer to the constants through their common type: Color::RED, Airline::AIR_FRANCE etc.
Note also that the enum name typically should be singular as in enum Color {...}. A plural name like enum Colors {...} may look fine when declaring the type, but it will look silly in use.
32. Exception classes should be suffixed with Exception.
Code: Select all
class AccessException
{
:
}33. Functions (methods returning something) should be named after what they return and procedures (void methods) after what they do.
Increase readability. Makes it clear what the unit should do and especially all the things it is not supposed to do. This again makes it easier to keep the code clean of side effects.
4 Files
4.1 Source Files
34. C++ header files should have the extension .h (preferred) or .hpp. Source files can have the extension .c++ (recommended), .C, .cc or .cpp.
MyClass.c++, MyClass.h
These are all accepted C++ standards for file extension.
35. A class should be declared in a header file and defined in a source file where the name of the files match the name of the class.
MyClass.h, MyClass.c++
Makes it easy to find the associated files of a given class. An obvious exception is template classes that must be both declared and defined inside a .h file.
36. All definitions should reside in source files.
Code: Select all
class MyClass
{
public:
int getValue () {return value_;} // NO!
...
private:
int value_;
}37. File content must be kept within 80 columns.
80 columns is a common dimension for editors, terminal emulators, printers and debuggers, and files that are shared between several people should keep within these constraints. It improves readability when unintentional line breaks are avoided when passing a file between programmers.
38. Special characters like TAB and page break must be avoided.
These characters are bound to cause problem for editors, printers, terminal emulators or debuggers when used in a multi-programmer, multi-platform environment.
39. The incompleteness of split lines must be made obvious [1].
totalSum = a + b + c +
d + e;
function (param1, param2,
param3);
setText ("Long line split"
"into two parts.");
for (int tableNo = 0; tableNo < nTables;
tableNo += tableStep) {
...
}
Split lines occurs when a statement exceed the 80 column limit given above. It is difficult to give rigid rules for how lines should be split, but the examples above should give a general hint.
In general:
- Break after a comma.
- Break after an operator.
- Align the new line with the beginning of the expression on the previous line.
4.2 Include Files and Include Statements
40. Header files must contain an include guard.
#ifndef COM_COMPANY_MODULE_CLASSNAME_H
#define COM_COMPANY_MODULE_CLASSNAME_H
:
#endif // COM_COMPANY_MODULE_CLASSNAME_H
The construction is to avoid compilation errors. The name convention resembles the location of the file inside the source tree and prevents naming conflicts.
41. Include statements should be sorted and grouped. Sorted by their hierarchical position in the system with low level files included first. Leave an empty line between groups of include statements.
#include <fstream>
#include <iomanip>
#include <qt/qbutton.h>
#include <qt/qtextfield.h>
#include "com/company/ui/PropertiesDialog.h"
#include "com/company/ui/MainWindow.h"
In addition to show the reader the individual include files, it also give an immediate clue about the modules that are involved.
Include file paths must never be absolute. Compiler directives should instead be used to indicate root directories for includes.
42. Include statements must be located at the top of a file only.
Common practice. Avoid unwanted compilation side effects by "hidden" include statements deep into a source file.
5 Statements
5.1 Types
43. Types that are local to one file only can be declared inside that file.
Enforces information hiding.
44. The parts of a class must be sorted public, protected and private [2][3]. All sections must be identified explicitly. Not applicable sections should be left out.
The ordering is "most public first" so people who only wish to use the class can stop reading when they reach the protected/private sections.
45. Type conversions must always be done explicitly. Never rely on implicit type conversion.
floatValue = static_cast<float>(intValue); // NOT: floatValue = intValue;
By this, the programmer indicates that he is aware of the different types involved and that the mix is intentional.
5.2 Variables
46. Variables should be initialized where they are declared.
This ensures that variables are valid at any time. Sometimes it is impossible to initialize a variable to a valid value where it is declared:
int x, y, z;
getCenter(&x, &y, &z);
In these cases it should be left uninitialized rather than initialized to some phony value.
47. Variables must never have dual meaning.
Enhance readability by ensuring all concepts are represented uniquely. Reduce chance of error by side effects.
48. Use of global variables should be minimized.
In C++ there is no reason global variables need to be used at all. The same is true for global functions or file scope (static) variables.
49. Class variables should never be declared public.
The concept of C++ information hiding and encapsulation is violated by public variables. Use private variables and access functions instead. One exception to this rule is when the class is essentially a data structure, with no behavior (equivalent to a C struct). In this case it is appropriate to make the class' instance variables public [2].
Note that structs are kept in C++ for compatibility with C only, and avoiding them increases the readability of the code by reducing the number of constructs used. Use a class instead.
51. C++ pointers and references should have their reference symbol next to the type rather than to the name.
float* x; // NOT: float *x;
int& y; // NOT: int &y;
The pointer-ness or reference-ness of a variable is a property of the type rather than the name. C-programmers often use the alternative approach, while in C++ it has become more common to follow this recommendation.
53. Implicit test for 0 should not be used other than for boolean variables and pointers.
if (nLines != 0) // NOT: if (nLines)
if (value != 0.0) // NOT: if (value)
It is not necessarily defined by the C++ standard that ints and floats 0 are implemented as binary 0. Also, by using an explicit test the statement gives an immediate clue of the type being tested.
It is common also to suggest that pointers shouldn't test implicitly for 0 either, i.e. if (line == 0) instead of if (line). The latter is regarded so common in C/C++ however that it can be used.
54. Variables should be declared in the smallest scope possible.
Keeping the operations on a variable within a small scope, it is easier to control the effects and side effects of the variable.
5.3 Loops
55. Only loop control statements must be included in the for() construction.
Code: Select all
sum = 0; // NOT: for (i = 0, sum = 0; i < 100; i++)
for (i = 0; i < 100; i++) sum += value[i];
sum += value[i];56. Loop variables should be initialized immediately before the loop.
isDone = false; // NOT: bool isDone = false;
while (!isDone) { // :
: // while (!isDone) {
} // :
// }
57. do-while loops can be avoided.
do-while loops are less readable than ordinary while loops and for loops since the conditional is at the bottom of the loop. The reader must scan the entire loop in order to understand the scope of the loop.
In addition, do-while loops are not needed. Any do-while loop can easily be rewritten into a while loop or a for loop. Reducing the number of constructs used enhance readbility.
58. The use of break and continue in loops should be avoided.
These statements should only be used if they give higher readability than their structured counterparts.
60. The form while(true) should be used for infinite loops.
Code: Select all
while (true) {
:
}
for (;;) { // NO!
:
}
while (1) { // NO!
:
}5.4 Conditionals
61. Complex conditional expressions must be avoided. Introduce temporary boolean variables instead [1].
bool isFinished = (elementNo < 0) || (elementNo > maxElement);
bool isRepeatedEntry = elementNo == lastElement;
if (isFinished || isRepeatedEntry) {
:
}
// NOT:
if ((elementNo < 0) || (elementNo > maxElement)||
elementNo == lastElement) {
:
}
By assigning boolean variables to expressions, the program gets automatic documentation. The construction will be easier to read, debug and maintain.
62. The nominal case should be put in the if-part and the exception in the else-part of an if statement [1].
bool isOk = readFile (fileName);
if (isOk) {
:
}
else {
:
}
Makes sure that the exceptions don't obscure the normal path of execution. This is important for both the readability and performance.
63. The conditional should be put on a separate line.
if (isDone) // NOT: if (isDone) doCleanup();
doCleanup();
This is for debugging purposes. When writing on a single line, it is not apparent whether the test is really true or not.
64. Executable statements in conditionals must be avoided.
File* fileHandle = open(fileName, "w");
if (!fileHandle) {
:
}
// NOT:
if (!(fileHandle = open(fileName, "w"))) {
:
}
Conditionals with executable statements are just very difficult to read. This is especially true for programmers new to C/C++.
5.5 Miscellaneous
65. The use of magic numbers in the code should be avoided. Numbers other than 0 and 1 should be considered declared as named constants instead.
If the number does not have an obvious meaning by itself, the readability is enhanced by introducing a named constant instead. A different approach is to introduce a method from which the constant can be accessed.
66. Floating point constants should always be written with decimal point and at least one decimal.
double total = 0.0; // NOT: double total = 0;
double speed = 3.0e8; // NOT: double speed = 3e8;
double sum;
:
sum = (a + b) * 10.0;
This emphasizes the different nature of integer and floating point numbers. Mathematically the two model completely different and non-compatible concepts.
Also, as in the last example above, it emphasizes the type of the assigned variable (sum) at a point in the code where this might not be evident.
67. Floating point constants should always be written with a digit before the decimal point.
double total = 0.5; // NOT: double total = .5;
The number and expression system in C++ is borrowed from mathematics and one should adhere to mathematical conventions for syntax wherever possible. Also, 0.5 is a lot more readable than .5; There is no way it can be mixed with the integer 5.
68. Functions must always have the return value explicitly listed.
Code: Select all
int getValue() // NOT: getValue()
{
:
}69. goto should not be used.
Goto statements violate the idea of structured code. Only in some very few cases (for instance breaking out of deeply nested structures) should goto be considered, and only if the alternative structured counterpart is proven to be less readable.
70. "0" should be used instead of "NULL".
NULL is part of the standard C library, but is made obsolete in C++.
6 Layout and Comments
6.1 Layout
71. Basic indentation should be 2.
Code: Select all
for (i = 0; i < nElements; i++)
a[i] = 0;72. Block layout should be as illustrated in example 1 below (recommended) or example 2, and must not be as shown in example 3 [4]. Function and class blocks must use the block layout of example 2.
Code: Select all
while (!done) {
doSomething();
done = moreToDo();
}Code: Select all
while (!done)
{
doSomething();
done = moreToDo();
}Code: Select all
while (!done)
{
doSomething();
done = moreToDo();
}73. The class declarations should have the following form:
Code: Select all
class SomeClass : public BaseClass
{
public:
...
protected:
...
private:
...
}74. Method definitions should have the following form:
Code: Select all
void someMethod()
{
...
}75. The if-else class of statements should have the following form:
Code: Select all
if (condition) {
statements;
}
if (condition) {
statements;
}
else {
statements;
}
if (condition) {
statements;
}
else if (condition) {
statements;
}
else {
statements;
}Code: Select all
if (condition) {
statements;
} else {
statements;
}76. A for statement should have the following form:
Code: Select all
for (initialization; condition; update) {
statements;
}77. An empty for statement should have the following form:
for (initialization; condition; update)
;
This emphasizes the fact that the for statement is empty and it makes it obvious for the reader that this is intentional. Empty loops should be avoided however.
78. A while statement should have the following form:
Code: Select all
while (condition) {
statements;
}79. A do-while statement should have the following form:
Code: Select all
do {
statements;
} while (condition);80. A switch statement should have the following form:
Code: Select all
switch (condition) {
case ABC :
statements;
// Fallthrough
case DEF :
statements;
break;
case XYZ :
statements;
break;
default :
statements;
break;
}81. A try-catch statement should have the following form:
Code: Select all
try {
statements;
}
catch (Exception& exception) {
statements;
}82. Single statement if-else, for or while statements can be written without brackets.
if (condition)
statement;
while (condition)
statement;
for (initialization; condition; update)
statement;
It is a common recommendation that brackets should always be used in all these cases. However, brackets are in general a language construct that groups several statements. Brackets are per definition superfluous on a single statement. A common argument against this syntax is that the code will break if an additional statement is added without also adding the brackets. In general however, code should never be written to accommodate for changes that might arise.
83. The function return type can be put in the left column immediately above the function name.
Code: Select all
void
MyClass::myMethod(void)
{
:
}6.2 White Space
84.
- Conventional operators should be surrounded by a space character.
- C++ reserved words should be followed by a white space.
- Commas should be followed by a white space.
- Colons should be surrounded by white space.
- Semicolons in for statments should be followed by a space character.
a = (b + c) * d; // NOT: a=(b+c)*d
while (true) // NOT: while(true)
{
...
doSomething(a, b, c, d); // NOT: doSomething(a,b,c,d);
case 100 : // NOT: case 100:
for (i = 0; i < 10; i++) { // NOT: for(i=0;i<10;i++){
...
Makes the individual components of the statements stand out. Enhances readability. It is difficult to give a complete list of the suggested use of whitespace in C++ code. The examples above however should give a general idea of the intentions.
85. Method names can be followed by a white space when it is followed by another name.
doSomething (currentFile);
Makes the individual names stand out. Enhances readability. When no name follows, the space can be omitted (doSomething()) since there is no doubt about the name in this case.
An alternative to this approach is to require a space after the opening parenthesis. Those that adhere to this standard usually also leave a space before the closing parentheses: doSomething( currentFile );. This do make the individual names stand out as is the intention, but the space before the closing parenthesis is rather artificial, and without this space the statement looks rather asymmetrical (doSomething ( currentFile ) ; ).
86. Logical units within a block should be separated by one blank line.
Matrix4x4 matrix = new Matrix4x4();
double cosAngle = Math.cos(angle);
double sinAngle = Math.sin(angle);
matrix.setElement(1, 1, cosAngle);
matrix.setElement(1, 2, sinAngle);
matrix.setElement(2, 1, -sinAngle);
matrix.setElement(2, 2, cosAngle);
multiply(matrix);
Enhance readability by introducing white space between logical units of a block.
87. Methods should be separated by three blank lines.
By making the space larger than space within a method, the methods will stand out within the class.
88. Variables in declarations can be left aligned.
AsciiFile* file;
int nPoints;
float x, y;
Enhance readability. The variables are easier to spot from the types by alignment.
89. Use alignment wherever it enhances readability.
Code: Select all
if (a == lowValue) compueSomething();
else if (a == mediumValue) computeSomethingElse();
else if (a == highValue) computeSomethingElseYet();Code: Select all
value = (potential * oilDensity) / constant1 +
(depth * waterDensity) / constant2 +
(zCoordinateValue * gasDensity) / constant3;Code: Select all
minPosition = computeDistance(min, x, y, z);
averagePosition = computeDistance(average, x, y, z);Code: Select all
switch (value) {
case PHASE_OIL : strcpy(phase, "Oil"); break;
case PHASE_WATER : strcpy(phase, "Water"); break;
case PHASE_GAS : strcpy(phase, "Gas"); break;
}6.3 Comments
90. Tricky code should not be commented but rewritten! [1]
In general, the use of comments should be minimized by making the code self-documenting by appropriate name choices and an explicit logical structure.
91. All comments should be written in English [2].
In an international environment English is the preferred language.
92. Use // for all comments, including multi-line comments.
// Comment spanning
// more than one line.
Since multilevel C-commenting is not supported, using // comments ensure that it is always possible to comment out entire sections of a file using /* */ for debugging purposes etc.
There should be a space between the "//" and the actual comment, and comments should always start with an upper case letter and end with a period.
93. Comments should be included relative to their position in the code. [1]
Code: Select all
while (true) { // NOT: while (true) {
// Do something // Do something
something(); something();
} }94. Class and method header comments should follow the JavaDoc conventions.
Regarding standardized class and method documentation the Java development community is more mature than the C/C++ one. This is due to the standard automatic Javadoc tool that is part of the development kit and that help producing high quality hypertext documentation from these comments.
There are Javadoc-like tools available also for C++. These follows the same tagging syntax as Javadoc. See for instance Doc++ or Doxygen.