C++ tutorial: keywords

Keyword reference This table lists all the C++ keywords with very brief explanations

Keyword Used Purpose

auto Very infrequently Defines a variable as 'automatic', that is, its memory is allocated by the compiler for the duration of its existence. For local variables this is the default anyway

break All the time The 'break' statment causes execution of the program to jump out of the current loop or 'case' statement. Use is almost compuslory with 'switch...case', and common with loops. However, the principles of structured programming suggest that use of 'break' to jump out of the middle of a loop should usully be avoided

bool Infrequently; should be used more 'bool' defines a variable as being of 'Boolean' type, i.e. able to take only 'true' or 'false' values. As C did not have a Boolean data type, many programmers have got used to using integers for this

case All the time Used with 'switch' to allow execution of one of a list of groups of statements

catch Infrequently; not well understood Used with 'try' and 'throw' to detect and process 'exceptions', that is, unusual circumstances like errors. The exception handling mechanism in C++ is not well understood by programmers

char All the time Indicates a character data type

class All the time Indicates that the following statements are the definition or declaration of a C++ class

const All the time by good programmers Indicates that a variable has a constant value, i.e., cannot be redefined; with a class operation, indicates that the operation does not modify the object's data

const_cast Very infrequently Changes the 'constant' or 'volatile' status of a data item

continue Too often Inside a loop, causes the rest of the statements after 'continue' to be skipped, and the next turn of the loop started. Use with caution.

default All the time Used with 'switch' to indicate the action that should be carried out if no others are appropriate

delete All the time Removes a data item that was constructed using 'new', freeing any memory it used

do All the time Used with 'while' to construct a loop that must be executed at least once

double All the time Indicates a real-number, double-precision value

dynamic_cast Very infrequently Used in management of class inheritance

else All the time Used with 'if' to execute statements according to the results of a comparison

enum Not often enough A data type whose possible values are listed explicitly, e.g., 'enum {red, amber, green} TrafficLightColour'

explicit Infrequently Prevents an object being intialized using a statement of the form 'X x=1', where x is an object of class X

extern All the time Indicates that something used in one program module (file) is defined in a different program module.

false Not frequently enough See 'bool'

float All the time Indicates a floating-point (real number) data element

friend All the time Within a class, indicates that a different class or function is to be allowed access to the class's data

for All the time Used to construct loops

goto Infrequently Jumps to a specified place in a function

if All the time Executes statements if a condition is true

inline Commonly in specific contexts Suggests to the compiler that a function should be inserted into the program whereever it is used, and not called as a function. This process is transparent to the programmer and does not change the result of the function; it makes some programs slightly faster at the expense of needing more memory

long All the time Indicates that a long version of a particular data type is to be used, i.e., one with a greater range of values

mutable Infrequently Indicates that a data element in an object can be changed, even by operations which claim not to change data

namespace Infrequently, except in large applications Defines a namespace, that is, a division of the program in which names will not be shared with other parts of the program

new All the time Creates a new variable or object

operator Sometimes (or all the time by some programmers) Used in extending the meanings of C++ operators

private All the time In a class, indicates an operation or attribute that is accessible only within the class (except as indicated by 'friend')

protected All the time In a class, indicates an operation or attribute that is accessible only classes of the same type (i.e., derived from this one)

public All the time In a class, indicates an operation or attribute that is accessible outside the class

register Suggests to the compiler that a variable should be held in a CPU register and not in memory. This is for speed optimization.

reinterpret_cast Very infrequently Used to improve the safety of certain types of cast operation

return All the time Exits a function, specifying the return value if appropriate

short All the time Indicates that a shorter version of a particular data type is to be used, i.e., one with a smaller range

signed Infrequently Indicates that the following data type is signed, i.e., accepts positive and negative values. In C++, integers are signed by default, so this keyword is uncommon

sizeof All the time When applied to a variable or data type, give the number of bytes used to store it

static All the time Various meanings, all concerned with the scope and lifetime of data and functions

static_cast Very infrequently Used in conversion between objects of different types

struct All the time Introduces a programmer-defined data structure

switch All the time Used with 'case' and 'default' to provide a mechanism where one of a list of sets of statements is executed

template All the time by programmers that like templates, or use template libraries Indicates that the following definitation is of a fucntion or class template, rather than a function or class

this All the time Used within an object to refer to the object itself

throw Infrequently; not well understood Used with 'try' and 'throw' to detect and process 'exceptions', that is, unusual circumstances like errors. The exception handling mechanism in C++ is not well understood by programmers

true Not frequently enough See 'bool'

try Infrequently; not well understood Used with 'catch' and 'throw' to detect and process 'exceptions', that is, unusual circumstances like errors. The exception handling mechanism in C++ is not well understood by programmers

typedef All the time Introduces a programmer-defined data type

typename Infrequently Used in the definition of general-purpose templates

typeid Infrequently, except in certain contexts Provides information about the type of an object

union Increasingly uncommonly Indicates that the same area of memory can be used to store a number of different variables at different times

using Infrquent, except in large applications Used with 'namespace' to distinguish between data in different namespaces

unsigned All the time Indicates that the following variable is unsigned, i.e., only takes positive values

virtual All the time In the definition of a class, indicates that an operation will be re-defined in classes derived from it

void All the time Indicates that something is unknown, or does not have a value, or does not accept parameters, or does not return a value

volatile Infrequently, except in certain contexts Indicates that a variable or object is liable to be modified by another part of the program in an unpredictable way. For example, in response to a background processing task

wchar_t Infrequently Indicates a 'wide charater', that is, a character type with a much greater range of symbols that the standard 'char'

while All the time Used to construct loops, alone or with 'do'

Keyword	Used	Purpose
auto	Very infrequently	Defines a variable as 'automatic', that is, its memory is allocated by the compiler for the duration of its existence. For local variables this is the default anyway
break	All the time	The 'break' statment causes execution of the program to jump out of the current loop or 'case' statement. Use is almost compuslory with 'switch...case', and common with loops. However, the principles of structured programming suggest that use of 'break' to jump out of the middle of a loop should usully be avoided
bool	Infrequently; should be used more	'bool' defines a variable as being of 'Boolean' type, i.e. able to take only 'true' or 'false' values. As C did not have a Boolean data type, many programmers have got used to using integers for this
case	All the time	Used with 'switch' to allow execution of one of a list of groups of statements
catch	Infrequently; not well understood	Used with 'try' and 'throw' to detect and process 'exceptions', that is, unusual circumstances like errors. The exception handling mechanism in C++ is not well understood by programmers
char	All the time	Indicates a character data type
class	All the time	Indicates that the following statements are the definition or declaration of a C++ class
const	All the time by good programmers	Indicates that a variable has a constant value, i.e., cannot be redefined; with a class operation, indicates that the operation does not modify the object's data
const_cast	Very infrequently	Changes the 'constant' or 'volatile' status of a data item
continue	Too often	Inside a loop, causes the rest of the statements after 'continue' to be skipped, and the next turn of the loop started. Use with caution.
default	All the time	Used with 'switch' to indicate the action that should be carried out if no others are appropriate
delete	All the time	Removes a data item that was constructed using 'new', freeing any memory it used
do	All the time	Used with 'while' to construct a loop that must be executed at least once
double	All the time	Indicates a real-number, double-precision value
dynamic_cast	Very infrequently	Used in management of class inheritance
else	All the time	Used with 'if' to execute statements according to the results of a comparison
enum	Not often enough	A data type whose possible values are listed explicitly, e.g., 'enum {red, amber, green} TrafficLightColour'
explicit	Infrequently	Prevents an object being intialized using a statement of the form 'X x=1', where x is an object of class X
extern	All the time	Indicates that something used in one program module (file) is defined in a different program module.
false	Not frequently enough	See 'bool'
float	All the time	Indicates a floating-point (real number) data element
friend	All the time	Within a class, indicates that a different class or function is to be allowed access to the class's data
for	All the time	Used to construct loops
goto	Infrequently	Jumps to a specified place in a function
if	All the time	Executes statements if a condition is true
inline	Commonly in specific contexts	Suggests to the compiler that a function should be inserted into the program whereever it is used, and not called as a function. This process is transparent to the programmer and does not change the result of the function; it makes some programs slightly faster at the expense of needing more memory
long	All the time	Indicates that a long version of a particular data type is to be used, i.e., one with a greater range of values
mutable	Infrequently	Indicates that a data element in an object can be changed, even by operations which claim not to change data
namespace	Infrequently, except in large applications	Defines a namespace, that is, a division of the program in which names will not be shared with other parts of the program
new	All the time	Creates a new variable or object
operator	Sometimes (or all the time by some programmers)	Used in extending the meanings of C++ operators
private	All the time	In a class, indicates an operation or attribute that is accessible only within the class (except as indicated by 'friend')
protected	All the time	In a class, indicates an operation or attribute that is accessible only classes of the same type (i.e., derived from this one)
public	All the time	In a class, indicates an operation or attribute that is accessible outside the class
register	Suggests to the compiler that a variable should be held in a CPU register and not in memory. This is for speed optimization.
reinterpret_cast	Very infrequently	Used to improve the safety of certain types of cast operation
return	All the time	Exits a function, specifying the return value if appropriate
short	All the time	Indicates that a shorter version of a particular data type is to be used, i.e., one with a smaller range
signed	Infrequently	Indicates that the following data type is signed, i.e., accepts positive and negative values. In C++, integers are signed by default, so this keyword is uncommon
sizeof	All the time	When applied to a variable or data type, give the number of bytes used to store it
static	All the time	Various meanings, all concerned with the scope and lifetime of data and functions
static_cast	Very infrequently	Used in conversion between objects of different types
struct	All the time	Introduces a programmer-defined data structure
switch	All the time	Used with 'case' and 'default' to provide a mechanism where one of a list of sets of statements is executed
template	All the time by programmers that like templates, or use template libraries	Indicates that the following definitation is of a fucntion or class template, rather than a function or class
this	All the time	Used within an object to refer to the object itself
throw	Infrequently; not well understood	Used with 'try' and 'throw' to detect and process 'exceptions', that is, unusual circumstances like errors. The exception handling mechanism in C++ is not well understood by programmers
true	Not frequently enough	See 'bool'
try	Infrequently; not well understood	Used with 'catch' and 'throw' to detect and process 'exceptions', that is, unusual circumstances like errors. The exception handling mechanism in C++ is not well understood by programmers
typedef	All the time	Introduces a programmer-defined data type
typename	Infrequently	Used in the definition of general-purpose templates
typeid	Infrequently, except in certain contexts	Provides information about the type of an object
union	Increasingly uncommonly	Indicates that the same area of memory can be used to store a number of different variables at different times
using	Infrquent, except in large applications	Used with 'namespace' to distinguish between data in different namespaces
unsigned	All the time	Indicates that the following variable is unsigned, i.e., only takes positive values
virtual	All the time	In the definition of a class, indicates that an operation will be re-defined in classes derived from it
void	All the time	Indicates that something is unknown, or does not have a value, or does not accept parameters, or does not return a value
volatile	Infrequently, except in certain contexts	Indicates that a variable or object is liable to be modified by another part of the program in an unpredictable way. For example, in response to a background processing task
wchar_t	Infrequently	Indicates a 'wide charater', that is, a character type with a much greater range of symbols that the standard 'char'
while	All the time	Used to construct loops, alone or with 'do'