1. 第一個字母必須是英文字或是"_"底線符號
2. 其它字母必須是英文字、數字或"_"底線符號
3. 不可以使用 C++ 關鍵字 ( 保留字 )
4. 大小寫是有區別的

※關鍵字
下表為 C++ 關鍵字，這些關鍵字都不可以被用來當做常數或變數

Keyword	Description
asm	insert an assembly instruction
auto	declare a local variable
bool	declare a boolean variable
break	break out of a loop
case	a block of code in a switch statement
catch	handles exceptions from throw
char	declare a character variable
class	declare a class
const	declare immutable data or functions that do not change data
const_cast	cast from const variables
continue	bypass iterations of a loop
default	default handler in a case statement
delete	make dynamic memory available
do	looping construct
double	declare a double precision floating-point variable
dynamic_cast	perform runtime casts
else	alternate case for an if statement
enum	create enumeration types
explicit	only use constructors when they exactly match
export	allows template definitions to be separated from their declarations
extern	tell the compiler about variables defined elsewhere
false	a constant representing the boolean false value
float	declare a floating-point variable
for	looping construct
friend	grant non-member function access to private data
goto	jump to a different part of the program
if	execute code based on the result of a test
inline	optimize calls to short functions
int	declare an integer variable
long	declare a long integer variable
mutable	override a const variable
namespace	partition the global namespace by defining a scope
new	allocate dynamic memory for a new variable
operator	create overloaded operator functions
private	declare private members of a class
protected	declare protected members of a class
public	declare public members of a class
register	request that a variable be optimized for speed
reinterpret_cast	change the type of a variable
return	return from a function
short	declare a short integer variable
signed	modify variable type declarations
sizeof	return the size of a variable or type
static	create permanent storage for a variable
static_cast	perform a nonpolymorphic cast
struct	define a new structure
switch	execute code based on different possible values for a variable
template	create generic functions
this	a pointer to the current object
throw	throws an exception
true	a constant representing the boolean true value
try	execute code that can throw an exception
typedef	create a new type name from an existing type
typeid	describes an object
typename	declare a class or undefined type
union	a structure that assigns multiple variables to the same memory location
unsigned	declare an unsigned integer variable
using	import complete or partial namespaces into the current scope
virtual	create a function that can be overridden by a derived class
void	declare functions or data with no associated data type
volatile	warn the compiler about variables that can be modified unexpectedly
wchar_t	declare a wide-character variable
while	looping construct