Lexical IN C++

Introduces the fundamental elements of a C++ program as they are meaningful to the compiler are used to construct statements, definitions, declarations, and so on, which are used to construct complete programs.

The fundamental elements of a C++ program are called “lexical elements” or “tokens” to construct statements, definitions, declarations, and so on, which are used to construct complete programs. The following lexical elements are discussed here:

1.    Tokens

A token is the smallest element of a C++ program that is meaningful to the compiler. The C++ parser recognizes these kinds of tokens: Identifiers, Keywords, Literals, Operators, Punctuators, and Other Separators. A stream of these tokens makes up a translation unit.

2.    Comments

A comment is text that the compiler ignores but that is useful for programmers. Comments are normally used to annotate code for future reference.

A C++ comment is written in one of the following ways:

The /* (slash, asterisk) characters, followed by any sequence of characters (including new lines), followed by the */ characters.

The // (two slashes) characters, followed by any sequence of characters. A new line not immediately preceded by a backslash terminates this form of comment. Therefore, it is commonly called a “single-line comment.”

/*This is an example of

  comments in a program */

FileName = String( "hello.dat" );   // Initialize file string 

3.    Identifiers

An identifier is a sequence of characters used to denote one of the following:

   

Object or variable name

   

Class, structure, or union name

   

Enumerated type name

 Member of a class, structure, union, or enumeration

 Function or class-member function

   typedef name

   Label name

The first character of an identifier must be an alphabetic character, either uppercase or lowercase, or an underscore ( _ ). Because C++ identifiers are case sensitive, fileName is different from FileName.

4.    C++ keywords

Keywords are predefined reserved identifiers that have special meanings. They cannot be used as identifiers in your program. The following keywords are reserved for C++:

asm	auto	bad_cast	bad_typeid
bool	break	case	catch
char	class	const	const_cast
continue	default	delete	do
double	dynamic_cast	else	enum
except	explicit	extern	false
finally	float	for	friend
goto	if	inline	int
long	mutable	namespace	new
operator	private	protected	public
register	reinterpret_cast	return	short
signed	sizeof	static	static_cast
struct	switch	template	this
throw	true	try	type_info
typedef	typeid	typename	union
unsigned	using	virtual	void
Volatile	while

5.    Punctuators

Punctuators in C++ have syntactic and semantic meaning to the compiler but do not, of themselves, specify an operation that yields a value. Some punctuators, either alone or in combination, can also be C++ operators or be significant to the preprocessor.  The punctuators are

! % ^ & * ( ) – + = { } | ~
[ ] \ ; ' : " < > ? , . / #

The punctuators [ ], ( ), and { } must appear in pairs

6.    Operators