C++ Basic Rules

C++ program structure

Let's look at a simple piece of code that prints the word Hello World .

#include <iostream>
using namespace std;
 
// main() is where the program starts executing
 
intmain ()
{
   cout << "Hello World"; // 输出 Hello World
   return 0;
}

Next, we explain the above program:

• The C++ language defines header files that contain information that is necessary or useful in a program. In the above program, the header file <iostream> is included .
• The next line using namespace std; tells the compiler to use the std namespace. Namespaces are a relatively new concept in C++.
• The next line // main() is where the program begins execution is a one-line comment. Single-line comments begin with // and end at the end of the line.
• The next line int main() is the main function, where the program execution begins.
• The next line cout << "Hello World"; will display the message "Hello World" on the screen.
• The next line returns 0; terminates the main( ) function and returns the value 0 to the calling process.

C++ identifiers

A C++ identifier is a name used to identify a variable, function, class, module, or any other user-defined item. An identifier begins with the letters AZ or az or the underscore _, followed by zero or more letters, underscores, and numbers (0-9).

Punctuation characters such as @, &, and % are not allowed within C++ identifiers. C++ is a case-sensitive programming language. Therefore, in C++, Manpower and manpower are two different identifiers.

Several valid identifiers are listed below:

mohd       zara    abc   move_name  a_123
myname50   _temp   j     a23b9      retVal

C++ keywords

The following table lists reserved words in C++. These reserved words cannot be used as constant names, variable names, or other identifier names.

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

trigram

A trigram is a sequence of three characters used to represent another character, also known as a three-character sequence. A three-character sequence always begins with two question marks.

Three-character sequences are less common, but the C++ standard allows certain characters to be specified as three-character sequences. In the past, this was an essential method to represent characters that were not on the keyboard.

Three-character sequences can appear anywhere, including strings, character sequences, comments, and preprocessing directives.

The most commonly used three-character sequences are listed below:

三字符组	替换
??=	#
??/	\
??'	^
??(	[
??)	]
??!	|
??<	{
??>	}
??-	~

 

 

 

 

 

 

 

 

 

 

If you want to have two consecutive question marks in the source program, and do not want to be replaced by the preprocessor, this situation occurs in character constants, string literals or program comments, the alternative is to use automatic string concatenation : "...?" "?..." or escape sequence: "...?\?...".

Starting with Microsoft Visual C++ version 2010, the compiler no longer automatically replaces trigrams by default. If you need to use trigraph substitution (eg for compatibility with old software code), you need to set the compiler command line option /Zc:trigraphs

g++ still supports trigrams by default, but will give compile warnings.

Exercises on input and output

#include <stdlib.h>
#include <iostream>
using namespace std;

// Requirement: Prompt the user to input an integer, and print the integer on the screen in octal, decimal, and hexadecimal respectively.
 // Requirement: Prompt the user to enter a boolean value (0/1), and in Boolean way The value is printed on the screen

int main(void)
{
    cout << " Please enter an integer: " << endl;
     int x = 0 ;
    cin >> x;
    cout << oct << x << endl; // octal 
    cout << dec << x << endl; // decimal 
    cout << hex << x << endl; // hex 

    cout < < " Please enter a boolean value (0, 1): " << endl;
     bool y = false ;
    cin >> y;
    cout << boolalpha << y << endl;

    system("pause");
    return 0;

}