C++ constants

Integer constants

Integer constants can be decimal, octal or hexadecimal constants. The prefix specifies the base: 0x or 0X for hexadecimal, 0 for octal, and without a prefix the default is decimal.

Integer constants can also have a suffix. The suffix is ​​a combination of U and L. U represents an unsigned integer (unsigned) and L represents a long integer (long). The suffix can be uppercase or lowercase, and U and L can be in any order.

The following are examples of several integer constants:

212         // 合法的
215u        // 合法的
0xFeeL      // 合法的
078         // 非法的：8 不是八进制的数字
032UU       // 非法的：不能重复后缀

The following are examples of various types of integer constants:

85         // 十进制
0213       // 八进制 
0x4b       // 十六进制 
30         // 整数 
30u        // 无符号整数 
30l        // 长整数 
30ul       // 无符号长整数

Floating point constants

Floating Point constants consist of an integer part, a decimal point, a fractional part, and an exponent part. You can represent floating-point constants in decimal or exponential form.

When expressed in decimal form, the decimal point, exponent, or both must be included. When expressed in exponential form, an integer part, a decimal part, or both must be included. Signed exponents are introduced with e or E.

The following are several examples of floating point constants:

3.14159       // 合法的 
314159E-5L    // 合法的 
510E          // 非法的：不完整的指数
210f          // 非法的：没有小数或指数
.e55          // 非法的：缺少整数或分数

Boolean constants

There are two Boolean constants, both of which are standard C++ keywords:

• true The value represents true.

• #false The value represents false.

We should not treat the value of true as 1 and the value of false as 0.

Character constants

Character constants are enclosed in single quotes. If a constant begins with L (only when uppercase), it means that it is a wide character constant (such as L'x'), in which case it must be stored in a variable of type wchar_t. Otherwise, it is a narrow character constant (such as 'x'), in which case it can be stored in a simple variable of type char.

Character constant can be an ordinary character (such as 'x'), an escape sequence (such as '\t'), or a universal character (such as '\u02C0').

In C++, there are some specific characters. When they are preceded by a backslash, they have special meanings and are used to represent newlines (\n) or tabs (\t )wait. The following table lists some such escape sequence codes:

##\vVertical tab\oooOne to three digits in octal number##\xhh . . .

The following example shows some escape sequence characters:

#include <iostream>
using namespace std;

int main()
{
   cout << "Hello\tWorld\n\n";
   return 0;
}

When the above code is compiled and executed, it produces the following results:

Hello   World

String constants

String literals or constants are enclosed in double quotes "". A string contains characters similar to character constants: ordinary characters, escape sequences, and universal characters.

You can use spaces as delimiters to separate a long string constant into lines.

The following example shows some string constants. The strings displayed in the following three forms are the same.

"hello, dear"

"hello, \

dear"

"hello, " "d" "ear"

Define constants

In C++, there are two simple ways to define constants:

• Use #define preprocessor.

• Use the const keyword.

#define preprocessor

The following is the form of using the #define preprocessor to define constants:

#define identifier value

Please see the following example for details:

#include <iostream>
using namespace std;

#define LENGTH 10   
#define WIDTH  5
#define NEWLINE '\n'

int main()
{

   int area;  
   
   area = LENGTH * WIDTH;
   cout << area;
   cout << NEWLINE;
   return 0;
}

When the above code is compiled and executed, it produces the following results:

50

const Keywords

You can use the const prefix Declare a constant of the specified type as follows:

const type variable = value;

Please see the following example for details:

#include <iostream>
using namespace std;

int main()
{
   const int  LENGTH = 10;
   const int  WIDTH  = 5;
   const char NEWLINE = '\n';
   int area;  
   
   area = LENGTH * WIDTH;
   cout << area;
   cout << NEWLINE;
   return 0;
}

When the above code is compiled and executed, it will produce the following results:

50

Please note that it is a good programming practice to define constants in uppercase letters.

#Escape sequence	Meaning
\\	\ characters
\'	' characters
\"	" Characters
\?	? Characters
\a	Alarm ringtone
\b	Backspace key
\f	Form feed
\n	Line break
\r	Enter
\t	Horizontal tab
One or more digits in hexadecimal number