High-quality C ++/C Programming Guide-Constant

A constant is an identifier whose value remains unchanged during running. The C language uses # define to define constants (called macro constants ). In addition to # define, the C ++ language can also use const to define constants (called const constants ).

1. Why constant?

If you do not use constants and enter numbers or strings directly in the program, what will happen?

(1) the readability (comprehensibility) of the program is deteriorated. Programmers forget what the numbers or strings mean, and users do not know where they come from and what they represent.

(2) Inputting the same number or string in many places of the program makes it difficult to avoid writing errors.

(3) If you want to modify a number or string, it will be changed in many places, which is both troublesome and error-prone.

 

Rule 5-1-1]Use constants with intuitive meanings to indicate the numbers or strings that will appear multiple times in the program.

For example:

# Define max 100/* macro constant of C language */

Const int max = 100; // const constant in C ++

Const float Pi = 3.14159; // const constant in C ++

2 const and # define comparison

The C ++ language can use const to define constants, or use # define to define constants. However, the former has more advantages than the latter:

(1) const constants have data types, while macro constants do not. The compiler can perform type security checks on the former. However, only character replacement is performed for the latter, and there is no type security check. In addition, replacement may produce unexpected errors (marginal effect ).

(2) Some integrated debugging tools can debug const constants, but cannot debug macro constants.

 

Rule 5-2-1]In the C ++ program, only the const constant is used instead of the macro constant, that is, the const constant completely replaces the macro.

3. Constant definition rules

Rule 5-3-1]Constants that need to be made public are placed in the header file, and constants that do not need to be made public are placed in the header of the definition file. For ease of management, constants of different modules can be stored in a public header file.

Rule 5-3-2]If a constant is closely related to other constants, this relationship should be included in the definition, rather than some isolated values.

For example:

Const float radius = 100;

Const float diameter = radius * 2;

Constants in class 4

Sometimes we want some constants to be valid only in the class. Since the macro constants defined by # define are global and cannot be achieved, it is assumed that we should use const to modify data members. The const data member does exist, but its meaning is not what we expected. The const data member is a constant only within the lifetime of an object, but it is variable for the entire class, because the class can create multiple objects, the values of the const data members of different objects can be different.

The const data member cannot be initialized in the class declaration. The following usage is incorrect because the compiler does not know what the size value is when the class object is not created.

Class

{...

Const int size = 100; // error, attempted to initialize the const data member in the class declaration

Int array [size]; // error, unknown size

};

 

The const data member initialization can only be performed in the initialization table of the class constructor, for example

Class

{...

A (INT size); // Constructor

Const int size;

};

A: A (INT size): size (size) // initialization table of the constructor

{

...

}

A A (100); // the size of object A is 100

A B (200); // the size of object B is 200

 

How can we create constants that are constant throughout the class? Don't count on the const data member. We should use the enumerated constants in the class. For example

Class

{...

Enum {size1 = 100, size2 = 200}; // enumerated constant

Int array1 [size1];

Int array2 [size2];

};

Enumerated constants do not occupy the storage space of objects. They are fully evaluated during compilation. The disadvantage of an enumerated constant is that its implicit data type is an integer, its maximum value is limited, and it cannot represent a floating point number (such as Pi = 3.14159 ).