Relationship between common objects and common member functions

(1) Common objects
 

Objects modified with const are called object constants in the following format:
 

<Class Name> const <Object Name> or const <Class Name> <Object Name> 〉
 

When declared as a common object, it must be initialized and cannot be rewritten to the data member of the object.

 

 

Example:Analyze the following errors and explain the reasons;

# Include <iostream. h>

Class Point

{Int X, Y;

Public:

 

Point (int A, int B) {x = A; y = B ;}

Void movepoint (int A, int B) {x + = A; y + = B ;}

 

Void print () {cout <"x =" <x <"Y =" <Y <Endl ;}

};

Void main ()
{

 

Const point point1 (10, 10); // constant object

Point1.movepoint (); // the data of the Common Object point1 cannot be changed, so the error occurs.

Point1.print (); // The this pointer of the print function cannot point to a common object.

}

The cause of the error is added. 
 

Common objects are often used in object reference. The so-called frequent reference refers to the useConst modifier. The referenced object cannot be changed.

 

New. It is generally used as a form parameter. Format:
 

Const <type description> & reference Name> 〉 

Example:According to the class in Example 5-11ProgramResult:

# Include <math. h>

Int length (const point & P1, const point & p2)

{Int L;

L = SQRT (p1.x-p2.x) * (p1.x-p2.x) + (p1.y-p2.y) * (p1.y-p2.y ));

Return L;

}

Void main ()

{

Point A (1, 1), B (4, 5 );

Cout <"the distance between two points is:" <length (a, B) <Endl;

}

Output of this program:

The distance between two points is: 5 

(2) common member functions

 

The function described in the const keyword is a common member function. The format is as follows:

<Type> <Function Name> (parameter table>) const;

Functions and functions of common member functions:

Common member functions cannot update object data or call non-const modified member functions. A common object can only call common member functions of a class.

And static member functions of the class. 

Example:The execution results of the following programs are given:

# Include <iostream. h>

Class Point

{Int X, Y;

Public:

Point (int A, int B) {x = A; y = B ;}

Void movepoint (int A, int B) {x + = A; y + = B ;}

Void print () const {cout <"x =" <x <"Y =" <Y <Endl ;}

};

Void main ()

{

Point point1 (1, 1 );

Const point point2 (2, 2); // constant object

Point1.print (); // a common object can call a common member function.

Point2.print (); // call a common member function for a Common Object

}

Execution result of this program:

X = 1 y = 1

X = 2 y = 2 

A member function described by the const keyword is a member function. The description format of a member function is as follows: 

<Return type description> <member function name> (<parameter table>) const;

 

Note the following when using common member functions: 

(1) const is an integral part of the function type. Therefore, the const keyword must be included in the function implementation part. 

(2) A common member function does not update the data member of an object, nor can it call a member function without const modification in this class. 

(3) A common object can only call its common member functions, but cannot call other member functions. The operation Relationship between member functions and objects is shown in figure

Table 12-1. 

(4) The const keyword can be used to distinguish between overloaded functions. For example: 
Void print (); 

Void print () const; 

These two functions can be used for overloading. The principle of overloading is that a common object calls a common member function, and a general object calls a common member function. 

(5) In the const member function: 

 

Member variables defined as mutable can be modified.