首页 > itarticle > surrogate class

surrogate class

admin 11月 11, 2020 0

Here is a question.

How could we design a type of class to enable it to include the classes which is different but have something in common ?

The solution is sorrogate

A demo about “Vehicle”: 

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
 
    Programmer  :  EOF
    Date        :  2015.05.18
    File        :  5.1.c
    E-mail      :  [email protected]
 */
class Vehicle
{
    public:
        
           It will be OK, if you don't use virtual function.
        double weight() const;
        void start();
        */
        virtual double () const = 0;
        virtual void start() = 0;
        // ...
};
class RoadVehicle: public Vehicle {/* ... */};
class AutoVehicle: public RoadVehicle {/* ... */};
class Aircraft: public Vehicle {/* ... */};
class Helicopter: public Aircraft {/* ... */};
Vehicle parking_lot[1000];

If you gona to compile this source file, you will got errors and warnings like this:

images

The reason why we got error is that Vihecle is a base class, member function @weight and @start is pure virtual function. With the = 0 in the declaration, we declare that that pure virtual function could without definitaion. Only the class which is inherit from base class Vihecle.

So, the object Vehicle is not existed and we can’t define an array of that type.

If we change the virtual function into non-virtual function, we could pass the compile process.

But it’s very dangerous.

Eg, if we delete all pure virtual functions in Vehicle, what would happen?

1
2
3
4
5
 
Vehicle parking_lot[1000];
Automobile x = /* ... */
Parking_lot[num_vihicles++] = x;

The answer is that there would be a cast on @x. @x is class Automobile which inhiret from base class Vehicle. @x will be cast into class Vehicle and then assign to object Parking_lot[num_vihicles]. In the process of casing, @x will lost anything which is not in Vehicle.

More explicit, the array Parking_lot is an array of class Vehicle but not Automobile.

It’s time release our scheme to solve that problem.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
 
/**********************************************
Programmer  :   EOF
Date        :   2015.05.18
File        :   5.4.cpp
E-mail      :   [email protected]
***********************************************/
class Vehicle
{
    public:
        
           It will be OK, if you don't use virtual function.
        double weight() const;
        void start();
        */
        virtual double () const = 0;
        virtual void start() = 0;
        virtual Vehicle* copy() const = 0;
        virtual ~Vehicle() { } ;
        // ...
};
class RoadVehicle: public Vehicle {/* ... */};
class AutoVehicle: public RoadVehicle {/* ... */};
class Aircraft: public Vehicle {/* ... */};
class Helicopter: public Aircraft {/* ... */};
class VehicleSurrogate
{
    public:
        
           Constructor
         */
        VehicleSurrogate();
        VehicleSurrogate(const Vehicle&);
        VehicleSurrogate(const VehicleSurrogate&);
        
           De-constructor
         */
        ~VehicleSurrogate();
        VehicleSurrogate& operator =(const VehicleSurrogate&);
        
           Operations from class Vehicle
         */
        double () const;
        void start();
        // ...
    private:
        Vehicle* vp;
};
VehicleSurrogate::VehicleSurrogate(): vp(0) { }
VehicleSurrogate::VehicleSurrogate(const Vehicle& v):
    vp(v.copy()) { }
VehicleSurrogate:: ~VehicleSurrogate()
{
    delete vp;
}
VehicleSurrogate::VehicleSurrogate (const VehicleSurrogate& v):
    vp(v.vp ? v.vp->copy() : 0) { }
VehicleSurrogate&
VehicleSurrogate::operator=(const VehicleSurrogate& v)
{
    if (this != &v)
    {
        delete vp;
        vp = (v.vp ? v.vp->copy() : 0);
    }
    return *this;
}
double VehicleSurrogate::weight() const
{
    if (vp == 0)
    {
        throw "empty VehicleSurrogate.weight()";
    }
    return vp->weight();
}
void VehicleSurrogate::start()
{
    if (vp == 0)
    {
        throw "empty VehicleSurrogate.start()";
    }
    vp->start();
}
class Automobile: public AutoVehicle
{
        double () const;
        void start();
        Vehicle* copy() const;
};
double
Automobile::weight() const
{
    return 0.0;
}
void
Automobile::start()
{
    std::cout << "Hello this is automobile" << std::endl;
}
Vehicle*
Automobile::copy() const
{
    return new Automobile(*this);
}
   Here is our test unit.
   Attention! 
   Never use assignment between class objects out of function block.
   If your assignment operation is not in function block, you must got 
   an error from compiler.
   But it's ok, if you would like to define the varibles outside of blocks.
 */
VehicleSurrogate parking_lot[1000];
Automobile x;
int num_vehicles = 0;
int main()
{
    
       These two assignment statement are doing the same thing.
     */
    parking_lot[num_vehicles++] = x;
    //parking_lot[num_vehicles++] = VehicleSurrogate(x);
    return 0;
}

You may noticed that the key point of our solution is to use the class to represent a concept.

We defined a contructor without any parameters, so we define an array of this type of class.

If you have understand what we have done, you may feel excited. Yes, we create a container which could be used to represent any type of classes which are have relationships of inhiretance.

parking_lot[num_vehicles++] = x; is equal to parking_lot[num_vehicles++] = VehicleSurrogate(x);

This statement create a copy of object @x and attach @VehicleSurrogate onto object @x. If we destory elements in @parking_lot, the corresponding copy will be deleted.

Finally, you have know that why this techonology named Surrogate.

The objects in this type of class could be used to represent others class objects which are have completly relationships of inhiretance.

Margaret Heafield Hamilton who is a very famous computer scientist, systems engineer, and business owner. Picture blew there is that hamilton during the Apollo Program.

images

微海报分享

近期文章

近期评论

标签

热门

文章归档

分类目录

功能