从零开始学C++之RTTI、dynamic_cast、typeid、类与类之间的关系uml

一、RTTI

Run-time type information (RTTI) is a mechanism that allows the type of an object to be determined during program execution.

There are three main C++ language elements to run-time type information:

The dynamic_cast operator.

Used for conversion of polymorphic types.
The typeid operator.

Used for identifying the exact type of an object.

The type_info class.

Used to hold the type information returned by thetypeid operator.

class type_info {
public:

virtual ~type_info();
bool operator==(const type_info& rhs) const;
bool operator!=(const type_info& rhs) const;
int before(const type_info& rhs) const;
const char* name() const;
const char* raw_name() const;

private:

void *_m_data;
char _m_d_name[1];
type_info(const type_info& rhs);
type_info& operator=(const type_info& rhs);
static const char _Name_base(const type_info *,__type_info_node* __ptype_info_node);

};

The result of typeid is a const type_info&. The value is a reference to a type_info object that represents either thetype-id or the type of theexpression, depending on which form oftypeid is used.

       C++ Code 
    

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

#include <iostream>
using namespace std;

class Shape
{
public:
     virtual void Draw() = 0;
     virtual ~Shape() {}
};

class Circle : public Shape
{
public:
     void Draw()
    {
        cout << "Circle Draw ..." << endl;
    }
};

class Square : public Shape
{
public:
     void Draw()
    {
        cout << "Square Draw ..." << endl;
    }
};

int main( void)
{
    Shape *p;
    Circle c;

    p = &c;
    p->Draw();

     //使用dynamic_cast 的条件
     //1、开启运行时类型信息；2、应用在具有多态关系的继承体系上；
     if ( dynamic_cast<Circle *>(p))
    {
        cout << "p is point to a Circle object" << endl;
        Circle *cp = dynamic_cast<Circle *>(p);      // 安全向下转型
        cp->Draw(); //效率没有 p->Draw(); 高
    }
     else if ( dynamic_cast<Square *>(p))
    {
        cout << "p is point to a Square object" << endl;
    }
     else
    {
        cout << "p is point to a Other object" << endl;
    }

    cout << typeid(*p).name() << endl;
    cout << typeid(Circle).name() << endl;
     if ( typeid(Circle).name() == typeid(*p).name())
    {
        cout << "p is point to a Circle object" << endl;
        ((Circle *)p)->Draw();
    }
     else if ( typeid(Square).name() == typeid(*p).name())
    {
        cout << "p is point to a Circle object" << endl;
        ((Square *)p)->Draw();
    }
     else
    {
        cout << "p is point to a Other object" << endl;
    }

     return 0;
}

如上所述，dynamic_cast 和 typeid 操作符都可以实现运行时类型识别。其中使用dynamic_cast 时需要开启运行时类型信息，在项目-》属性-》C/C++-》语言-》启用运行时类型信息。在使用typeid时需要注意的是返回的是type_info 对象的引用，且type_info 类的拷贝构造函数和赋值运算符都声明为私有，故不能这样写： type_info tf = typeid(Circle);

二、类与类之间的关系

Unified Modeling Language (UML)又称统一建模语言或标准建模语言,是始于1997年一个OMG标准，它是一个支持模型化和软件系统开发的图形化语言。

1、继承（泛化）Generalization