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

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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

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#include <iostream>

using
namespace std;

class Shape

{

public:

virtual
void Draw() =
;

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
;

}

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