【C++ Primer | 19】运行类型识别

运行类型识别

一、使用RTTI

dynamic_cast运算符的调用形式如下所示：

dynamic_cast<type*>(e)   //e是指针
dynamic_cast<type&>(e)   //e是左值
dynamic_cast<type&&>(e)  //e是右值

e能成功转换为type*类型的情况有三种：

1）e的类型是目标type的公有派生类：派生类向基类转换一定会成功。

2）e的类型是目标type的基类，当e是指针指向派生类对象，或者基类引用引用派生类对象时，类型转换才会成功，当e指向基类对象，试图转换为派生类对象时，转换失败。

3）e的类型就是type的类型时，一定会转换成功。

1. 测试代码：

 1 class Rect : public Shape {
 2 public:
 3     void fun() {};
 4     void prin() { cout << "我是方形" << endl; }
 5 };
 6 
 7 class Circle : public Shape {
 8 public:
 9     void fun() {};
10     void prin() { cout << "我是圆形" << endl; }
11 };
12 
13 void fun(Shape *p)
14 {
15     if (typeid(*p) == typeid(Rect))
16     {
17         Rect *r = dynamic_cast<Rect *>(p);
18         r->prin();
19     }
20     if (typeid(*p) == typeid(Circle))
21     {
22         Circle *c = dynamic_cast<Circle *>(p);
23         c->prin();
24     }
25 }
26 int main()
27 {
28     Rect r;
29     cout << "第一次执行fun函数：";
30     fun(&r);
31 
32     Circle c;
33     cout << "第二次执行fun函数：";
34     fun(&c);
35 
36     return 0;
37 }

View Code

输出结果：

二、type_info类

1. 测试代码：

 1 #include <iostream>  
 2 #include <typeinfo.h>  
 3 using namespace std;
 4 
 5 class Base {
 6 public:
 7     virtual void vvfunc() {}
 8 };
 9 
10 class Derived : public Base {};
11 
12 int main() 
13 {
14     Derived* pd = new Derived;
15     Base* pb = pd;
16     cout << typeid(pb).name() << endl;   //prints "class Base *"  
17     cout << typeid(*pb).name() << endl;   //prints "class Derived"  
18     cout << typeid(pd).name() << endl;   //prints "class Derived *"  
19     cout << typeid(*pd).name() << endl;   //prints "class Derived"  
20     delete pd;
21 }

View Code

输出结果：

2. 代码示例

 1 #include <iostream>
 2 #include <typeinfo>
 3 
 4 int main()
 5 {
 6     if (typeid(int).before(typeid(char)))
 7         std::cout << "int goes before char in this implementation.
";
 8     else
 9         std::cout << "char goes before int in this implementation.
";
10 }

运行结果：

3. 代码示例

 1 #include <iostream>
 2 #include <typeinfo>
 3 #include <string>
 4 #include <utility>
 5 
 6 class person
 7 {
 8 public:
 9 
10     person(std::string&& n) : _name(n) {}
11     virtual const std::string& name() const { return _name; }
12 
13 private:
14 
15     std::string _name;
16 };
17 
18 class employee : public person
19 {
20 public:
21 
22     employee(std::string&& n, std::string&& p) :
23         person(std::move(n)), _profession(std::move(p)) {}
24 
25     const std::string& profession() const { return _profession; }
26 
27 private:
28     std::string _profession;
29 };
30 
31 void somefunc(const person& p)
32 {
33     if (typeid(employee) == typeid(p))
34     {
35         std::cout << typeid(p).name() << std::endl;
36         std::cout << p.name() << " is an employee ";
37         auto& emp = dynamic_cast<const employee&>(p);
38         std::cout << "who works in " << emp.profession() << '
';
39     }
40 }
41 
42 int main()
43 {
44     employee paul("Paul", "Economics");
45     somefunc(paul);
46 }

View Code

运行结果：

【C++ Primer | 19】运行类型识别

运行类型识别

参考资料