在学习java中的Class类，有getClass方法，其特性与c++中的typeid很类似，java中调用getClass方法和c++中的typeid运算符等价。
typeid是C++的一个关键字，其作用是在程序运行时，返回对象的类型，即所谓的RTTI(Run Time Type Identification), 它使程序能够获取由指针或引用所指向的对象的实际类型，即允许“用指向基类的指针或引用来操作对象”的程序能够获取到“这些指针或引用所指对象”的实际派生类型。在C++中，为了支持RTTI提供了两个操作符：dynamic_cast和typeid。
typeid操作符的返回结果是名为type_info的标准库类型的对象的引用。 type_info在typeinfo头文件中定义：
class type_info {public:
virtual ~type_info();
bool operator== (const type_info& rhs) const;
bool operator!= (const type_info& rhs) const;
bool before (const type_info& rhs) const;
const char* name() const;private:
type_info (const type_info& rhs);
type_info& operator= (const type_info& rhs);
}; 如下内容摘自http://www.cplusplus.com/reference/typeinfo/type_info/:
typeid can be applied to any typed expression, including a type itself, to retrieve the its type_info.
When typeid is applied to a reference or dereferenced pointer to an object of a polymorphic class type (a class declaring or inheriting a virtual function), it considers its dynamic type (i.e., the type of the most derived object). This requires the RTTI (Run-time type information) to be available.
When typeid is applied to a dereferenced null pointer a bad_typeid exception is thrown.
type_info成员介绍如下：
- operator==
operator!= - Comparison operators. They return whether the two types describe the same type.
A derived type is not considered the same type as any of its base classes.
- before
- Returns true if the type precedes the type of rhs in the collation order.
The collation order is just an internal order kept by a particular implementation and is not necessarily related to inheritance relations or declaring order. - name
- Returns a null-terminated character sequence with a human-readable name for the type.
- copy constructor and copy operator
- These are private, preventing type_info values from being copiable.
示例代码：
// type_info example
#include <iostream>
#include <typeinfo>
using namespace std;
struct Base {};
struct Derived : Base {};
struct Poly_Base {virtual void Member(){}};
struct Poly_Derived: Poly_Base {};
int main() {
// built-in types:
int i;
int * pi;
cout << "int is: " << typeid(int).name() << endl;
cout << " i is: " << typeid(i).name() << endl;
cout << " pi is: " << typeid(pi).name() << endl;
cout << "*pi is: " << typeid(*pi).name() << endl << endl;
// non-polymorphic types:
Derived derived;
Base* pbase = &derived;
cout << "derived is: " << typeid(derived).name() << endl;
cout << " *pbase is: " << typeid(*pbase).name() << endl;
cout << boolalpha << "same type? ";
cout << ( typeid(derived)==typeid(*pbase) ) << endl << endl;
// polymorphic types:
Poly_Derived polyderived;
Poly_Base* ppolybase = &polyderived;
cout << "polyderived is: " << typeid(polyderived).name() << endl;
cout << " *ppolybase is: " << typeid(*ppolybase).name() << endl;
cout << boolalpha << "same type? ";
cout << ( typeid(polyderived)==typeid(*ppolybase) ) << endl << endl;
}
输出：
int is: int
i is: int
pi is: int *
*pi is: int
derived is: struct Derived
*pbase is: struct Base
same type? false
polyderived is: struct Poly_Derived
*ppolybase is: struct Poly_Derived
same type? true
