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最新评论
typeid is the one of the meager offering of runtime type information hierarchy functionality. what it offers is the ability to determine at runtime the real type of an object (a pointer is a special type of variable, which we will lsee shorty in this post )
what is the typeid operator used for?
it is used in advanced programming developement, when building debugger for example, or it is used to deal with persistent object through a pointer or a reference to a base class.
The program needs to find out the actual type of the object manipulated to list the properties of an object properly during a debugging session or to store and retrieve an object property
During a debugging session or to store and retrieve an object property properly to find out the actual type of an object, the typeid operator can be used.
For better describe what we will describe, here is the class hiearchy that we will use in this example.
class employee { }; class manager : public employee {}; class programmer : public employee { };
so, in the simplest case, what is the output of the followiong.
void typeid_test() { programmer obj ; programmer &re = obj; // we look at name() in the subsection on type_info // below, name() returns the C-style string: "programmer" cout << typeid(re).name() << endl; }
the typeid can accept two types of operand, one is an expression and the other is the name of the type. Let's first see the first type of operand (the later case will be covered in example later) .
/** * the operand of the typeid operator can be expression or type names of any type * */ void typeid_operand() { int iobj; cout << typeid(iobj).name() << endl; // print : ints cout <<typeid(8.16).name() << endl; // prints : double }
when the oeprand to typeid operator is a class, but not a class of virtual function, the result of the typeid operator is well, not the type of underlying object.
/** * if the typeid operand is a class type, but not of a class type with virtual functions. then the typeid operator indicates the type of * the operand is well, not the type of the underlying object */ class Base { }; class Derived : public Base { }; void typeid_non_virtual_class() { Derived dobj; Base *pb = &dobj; cout << typeid(*pb).name() << endl; // prints : Base }
but, if the class does have virtual function, then the real derived object will be retrieved and its true type will be returned.
/* * if the typeid oeprand is a class type , where if class to be examined is of virutal functions, then the * real derived classes will be returned. */ void typeid_result_virtual_class() { employee *pe = new manager(); employee &re = *pe; if (typeid(*pe) == typeid(manager)) cout << "typeid(*pe) == typeid(manager)" << endl; // true if (typeid(re) == typeid(employee)) cout << "typeid(re) == typeid(employee)" << endl; // true if (typeid(&re) == typeid(employee *)) cout << "typeid(&re) == typeid(employee *)" << endl; // true if (typeid(&re) == typeid(manager *)) cout << "typeid(&re) == typeid(manager *)" << endl; // false }
you may have seen already that there are some difference between the object and poiner, the pointer type itself is fixed, so to get the dynamic behavior of typeid, you'd better pass in the real object or reference (which will be treated as the data object)
type_info class
You may find the type_info class declaration as follow. actually the real implementation can vary from implemenation to implemenation.
class type_info { private: type_info(const type_info&); type_info& operator = (const type_info &); public: virtual ~type_info(); int operator ==(const type_info&) const; int operator!=(const type_info &) const; const char *name() const; };
the only way to construct the type_info object is to use the typeid operator.
what is containd in the type_info class is implementation dependant. the name() const char * is the only member guaranteed to be provided by all C++ implementation , though it is possible that implementaion dependant may provide additional support. what can be added, basically any information that a compiler can provide about a type about a type can be added.
- a list of the class member functions.
- what the layout of an object of this clases type looks like in storage, that is , how the member and base subobjects are mapped
class extended_type_info :public type_info {}; typedef extended_type_info eti; void func(employee * p) { // downcast from type_info * to extended_type_info* if (const eti * eti_p = dynamic_cast<const eti *>(&typeid(*p))) { // if dynamic_cast succeeds // use extended_type_info information through eti_p } else{ // if dynamically fails // use standard type_info information } }
发表评论
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不安装Visual Studio,只用Windows SDK搭建VC环境
2013-12-31 21:52 15342首先你需要下载的是 Microsoft Windows S ... -
rpath - runtime search path
2013-04-03 11:36 1013RPath is a very interesting to ... -
C++ - autogenerated copy constructor and assignment operator gotchas
2013-01-24 13:32 772It has been changed that the s ... -
c++ - rethrow a exception gotchas
2012-12-23 10:57 964As in my prevoius example in j ... -
c++ - dynamic_cast revisit
2012-10-14 21:21 775There are several built-in type ... -
c++ - virtual inheritance example 1
2012-10-14 15:25 826we have discussed the virtual i ... -
c++ - virtual inheritance
2012-10-12 08:58 980As we have discussed in the pos ... -
c++ type of inheritance
2012-09-28 08:58 754There are 3 types of inheritanc ... -
c++ - vritually virtual new
2012-09-27 23:59 961Let's see what if we want to cl ... -
c++ - virtual destructor
2012-09-27 22:01 976As we all know that virtual des ... -
c++ - vritual function and default arguments
2012-09-27 08:56 996As we all know that we virtual ... -
c++ - template specialization and partial specialization
2012-09-26 22:38 1331in this post, we are going to e ... -
c++ - member template in class template
2012-09-26 08:19 940class member template can be us ... -
c++ template class and the pattern to use its friends
2012-09-25 23:47 987template class may defined thei ... -
c++ - Friend declaration in class Template
2012-09-25 08:47 1213There are three kinds of friend ... -
c++ - class template default parameters
2012-09-25 08:18 856the template has parameter, it ... -
c++ - operator new and delete and an example of linked list stores by new/delete
2012-09-24 07:53 589The operator new and delete ope ... -
c++ - delete(void *, size_t) or delete(void *)
2012-09-24 07:18 1172In my previous dicuss, we have ... -
c++ - placement operator new() and the operator delete()
2012-09-23 15:22 875A class member operator new() c ... -
c++ - overloaded subscript operator - []
2012-09-23 08:50 1189You can overload the subscript ...
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