typeid operator

kfaday

Hello I've got a vehicle class (cVehicle), and two 'heirs', car (cCar)and van (cVan). I store cars and vans in a list. The cvehicle class has a vehicle_type atribute (a char that is 'c' when it's a car and 'v' if it's a van). I want to take it out. in the following code, i get a vehicle from a list. I don;t know if it's a car or it's a van. I use the vehicle_type atribute to see if it's a car, and if it is, i must return it. cvehicle *vtemp=(list->Getvehicle()); if (vtemp->get_vehicle_type()=='c') return vtemp; i've heard of a more elegant way to solve it, that is with the typeid operator. Do you know how to use it? thanks!

toxcct

do you know MSDN ? i cite :

typeid Operator
C++ Specific —>

typeid( type-id ) typeid( expression )

The typeid operator allows the type of an object to be determined at run-time.

The result of a typeid expression is a const type_info&. The value is a reference to a type_info object that represents either the type-id or
the type of the expression, depending on which form of typeid is used.
See type_info Class for more information.

END C++ Specific

here you are

TOXCCT >>> GEII power

kfaday

sorry, i'm new to c++ and i don't have msdn cds. would you mind giving me an example of typeid usage, if it's not too much trouble?

jmkhael

string clss; if (typeid(obj)==typeid(int)) clss = "I"; else if (typeid(obj)==typeid(unsigned int)) clss = "i"; else if (typeid(obj)==typeid(float)) clss = "F"; else if (typeid(obj)==typeid(double)) clss = "D"; else if (typeid(obj)==typeid(short)) clss = "B"; else if (typeid(obj)==typeid(unsigned short)) clss = "b"; else if (typeid(obj)==typeid(long)) clss = "I"; else if (typeid(obj)==typeid(unsigned long)) clss = "i"; got the idea? :) Papa while (TRUE) Papa.WillLove ( Bebe ) ;

kfaday

i tried this but it doesn't work cvehicle *vtemp=(list->Getvehicle()); if (typeid(vtemp)==typeid(ccar))) return vtemp;

jmkhael

just tried this and it works CString temp; CString temp1; const type_info& t = typeid(temp); AfxMessageBox (t.name ()); // It gave class CString if(typeid(temp1) == typeid(CString)) AfxMessageBox ("Matched"); // It said matched Check this too: class Base { ... }; class Derived : public Base { ... }; void f() { Derived* pd = new Derived; Base* pb = pd; ... const type_info& t = typeid(pb); // t holds pointer type_info const type_info& t1 = typeid(*pb); // t1 holds Derived info ... } (This is your prob, you should dereference the pointer) What does yours do? Papa while (TRUE) Papa.WillLove ( Bebe ) ;

kfaday

t and t1 are always diferent! from the debugger: t: + _m_d_name 0x00433890 ".PAVcvehicle@@" t1: + _m_d_name 0x00433850 ".?AVccar@@"

jmkhael

yes so check it against t1 dereference first the pointer you receive and then compare it to typeid(ccar) Papa while (TRUE) Papa.WillLove ( Bebe ) ;

kfaday

Papa wrote: dereference first the pointer you receive and then compare it to typeid(ccar) what do you mean by this? sorry, english isn't my first language

jmkhael

cvehicle *vtemp=(list->Getvehicle()); if (typeid(*vtemp)==typeid(ccar))) return vtemp; Papa while (TRUE) Papa.WillLove ( Bebe ) ;

kfaday

it gives me an error: (runtime error, it builds alright) unhandled exception

jmkhael

"If the expression is dereferencing a pointer, and that pointer’s value is zero, typeid throws a bad_typeid exception. If the pointer does not point to a valid object, a __non_rtti_object exception is thrown." Do you have a NULL pointer? Papa while (TRUE) Papa.WillLove ( Bebe ) ;

kfaday

no, *vtemp isn't null it gives me this warning warning C4541: 'typeid' used on polymorphic type 'class ccar' with /GR-; unpredictable behavior may result

toxcct

you deference a pointer when you don't get the address it contains but when you get what is at the pointed address.

char* pc = (char*)malloc(sizeof(char)); // my pointer (something like 0x0A78DQ43)
*pc = 'a'; // dereferenced pointer (acsii code for 'a' caracter)

pc : simple pointer (cointaining an address)
*pc : dereferenced pointer (value at the pointed address)

TOXCCT >>> GEII power

antlers

typeid depends on RTTI being turned on. RTTI is part of the language standard but C++ turns it off by default. Change your project settings to enable RTTI and you will get rid of the compile-time warning and the run-time exception.

kfaday

that did it!!!!!!!!!!!!!1 thanks!!