RTTI only works when there is at least 1 virtual function / is it really?
Hi guys,
I read everywhere that RTTI can only work if you have at least 1 virtual function.
I use this code:
#include <iostream.h>
#include <typeinfo.h>
class X
{
int i;
public:
void f() {i++;}
};
void main()
{
X x1,x2;
if(typeid(x1)==typeid(x2))
cout<<"same!"<<endl;
}
As you can see I'm not using any virtual functions, and still everything works fine! how can it be?
Many thanks.....
# 1 Re: RTTI only works when there is at least 1 virtual function / is it really?
Well, RTTI/typeof need the vtable to determine the type of object. Here is an example to play with:
#include <iostream>
class SomeType
{
public:
// try commenting the next line
virtual void test() { }
};
class SomeOtherType : public SomeType { };
int main(int argc, char* argv[])
{
SomeType *a = new SomeOtherType();
SomeOtherType *b = new SomeOtherType();
if (typeid(*a) == typeid(*b))
std::cout << "same type!" << std::endl;
return 0;
}
- petter
# 2 Re: RTTI only works when there is at least 1 virtual function / is it really?
Hi Peter,
Thank you for your comment.
It only proove my point.
If I use:
#include <iostream.h>
#include <typeinfo.h>
class A
{
public:
// try commenting the next line
virtual void test() { }
};
class B : public A {};
int main(int argc, char* argv[])
{
A *a = new B;
B *b = new B;
if (typeid(*a) == typeid(*b))
cout << "same type!" <<endl;
return 0;
}
I will get a compile warning and my program will crash:
"warning C4541: 'typeid' used on polymorphic type 'class A' with /GR-; unpredictable behavior may result"
The only way I can run it is by removing the code line
virtual void test() { }
Why?
# 3 Re: RTTI only works when there is at least 1 virtual function / is it really?
Well, of course you need to compile your project with RTTI enabled. I assumed you did. Using typeof with RTTI disabled has somewhat limited functionality, but you could typically use it to compare types in a template function/class (where teh type is known at compile-time).
The only way I can run it is by removing the code lineAnd does the type comparison behave as expected?
- petter
# 5 Re: RTTI only works when there is at least 1 virtual function / is it really?
Well, you would have if you looked up the warning in the documentation ;)
Open the the projects property page (right-click the project in the solution explorer). Then navigate down to Configuration Properties / C/C++ / Language / Enable Run-time Type Info.
- petter
