A easy problem about Polymorphic

[Osamede.Zhang]

Compiler store a pointer table in each object with virtual fuction to
implement polymorphic,isn 't it? But where is the pointer table?
It seems that object only store a pointer to the table,the output of
this code is 88.
#include<iostre am>
using namespace std;
class MyClass{
public:
MyClass(){ a=0;}
virtual void tst1() {a-=1;}
virtual void tst2(){a*=2;}
virtual void tst3(){a*=1;}
private:
int a;
};
class Tst:public MyClass{
void tst1() {std::cout<<"Ts t"<<endl;}
void tst2(){std::cou t<<"Tst"<<endl; }
void tst3(){std::cou t<<"Tst"<<endl; }
};
int main()
{
//Tst tstObject;
//MyClass *tstp=&tstObjec t;
//tstp->sub();
Tst tstObject;
MyClass myObject;
cout<<sizeof(my Object);
cout<<sizeof(ts tObject);
return 1;
}

[Bluse.huang]

Re: A easy problem about Polymorphic

when base class has virtual functions, a v_ptr that point to the
v_table will be add
so, the object's size is the sizeof(v_ptr) + sizeof(data member)


"Osamede.Zh ang дµÀ£º
"

Compiler store a pointer table in each object with virtual fuction to
implement polymorphic,isn 't it? But where is the pointer table?
It seems that object only store a pointer to the table,the output of
this code is 88.
#include<iostre am>
using namespace std;
class MyClass{
public:
MyClass(){ a=0;}
virtual void tst1() {a-=1;}
virtual void tst2(){a*=2;}
virtual void tst3(){a*=1;}
private:
int a;
};
class Tst:public MyClass{
void tst1() {std::cout<<"Ts t"<<endl;}
void tst2(){std::cou t<<"Tst"<<endl; }
void tst3(){std::cou t<<"Tst"<<endl; }
};
int main()
{
//Tst tstObject;
//MyClass *tstp=&tstObjec t;
//tstp->sub();
Tst tstObject;
MyClass myObject;
cout<<sizeof(my Object);
cout<<sizeof(ts tObject);
return 1;
}

[Osamede.Zhang]

Re: A easy problem about Polymorphic


"Bluse.huan g дµÀ£º
"

when base class has virtual functions, a v_ptr that point to the
v_table will be add
so, the object's size is the sizeof(v_ptr) + sizeof(data member)

But where is v_table ?heap??textseg?

[Ron Natalie]

Re: A easy problem about Polymorphic

Osamede.Zhang wrote:

Compiler store a pointer table in each object with virtual fuction to
implement polymorphic,isn 't it? But where is the pointer table?
It seems that object only store a pointer to the table,the output of
this code is 88.
>

Making a class polymorphic (at least one virtual function) is
likely to incur overhead. The nature of the overhead is
implementation specific. While vtables and vptrs are a
common way to do this, do not be fooled into believing that
this is defined by the language.

In the common implementation, each object of a polymorphic
type will have a pointer to a table. Each defined class
will create this table somewhere in memory and set it's
objects pointer to it during construction.

[Ron Natalie]

Re: A easy problem about Polymorphic

Bluse.huang wrote:

when base class has virtual functions, a v_ptr that point to the
v_table will be add
so, the object's size is the sizeof(v_ptr) + sizeof(data member)
>
>

There might also be some padding. The object size is not necessarily
the sum of it's non-static members.