C ++ vtable Problems

(1) Impact of vtable

This increases the size of an object instance by 4 bytes.

# Include <cstdlib>
# Include <iostream>

Using namespace STD;

Class
{
Int;
};

Class B
{
Virtual void say ()
{
}

Int B;
};

Int main (INT argc, char * argv [])
{
A;
B;

Cout <"A:" <sizeof (a) <Endl;
Cout <"B:" <sizeof (B) <Endl;

System ("pause ");
Return exit_success;
}

 

Memset cannot be used for instances of objects containing virtual tables.
The generation time and location of the virtual table are affected by placement new.

# Include <cstdlib>
# Include <iostream>

Using namespace STD;

Class
{
Public:
A ()
{
Show ();
}

Virtual void show ()
{
Cout <"A" <Endl;
}
};

Class B: public
{
Public:
B ()
{
Show ();
}

Void show ()
{
Cout <"B" <Endl;
}
};

Int main (INT argc, char * argv [])
{
B;

System ("pause ");
Return exit_success;
}

(2) simulate vtable in C Language

 

# Include <cstdlib>
# Include <iostream>

Using namespace STD;

Void func1 (int)
{
Cout <"func1" <Endl;
}

Void func2 (int B)
{
Cout <"func2" <Endl;
}

Struct vtable
{
Void (* func) (int v );
};

Struct
{
Struct vtable;
Void show (int v)
{
Table. func (v );
}
};

Int main (INT argc, char * argv [])
{
A A, B;
A. Table. func = func1;
B. Table. func = func2;
A. Show (1 );
B. Show (1 );

System ("pause ");
Return exit_success;
}