Default argument ,static variable using Friend function

#include <iostream.h>
#include<conio.h>
static a=30,b=20;
void setvalue(int a=10,int b=10);
void setvalue(int a,int b)
{
  cout<<"A="<<a<<endl<<"B="<<b<<endl;
}
class sample
{
int a,b;
public:
void getvalue(){a=25;b=40;}
friend void display(sample s);
};
void display(sample s)
 {
   s.getvalue();
   cout<<"A Value is "<<s.a<<endl;
   cout<<"B Value is "<<s.b<<endl;
   cout<<"Addition value is "<<s.a+s.b<<endl;
 }
 void  main()
 {
 clrscr();
 sample X;
 cout<<"Default Argument:"<<endl;
 setvalue();
 setvalue(a);
 cout<<"Using Friend Function:"<<endl;
 setvalue(a,b);
 display(X);
 getch();
 }