Programming

class B
{
public:
void fns(void){//base def;}
};
class D:public B
{
public:
void fns(void) {//new def;}
};

Overriding has different scopes, same name, same signatures, and virtual is required.

#include <stdio.h>

class B
{
public:
        void fns(void){ fprintf(stderr, "B::fns\n"); }
};

class D:public B
{
public:
        void fns(void) {fprintf(stderr, "D::fns\n"); }
};

void call_fns(B &obj)   {       obj.fns();      }

int main(void)
{
        D d;
        call_fns(d);
}

#include <stdio.h>

class B
{
public:
        virtual void fns(void){ fprintf(stderr, "B::fns\n"); }
};

class D:public B
{
public:
        virtual void fns(void) {fprintf(stderr, "D::fns\n"); }
};

void call_fns(B &obj)   {       obj.fns();      }

int main(void)
{
        D d;
        call_fns(d);
}