template<class ForwIter,class Predicate>
ForwIter std::remove_if(ForwIter beg,ForeIter end,Predicate op)
{
       //A copy of "op" is created inside find_if effectively
       // as a local variable.  This is how pass-by-value works, rather
       // than pass-by-reference.  changes made in find_if aren't reflected
       // in our original "op".
       ForwIter pos = find_if(beg,end,op);
       if(pos == end)
            return beg;
       else{
            ForwIter next = beg;
            return remove_copy_if(++next,end,beg,op);
       }
}

template<class ForwIter,class Predicate>
ForwIter std::remove_if(ForwIter beg,ForeIter end,Predicate op)
{
       // Here we use 'op' directly, not a copy of it.
       while(beg != end && !op(*beg))
            beg++;
       if(pos == end)
            return beg;
       else{
            ForwIter next = beg;
            //Now that we've modified it in the manner we wanted,
            // we pass the modified version into remove_copy_if.
            return remove_copy_if(++next,end,beg,op);
       }
}

#include <stdio.h>

// val is effectively a local variable
void pass_by_value(int val)
{
        val++;
}

// val is effectively the same variable you passed it
// This style of reference isn't found in C, just C++
void pass_by_reference(int &val)
{
        val++;
}

// Another way of pass-by-reference.
void pass_by_pointer(int *val)
{
        (*val)++;
}

int main(void)
{
        int q=5;
        printf("q=%d\n", q);
        pass_by_value(q);
        printf("q=%d\n", q);
        pass_by_reference(q);
        printf("q=%d\n", q);
        pass_by_pointer(&q);
        printf("q=%d\n", q);
        return(0);
}

q=5
q=5
q=6
q=7