conversion

[raccio salviati]

hi all

how can we write this in c++?

#include <stdio.h>
#include <stdlib.h>
typedef struct {
int x, y;
} * Point;

void init (Point p) {p->x = 0; p->y = 0;}
void set (Point p, int a, int b) {p->x = a; p->y = b;}
void showw (Point p) {printf("%d %d\n", p->x, p->y);}
main(){
Point p = (Point) malloc(8),
q = (Point) malloc(8);
init(p);
set(q, 1, 2);
showw(p);
printf("%d %d\n", q->x, q->y);
}


thanks in advance

[andywestken]

#include <iostream>
#include <cstdlib>
using namespace std;

// no longer a pointer typedef
class Point {
private:
    int m_x;
    int m_y;

public:
    Point() : m_x(0), m_y(0) {}
    Point(int x, int y) : m_x(x), m_y(y) {}

    void clear() // was init
    {
        m_x = 0; m_y = 0;
    }

    void set(int x, int y)
    {
        m_x = x; m_y = y;
    }

    int getX() const
    {
        return m_x;
    }

    int getY() const
    {
        return m_y;
    }

    void show() const
    {
        cout <<  m_x <<  " " << m_y << endl;
    }
};

ostream& operator<<(ostream& os, const Point& p)
{
    os << p.getX() << " " << p.getY();
    return os;
}

int main(){
    Point* p = new Point;
    Point* q = new Point(1, 2);
    Point* r = new Point;
    r->set(3, 4); 

    p->show();
    cout << q->getX() << " " << q->getY() << endl;
    cout << *r << endl;

    delete p; // C version should have been calling free() !
    delete q;
    delete r;

    return 0;
}

[raccio salviati]

thanks for help

but what about malloc(8)??

[andywestken]

No need. operator new knows how big class Point is.

Point* p = new Point(1, 2);

does the same work as

Point* p = (Point*) malloc(sizeof(Point)),
set(p, 1, 2);

Inidentally, the C could be better as

#include <stdio.h>
#include <stdlib.h>

typedef struct {
int x, y;
} Point, * PointPtr; // or you could just use Point*

void init (PointPtr p) {p->x = 0; p->y = 0;}
void set (PointPtr p, int a, int b) {p->x = a; p->y = b;}
void showw (PointPtr p) {printf("%d %d\n", p->x, p->y);}

main(){
    PointPtr p = (PointPtr) malloc(sizeof(Point)),
    q = (PointPtr) malloc(sizeof(Point));
    init(p);
    set(q, 1, 2);
    showw(p);
    printf("%d %d\n", q->x, q->y);
    free(p);
    free(q);
}

That is, typedef the struct to Point and the pointer to PointPtr, so you can use sizeof(Point), rather than 8. Then if you add a new member to Point (e.g. color) you don't have to touch the code to change the size passed to malloc.

[raccio salviati]

thanks once again
But

we can write:

Point* p = (Point*) malloc(8*sizeof(Point),set(p,1,2));
??

by the way what is the difference between

Point* p= (Point) malloc(8)

and

Point* p= (Point) malloc(8*sizeof(Point))

isn'it the same??

[andywestken]

1. no - malloc() takes only one param: the number of bytes you want to allocate

2. Point* p = (Point)malloc(8); is a typo. The cast must be the same type

Point p = (Point)malloc(8);

or

Point* p = (Point*)malloc(8);

3. Also a typo, due to the cast

But

Point* p= (Point*) malloc(8*sizeof(Point));

allocates enough space for 8 points. That is, it allocates an array of 8 points.

[raccio salviati]

thank you once again


the 8 was the size of the two integers x and y (sizeof(int)=4)

[andywestken]

I could see what 8 meant in your code. But I always use sizeof() to achieve the same aim myself.

[Veltas]

I know this isn't what they were asking for but I'm wondering; could they just swap out <stdio.h> & <stdlib.h> for <cstdio> & <cstdlib>?
[edit]: And 'using namespace std;' too.

[andywestken]

If you're writing pure C, and have a .c extension, then you cannot use namespaces. Hence <cstdio>, etc are unusable (they just wrap <stdio.h>, etc in the std namespace anyway)

[Veltas]

I mean to compile it as C++.

[andywestken]

If you're compiling as C++, then it makes no odd if you use <stdio.h>, etc or <cstdio>, etc plus using namespace std.

I would prob. do the former if I'm just using C functions. I switch to <cstdio>, etc when I'm including things like <string>, <vector>, etc; to be consistent.