How to dynamically append new object at run time in C++ array of object and print them

Question

I have a class point.

I have an event call onPoint change. This may occur any number of times. In each event a new point is given by the event. So I don't know in advance how may points to be allocated in advance. It may but 1 2 or hundreds.

So I thought of using dynamic memory allocation. So I created a pointer p of type Point.

I allocated a reference memory by point* p = (point*)malloc(sizeof(p));

Inside the main method the first point is inserted (0, 0). All other points are inserted in the onPointChange Event.

Now I want, the point that are detected in onPointChange event to be appended in my pointer (p) also all of its previous values are preserved. And finally print them.

#include <iostream>
#include <string>
#include <array>

using namespace std;

class point {
    public:
    int x;
    int y;
};

point* p = (point*)malloc(sizeof(p));
int main()
{
   p.x =0;
   p.y =0;

  // Now all other points to be filled in onPointChange event
}

void onPointChange(){
 point newpoint;
 newpoint.x = newXValue;
 newpoint.y = newYValue;
 // Here I need to reallocate my pointer p such that the new point (newpoint) is appended to the p and also all of its earlier elements are preserved.
// And after meeting any condition, print the value of x and y of all points in p
}

Don't use `malloc()` or raw pointers in c++. Check `std::vector` instead please. — πάντα ῥεῖ, Sep 20 '18 at 03:17
Reason for not using `malloc`: `malloc` cannot handle complex objects. It will provide storage, but will not call the constructors of any objects that need constructing. This isn't a problem with `point`, it's too simple to care, but there is no point to developing bad habits early in your career. — user4581301, Sep 20 '18 at 03:35

score 2 · Answer 1 · edited Sep 20 '18 at 03:20

2

You cannot "append" to a pointer. You can use a std::vector to store the points as needed, and then print the points in the vector when your condition is met.

edited Sep 20 '18 at 03:20

πάντα ῥεῖ

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answered Sep 20 '18 at 03:19

user18359

76
2

can you provide me with example that accommodates my need ? – ShresthaGanesh Sep 20 '18 at 03:26
1

@ShresthaGanesh See the example at the bottom of https://en.cppreference.com/w/cpp/container/vector . In your case you will want to use `point`s in place of `int`s. – user4581301 Sep 20 '18 at 03:28
@ShresthaGanesh Sure: https://en.cppreference.com/w/cpp/container/vector – πάντα ῥεῖ Sep 20 '18 at 03:31

How to dynamically append new object at run time in C++ array of object and print them

1 Answers1