C++ Struct on Class

Question

need some guide here

I got a code which is

struct Point
{
   int x;
   int y;
};

then at my class i got a function

class MyClass
{
   private:
      Point myPoint[4];
   public:
     void setPoint();
};

void MyClass::setPoint()
{
   int xData,yData;

   for (int i=0;i<4;i++)
   {
      cout << "Please enter x-ordinate:";
      cin >> xData;

      cout << "Please enter y-ordinate:";
      cin >> yData;

      //at this part the code throw a segmentation core dump.
      myPoint[i].x = xData;
      myPoint[i].y = yData;
   }

}

On first run nothing happen, but on 2nd loop, segmentation core dump occur. whats wrong with my code?

Additional code on main.cpp

#include "MyClass.h"

int main()
{
MyClass *mClass;

mclass->setPoint();
}

Thanks for helping.

What does "run" mean to you? Execute `setPoint`? Create a `MyClass`? — Luchian Grigore, Oct 26 '12 at 16:35
compile the code and execute, then i run the setPoint function at main class which then i key in the value, and on 2nd prompting of x-ordinate , got thrown a segmentation core dump error. — user1777711, Oct 26 '12 at 16:38
Don't know if this will fix your problem, but you might want to validate that you were able to convert the user input successfully. `do { cout << "Please enter x-ordinate: "; cout.flush(); } while( cin >> xData );` And please post the code where you create an instance of `MyClass` and call `setPoint` — Praetorian, Oct 26 '12 at 16:41
you should show us you 'main' function or whereever you create your instance of MyClass and call setPoint. — cellcortex, Oct 26 '12 at 16:41
I'm hesitant to look beyond the unchecked input operations. Fix the input logic first; then we can see about the rest. — Kerrek SB, Oct 26 '12 at 16:42
The code that you have shown us is not the culprit. If your program is failing, it is because of **some other part** of your code. Please reduce your original program to the shortest complete program that demonstrates the error and copy-paste that into the question. See http://sscce.org for more details. — Robᵩ, Oct 26 '12 at 16:43
@user1777711 that doesn't mean it's the culprit. That could be the location of the error caused by a coding error in a different location. I'd take Rob's advice — im so confused, Oct 26 '12 at 17:11

score 4 · Accepted Answer · answered Oct 26 '12 at 16:47

4

You must allocate memory before use pointer:

int main()
{
    MyClass *mClass = new MyClass ();
    mclass->setPoint();
    return 0;
}

answered Oct 26 '12 at 16:47

Denis Ermolin

5,530
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Also, it's a good practice to use the destructor when allocating new objects with `new`. – EyalAr Oct 26 '12 at 16:50
3

Why new? `MyClass mClass; mclass.setPoint();` – Aubin Oct 26 '12 at 17:03
I guess he want to use pointer so why not? – Denis Ermolin Oct 26 '12 at 17:31

score 0 · Answer 2 · answered Oct 26 '12 at 16:48

MyClass *mClass;

mclass->setPoint();

There's your problem right there. You have never allocated an object, just a pointer.

int main () 
{
  MyClass mclass;
  mclass.setPoint();
}

As an alternative, if you require an object whose lifetime extends beyond the scope of your variable, you could allocate the object with new.

score 0 · Answer 3 · answered Oct 26 '12 at 16:50

0

The problem lies in creating the object. More precisely in your code you have not created a object.Its merely a pointer for which there is memory allocation happened.It should be like this:

MyClass *mClass = new MyClass();
mClass->setPoint();

answered Oct 26 '12 at 16:50

facebook-100001358991487

371
4
12

score 0 · Answer 4 · answered Oct 26 '12 at 16:59

0

you need memory allocation for your object

MyClass *obj = new MyClass();
                ^

new is a language construct that dynamically allocates memory

answered Oct 26 '12 at 16:59

b3h3m0th

1,330
1
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score 0 · Answer 5 · answered Oct 26 '12 at 17:00

related to the construction of an object.

MyClass *mClass;

means you get a Myclass pointer, this pointer has ability to deal with a Myclass-type object, just has ability. but you haven't got an "real object", "real object" means you apply and get a block of memory on the system heap. then

mclass->setPoint();

this call means call the function of a Myclass-type object instead of call the function of a Myclass-class.

So, the code may be correct is:

MyClass *mClass = new MyClass ();
mclass->setPoint();

the new keyword means apply a block of memory for a Myclass-type object.

C++ Struct on Class

5 Answers5

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