C++ create primitive array class - c++

Question

As an exercise, I'm trying to create a class myArray that acts as a simplified array class. Here is my header:
#ifndef myArray_h
#define myArray_h
typedef double ARRAY_ELEMENT_TYPE;
class myArray {
public:
//--constructors
myArray(int initMax);
// post: Allocate memory during pass by value
myArray(const myArray & source);
// post: Dynamically allocate memory during pass by value
//--destructor
~myArray();
// post: Memory allocated for my_data is deallocated.
//--modifier
void set(int subscript, ARRAY_ELEMENT_TYPE value);
// post: x[subscript] = value when subscript is in range.
// If not, an error message is displayed.
//--accessor
ARRAY_ELEMENT_TYPE sub(int subscript) const;
// post: x[subscript] is returned when subscript is in range.
// If not, display an error message and return [0].
private:
ARRAY_ELEMENT_TYPE* my_data;
int my_capacity;
};
#endif
Here is my implementation:
#include "myArray.h"
#include <iostream>
#include <cstring>
using namespace std;
typedef double ARRAY_ELEMENT_TYPE;
//--constructors
myArray::myArray(int initMax)
{
my_capacity = initMax;
}
myArray::myArray(const myArray & source)
{
int i;
my_data = new ARRAY_ELEMENT_TYPE[source.my_capacity];
for(i=0; i < my_capacity; i++)
my_data[i] = source.sub(i);
}
//--destructor
myArray::~myArray()
{
delete [ ] my_data;
}
//--modifier
void myArray::set(int subscript, ARRAY_ELEMENT_TYPE value)
{
if(subscript > my_capacity - 1)
{
cout << "**Error: subscript " << subscript << " not in range 0.." << my_capacity-1 << ". The array is unchanged." << endl;
}
else
my_data[subscript] = value;
}
//--accessor
ARRAY_ELEMENT_TYPE myArray::sub(int subscript) const
{
if(subscript >= my_capacity)
{
cout << "**Error: subscript " << subscript << " not in range 0.." << my_capacity-1 << ". Returning first element." << endl;
cout << my_data[0];
}
else
{
return my_data[subscript];
}
}
And I'm using this as a test driver:
#include <iostream>
using namespace std;
typedef double ARRAY_ELEMENT_TYPE;
#include "myArray.h"
void show (const myArray & arrayCopy, int n)
{
for(int j = 0; j < n; j++)
cout << arrayCopy.sub(j) << endl;
}
int main()
{
int n = 6;
myArray a(6);
a.set(0, 1.1);
a.set(1, 2.2);
a.set(2, 3.3);
a.set(3, 4.4);
a.set(4, 5.5);
a.set(5, 6.6);
show(a, n);
cout << a.sub(11) << endl;
a.set(-1, -1.1);
return 0;
}
The problem is that when I run this, I get nothing for a bit, then the "Press any key to continue..." prompt. What's going wrong?

Answer 1

The myArray constructor doesn't allocate memory for my_data. The first time you call set, it attempts to write to an uninitialised pointer. This results in undefined behaviour but a crash is likely.
You should change the constructor to
myArray::myArray(int initMax)
{
my_capacity = initMax;
my_data = new ARRAY_ELEMENT_TYPE[my_capacity];
}
There are a couple of other issues with the code you could also consider
In 'set', the test
if(subscript > my_capacity - 1)
should be
if(subscript < 0 || subscript > my_capacity - 1)
Or you could change the subscript argument to have type unsigned int.
In sub, the line cout << my_data[0]; should presumably be return my_data[0];

Answer 2

myArray::myArray(int initMax)
{
my_capacity = initMax;
my_data = new ARRAY_ELEMENT_TYPE[my_capacity]; //You missed this
}

Answer 3

In addition to missing your allocation in your current implementation, you are also dynamically allocating memory. A simple array type does not need to be allocated on the heap. The std::array collection does exactly what you are looking to do. I would urge you to look at its implementation for an example (if this is just an academic exercise). If this is for a production codebase, use what is already written and tested.
http://en.cppreference.com/w/cpp/container/array