Multi Dimensional Array Implementation - c++

Question

I've been working on some code for an assignment and I'm having an issue with nested templated types.
I need the following code to create a 3 element array of 3 element arrays (sort of like int b[3][3]):
Array< Array<int> > b(3);
Here are the relevant parts of my Array.h:
template <class T>
class Array{
public:
Array() : size(0){ data = NULL; }
Array(int s) : size(s) { data = new T[size]; }
Array(const Array & a) : size(a.length()) {
data = new T[a.length()];
for(int i = 0; i < a.length(); ++i)
data[i] = a[i];
}
~Array(){ delete[] data; }
T & operator[](int i) {
if (i >= 0 && i < size){
return data[i];
} else {
throw ArrayOutOfBounds(i);
}
}
T operator[](int i) const{
if (i >= 0 && i < size){
return data[i];
} else {
throw ArrayOutOfBounds(i);
}
}
Array<T> & operator=(const Array<T> &a){
if(this == &a) return *this;
delete[] data;
data = new T[a.length()];
for(int i = 0; i < a.length(); ++i)
data[i] = a[i];
size = a.length();
}
int length() const { return size; }
// Members
private:
int size;
T * data;
}
Update 6/1 the full driver code:
// Test driver for generic Array object with assignment and bounds checking
#include "Array.h"
int main() {
Array<int> a1(10);
for (int i = 0; i < a1.length(); ++i)
a1[i] = i * i;
Array<int> a2 = a1;
try {
for (int i = 0; i <= a2.length(); ++i)
cout << a2[i] << " ";
cout << endl;
}
catch (const ArrayOutOfBounds & e) {
cout << endl << "ArrayOutOfBounds index=" << e.index << endl;
}
Array< Array<int> > b(3);
for (int i = 0; i < b.length(); ++i) {
for (int j = 0; j < b[i].length(); ++j)
b[i][j] = i*b[i].length() + j;
}
for (int i = 0; i < b.length(); ++i) {
cout << "b[" << i << "]= ";
for (int j = 0; j < b[i].length(); ++j)
cout << b[i][j] << " ";
cout << endl;
}
Array<const char *> c(3);
c[0] = "moe"; c[1] = "curly"; c[2] = "larry";
Array<const char *> d(10);
d = c;
for (int i = 0; i < d.length(); ++i)
cout << "d[" << i << "]=" << d[i] << " ";
cout << endl;
return 0;
}
Expected output:
0 1 4 9 16 25 36 49 64 81
ArrayOutOfBounds index=10
b[0]= 0 1 2
b[1]= 3 4 5
b[2]= 6 7 8
d[0]=moe d[1]=curly d[2]=larry
Update 6/2
Per Guillaume's solution, here is the resize method I used:
Array<T> & resize(int newsize){
delete[] data;
data = new T[newsize];
size = newsize;
for(int i = 0; i < size; ++i)
init(data[i], size);
}
Recursive resizing works for higher dimensions, such as Array< Array< Array<int> > > q(3);

Answer 1

Based on the code, you pasted, this should not crash. Did you forget the destructor? You should have a destructor that deletes the memory. When you have one, you need to make sure that data is initialized to nullptr (or NULL) so it does not crash when deleting an empty array.
But your approach is confusing. Is Array size supposed to be determined at runtime or at compile time? int b[3][3] is determined at compile time. If you want that, you should make the size a template argument like std::array in C++11, see http://en.cppreference.com/w/cpp/container/array
If you want to detemrine the size at runtime, you'll need a resize method to determine the size of the the 2nd dimension.
EDIT:
Based on the driver code, you need to do something different in the constructor (passing the int to T if T is an Array). To be quite honest, this seems almost like a bug. This kind of specification makes it really hard to describe what the constructor do (call the default ctor for all types T except for Array)
I'd something like this:
private:
template <typename U>
void init(U&, int) {}
template <typename U>
void init(Array<U>& a, int sz)
{
a.resize(sz);
}
public:
Array(int s) : size(s) {
data = new T[size];
for (int i = 0 ; i < size; ++i) {
init(data[i], s);
}
}
It works but this is ugly. If you can use C++11, you can something nicer with std::enable_if