whats wrong with my code for arrays [closed] - c++

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i am trying to generate an array which may be independant of dimensions. i tried doing this
#include <iostream>
using namespace std;
class array3d
{
public:
array3d(size_t* d, int dims)
{
int all = 1;
size_t* dimensions;
int* array;
for (size_t i = 0; i < dims; i++) {
all = d[i];
dimensions = new size_t[dims];
array = new int[all];
std::cout << array[i] << std::endl;
}
}
};
int main()
{
size_t d[6];
d[0] = 2;
d[1] = 3;
d[2] = 4;
d[3] = 2;
d[4] = 3;
d[5] = 4;
array3d arr(d, 6);
return 0;
}
when i compile it i end up with an array of zeros alone, i am not able to find where i going wrong. can anyone help?

Answer 1

I cannot really understand the logic behind you code, but if you simply want to see something printed you probably meant to do this:
array = new int [all];
//write something here first!
array[i] = some_value; <--- Note that this may access past the end of the array
std::cout << array[i] << std::endl;
Or more likely you wanted this:
std::cout << d[i] << std::endl;
Note1: You should really listen to 0d0a and use std::vector unless you really need arrays
Note2: You're doing some signed to unsigned comparisons (i < dims) - you might want to take care of those too

Answer 2

array = new int[all]; does not initialize the allocated memory. It will just make the allocation from heap, but leave the contents of memory as it was (or, actually, accessing the allocated but uninitialized memory may be Undefined Behaviour, strictly speaking).
You see zeros only by chance, because the memory happens to contain zeros. It is zeros probably because it has not been allocated, used and released by your program yet, and OS filled it with zeros before giving it to your program.
Additionally, std::cout << array[i] << std::endl; will be buffer overflow, if i>=all. Why do you do that anyway?
And finally, your code leaks memory like crazy. Your loop loops dims times, and with each iteration you do two allocations with new, but then you lose the returned pointers.
In short, use std::vector in C++, (almost) never use plain C arrays. If you have a clear use case for a fixed length array, use std::array. Furthermore, most of the time, if you are using naked pointers in C++ application code, you are not doing it right. Use smart pointers. This applies especially, if you are learning C++ today. Learn the modern way of doing things, it will make your life so much easier, while improving quality of your code.