How to create dynamic array from class objects?

Question

C++ / C#

You need to create a dynamic array of class objects whose only constructor takes a value. C++ without using the vector!
Class description:

class Test {
  int k;
public:
  Test(int _i) {k = _i; }
  ~Test();

  void put_k(int i) {k = i; }
  int get_k() {return k; }
};

Implementation

#define COUNT_OBJ 4
int main()
{
  // Данное объявление массива не является верным, т.к. конструктору класса Test в обязательном порядке нужно передать параметр типа int
  Test *objPtr = new Test[COUNT_OBJ];

  // Со статическим объявлением массива все проще
  Test objArray[COUNT_OBJ] = {5, 6, 7, 8};

  return 0;
}

Answer 1

And so, I found several ways:
2. Implementation of what was said by GavriKos

Test *objPtr = (Test*)calloc(COUNT_OBJ, sizeof(Test));
memset(objPtr, 0, COUNT_OBJ * sizeof(Test));
for (int i = 0; i < COUNT_OBJ; ++i) objPtr[i].put_k(i + 5);

This option should be used with caution as memory is allocated but the object's constructor and destructor are not called.
3. Option provided by Rsa97 in the comments

Test **objPtr = new Test*[COUNT_OBJ];
for (int i = 0; i < COUNT_OBJ; ++i) objPtr[i] = new Test(i + 5);

4. In my opinion, the most successful implementation using Placement new . This method was pointed out by Igor Ivanov

Test *objPtr = static_cast<Test*>(operator new[] (COUNT_OBJ * sizeof(Test)));
for (int i = 0; i < COUNT_OBJ; ++i) new (objPtr + i) Test(i + 5);

Answer 2

Use vector and don't think about anything

std::vector<Test> myTests(4, Test(777));
std::cout << myTests[0].get_k(); // 777

Answer 3

Options:
1) Use a vector
2) Just allocate a piece of memory (malloc and others), and then fill it in a loop
3) Do not use a constructor with a parameter, but make a method of type Initialize.

Answer 4

You are only allocating memory. You must initialize manually.

Answer 5

Test **objPtr = new (Test *)[COUNT_OBJ];
for (int i = 0; i < COUNT_OBJ; i++)
    objPtr[i] = new Test(i);

Answer 6

But what if we make an array not of objects of this class, but an array of pointers to objects? And then in the loop, initialize each element already with a constructor call.
In general, use a vector, what problems.

Answer 7

Is there an assignment or move operation? Let even created automatically? If there is, look here.

#include <iostream>
#include <vector>

class Test {
  int k;
public:
  Test(int _i) {k = _i; }

  void put_k(int i) {k = i; }
  int get_k() {return k; }
};


struct TestWrapper {
    Test payload;
    TestWrapper() : payload(0) {}
    TestWrapper(int i) : payload(i) {}
    TestWrapper(const Test& i) : payload(i) {}
};

int main()
{
    Test test(10);
    std::vector<TestWrapper> v;
    v.push_back(test);
    std::cout << v[0].payload.get_k() << std::endl;
    return 0;
}

If not, then a dynamic array cannot work so simply (the trick of a dynamic array is to create a new array and copy information from the old one). Only using smart pointers.

#include <iostream>
#include <vector>
#include <memory>


class Test {
  int k;
public:
  Test(int _i) {k = _i; }
  Test& operator=(const Test&) = delete;
  Test& operator=(Test&&) = delete;

  void put_k(int i) {k = i; }
  int get_k() {return k; }
};


int main()
{
    std::vector<std::unique_ptr<Test>> v;
    v.push_back(std::unique_ptr<Test>(new Test(10)));
    std::cout << v[0]->get_k() << std::endl;
    return 0;
}