How to Initialize Object Arrays Without a Default Constructor?

Initializing Object Arrays Without Default Constructor

Object arrays are commonly used in programming, but issues may arise when initializing them with classes lacking default constructors. Consider the following code:

class Car {
private:
  Car(){};  // Private default constructor
  int _no;
public:
  Car(int no) {
    _no = no;
  }
};

Within this class, the default constructor (Car()) is private, which means objects of type Car cannot be created without providing arguments. However, when initializing an array of Car objects, the compiler attempts to call the default constructor for each element, leading to the error:

cartest.cpp:5: error: ‘Car::Car()’ is private
cartest.cpp:21: error: within this context

Solution Using Placement-New

To overcome this issue, placement-new can be employed. Placement-new allows us to allocate memory and construct objects directly in that memory location without invoking the object's constructor. Here's how it can be implemented:

int main() {
  void *raw_memory = operator new[](NUM_CARS * sizeof(Car));  // Allocate raw memory
  Car *ptr = static_cast<Car *>(raw_memory);  // Cast to a pointer to Car
  for (int i = 0; i < NUM_CARS; ++i) {
    new(&amp;ptr[i]) Car(i);  // Call placement-new to construct Car objects in-place
  }
}

By using placement-new, we can initialize the object array without explicitly calling the default constructor. The objects are created and placed directly into the allocated memory.

Advantages of Avoiding Default Constructors

As mentioned in Item 4 of Scott Meyers' "More Effective C ", avoiding gratuitous default constructors can enhance program correctness and maintainability. Default constructors often lead to ambiguity in class design and can make it challenging to enforce class invariants. By explicitly defining constructors, the programmer asserts that objects of that class can only be created with specific sets of arguments, ensuring greater control and clarity.

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