C++ Static and Dynamic Array Initialization

Question

Let's have a following code:

#include <iostream>

struct TStruct{
  int x;
  int y;
  int * ptr;

  TStruct( void ) : x( 0 ), y( 0 ), ptr( nullptr ) { }
};

int main(){
  TStruct arr[100];
  TStruct * arr2 = new TStruct[200];

  // some code working with these arrays

  for( int i = 0; i < 100; i ++ )
    if( arr[i] . ptr != nullptr )  
      delete [] arr[i] . ptr;
  for( int i = 0; i < 200; i ++ )
    if( arr2[i] . ptr != nullptr )  
      delete [] arr2[i] . ptr;      
  delete [] arr2;
  return 0;
}

How does initialization of both of these arrays - static and dynamic - works? Are they automatically initialized by TStruct constructor or do I have to initialize them manually by looping both of them and setting values for each TStruct member?

Answer 1

The TStruct constructor will be called 300 times with the code you're showing. A hundred times for each element in the array arr, and two hundred times when you allocate memory for arr2.

For the array arr the compiler generates code to call the constructor, and when you use new[] the operator will make sure the constructor is called.

Answer 2

The default constructor is being called for every instance of TStruct. In your code snippet, there are 300 instances and 300 default constructor calls.

There's no difference between an instance created by new, new[] or the "normal" creation on the stack, when it comes to constructor calls.

In your example you created a default constructor, which is called for each instance.

If there wouldn't exist such a self-defined default constructor, the system would create an auto-generated default constructor. The generated one would call the default constructors of each data element.

In case one defines classes with dynamic parts, like in the question (using new to reserve memory and assign the memory adress to the pointer), you need to care about the pointer value: Setting the pointers value to NULL / nullptr at the time of instantiation is very important.

The reason is, that you (or somebody using the class) could suddenly apply delete to the pointers value of his instance. The default value for a pointer is not null, but a random adress in the memory. Deleting the object at this default random adress can lead to unexpected behaviour or crash of the program.

Answer 3

How does initialization of both of these arrays - static and dynamic - works? Are they automatically initialized by TStruct constructor or do I have to initialize them manually by looping both of them and setting values for each TStruct member?

Both statements will call the TStruct default constructor for each element instantiated, you don't need to do anything manually.