What is dynamic initialization of object in c++?

Question

What is dynamic initialization of objects in c++?

Please explain with an simple example...

Answer 1

Dynamic initialization is that in which initialization value isn't known at compile-time. It's computed at runtime to initialize the variable.

Example,

int factorial(int n)
{
     if ( n < 0 )       return -1; //indicates input error
     else if ( n == 0 ) return 1;
     else               return n * factorial(n-1);
}

int const a = 10 ; //static initialization 
             //10 is known at compile time. Its 10!

int const b = factorial(8); //dynamic initialization 
                      //factorial(8) isn't known at compile time,
                      //rather it's computed at runtime.

That is, static-initialization usually involves constant-expression (which is known at compile-time), while dynamic-initialization involves non-constant expression.

static int c;//this is also static initialization (with zero)!

§3.6.2/1 from the C++ Standard (2003) says,

Objects with static storage duration (3.7.1) shall be zero-initialized (8.5) before any other initialization takes place. Zero-initialization and initialization with a constant expression are collectively called static initialization; all other initialization is dynamic initialization.

So there are two kind of initializations:

• Static initialization : Its either zero-initialization or initialization with a constant expression
• Any other initialization is dynamic initialization.

Also note that the same variable can be dynamically-initialized after it has been statically-initialized. For example, see this code:

int d = factorial(8);
int main()
{
}

Since d is a global variable, it has static storage. That means, according to §3.6.2.1 it's initialized to 0 at the static-initialization phase which occurs before any other initialization takes place. Then later, at runtime, it's dynamically-initialized with the value returned from the function factorial().

That means, global objects can be initialized twice: once by static initialization (which is zero-initialization) and later, at runtime, they can be dynamically-initialized.