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.