Not able to understand declaration in c++

Question

I am not able to understand what this declaration does in C++? What does the 0 in parantheses do?

unsigned long num (0);

Answer 1

There are several types of variable initialization in C++:

1. C-like initialization type identifier = initial_value; — example: int x = 0;
2. Constructor initialization type identifier (initial_value); — example int x (0);
3. Uniform initialization type identifier {initial_value}; — example int x {0};

Your example is for the second initialization type.

Answer 2

Initializes the variable num with 0.

It's also a definition, not only declaration in this case..

Answer 3

It initializes the variable to zero.