How object can be created outside main function?

Question

I don't understand the output of this code:

• How the object a1 of class A is created and constructor is called?
• Why the object a1 is created outside the main function?

#include<iostream>
using namespace std;     
class A
{
    public:
    A(int i)
    {
        std::cout<<"I am in A Class "<< i <<endl;
    }
};
 A a1(8);

int main()
{
    A a(9);
    return 0;
}

Output:

I am in A class 8
I am in A class 9

Answer 1

whats the reason that object can create first outside main function

In your example a1 has global namespace scope and has static storage duration .
It is constructed at program startup and therefore you see

I am in A class 8

printed out before

I am in A class 9

Answer 2

a1 is a global variable. Globals are constructed before main is invoked.

Answer 3

It's technically implementation dependent.

Except that a1 has to be constructed before it gets used.

In your example, main() isn't using a1. However, a simple way for a C++ implementation to make sure that a1 is constructed before it gets used is to just have a1 (and any non-local non-inline variables with static storage) be constructed/initialized before main().

Reference: "It is implementation-defined whether the dynamic initialization of a non-local non-inline variable with static storage duration is sequenced before the first statement of main or is deferred. If it is deferred, it strongly happens before any non-initialization odr-use of any non-inline function or non-inline variable defined in the same translation unit as the variable to be initialized"