Question

In principle, a variable defined outside any function (that is, global, namespace, and class static variables) is initialized before main() is invoked. Such nonlocal variables in a translation unit are initialized in their declaration order

Above are the lines from the class notes given by my lecturer.

#include <iostream>

using namespace std;
int a=99;
int main(int argc, char *argv[]) 
{
  cout<<a<<endl;
  cout<<b<<endl;
  return 0;
}
int b=100;

There is an error while I run this. Isn’t it true that b assigned to 100 before main() is called?

Answer 1

The problem here is not initialisation order: b is indeed initialised before main starts running.

The problem is the “visibility” of b. At the point where main is being compiled, there is no b.

You can fix it by either moving the definition/initialisation of b to before main:

#include <iostream>

using namespace std;
int a = 99;
int b = 100;
int main (int argc, char *argv[]) {
    cout << a << '\n';
    cout << b << '\n';
    return 0;
}

or simply indicate that b exists:

#include <iostream>

using namespace std;
int a = 99;
extern int b;
int main (int argc, char *argv[]) {
    cout << a << '\n';
    cout << b << '\n';
    return 0;
}
int b = 100;

Neither of those two solutions change when b is created or initialised at run-time, they simply make b available within main.