compiler order with gcc 4.8.5 - gcc

Question

My gcc version is 4.8.5
posix gcc version 4.8.5 20150623 (Red Hat 4.8.5-4) (GCC)
I have two cpp files：
1.cpp and 2.cpp
while there is a static function in 1.cpp called by 2.cpp。
In most machine，we should like this：
g++ 2.cpp 1.cpp
or it will cause compile error or runtime error。
However，in my machine with gcc 4.8.5，I must compile using “g++ 1.cpp 2.cpp” to make it run successful。
Is this the property of gcc4.8.5？ or there is something wrong on my soft，or I used it wrong？
==============================================================
My machine is centos7 installed on virtulbox of mac. Here is my code:
1.h
#include <map>
using namespace std;
class A {
private:
A();
static A _instance;
map<int, int> test_map;
public:
static A& get_instance();
static void fun();
};
1.cpp
#include <iostream>
#include "1.h"
using namespace std;
A A::_instance;
A::A() {
cout << "A::A()\n";
}
A& A::get_instance() {
cout << "A::get_instance()\n";
return A::_instance;
// static A instance;
// return instance;
}
void A::fun() {
cout << "A::fun()\n";
get_instance().test_map[1];
}
main.cpp
#include <iostream>
#include "1.h"
using namespace std;
int test() {
cout << "test()\n";
A::fun();
return 0;
}
int y = test();
int main() {
cout << "main()\n";
A::fun();
}
In most machine and in what I see in the web, we should compile like this:
g++ main.cpp 1.cpp
But in my machine, I must compile like this:
g++ 1.cpp main.cpp
what's wrong with my machine or my gcc?

Answer 1

I think you are facing the static initialization order fiasco which is a classical C++ bug. If initialization function test() in main.cpp is called before constructor for A::_instance has been called, your code will access uninitialized A::_instance::test_map field which is likely to cause segmentation fault. I suggest you rewrite getInstance to create instance when needed:
A *A::_instance;
A& A::get_instance() {
cout << "A::get_instance()\n";
if(!_instance)
_instance = new A;
return *A::_instance;
}
As a side note, I suggest you to use AddressSanitizer to autodetect this and similar types of errors.