Question

Having a simple class:

class A {
public:
  A() {}
  void set(int value) { value_ = value; }

private:
  int value_;
};

and its global instance:

A a;

1. Is it OK to call method set on a not yet constructed object a? That can happen when for example a.set(123) is called from a constructor of another global object in another translation unit.

2. Will the value in the object a set by calling a.set(123) remain when the non-parametric and empty constructor of A is later called for object a?

Answer 1

Is it ok to call method set on a not yet constructed object a?

No. You may not call member functions for an object that has not yet begun construction.

(Since the answer is no, your second question requires no answer.)

---

If you may need to access this global instance from multiple translation units during dynamic initialization, you can use the Meyers singleton technique:

A& global_a()
{
    static A a;
    return a;
}

a will be initialized when global_a() is first called. Note that in a multithreaded program you may need to concern yourself with synchronization of the initialization.