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Only pointers can be null. As static object B::a was constructed by default CTOR even prior main().

You can see this line:
A's constructor called.

since "A a " is a static object, so first all static objects are constructed.

But in the line "A a1 = b1.getA(); " the copy of static object returned is copied in "a1" object , and first a1 object should be constructed using a default constructor of class A, correct me if i am wrong

Theoretically, temporary object will be created and returned from b1.getA(), and then in "A a1 = b1.getA();" copy CTOR will be called again to construct a1.

In practice creation of temporary object will be optimized by compiler.

What you describe is operation of assignment operator (CTOR can't be called twice):
A a1;
a1 = b1.getA();