what changes to class lead to recompile?

[nuzzle]

So, seems like addiing constructor would require recompile of lib. Same for (d), adding default value to a function.

correct?

You're probably right about a. I didn't consider the case when the added constructor overloaded an existing one.

In d on the other hand, if you have

class A {
void a(double d) {...}
}

and change to,

class A {
void a(double d=0.0) {...}
}

that would be syntactically equivalent to this,

class A {
void a(double d) {...}
void a() {a(0.0);}
}

Although adding default to a parameter corresponds to adding an overloaded method, the existing code cannot have any calls to the added overload, in this case a(). So it shouldn't require a recompilation.