member variable in C++

The initialisation order of member variables is determined by their order of declaration, not their order in the initialisation list.
```
#include <iostream>
struct A {
  A() { std::cout << "A"; }
};
struct B {
  B() { std::cout << "B"; }
};

class C {
public:
  C() : a(), b() {}
private:
  B b;
  A a;
};

int main() {
    C();
}
```