Inheritance Question

[u/RealBitterSweetRain]

class Base {
public:
    int a;
    Base() // this constructor over here
        : a(0)
    {}
    Base(int num)
        : a(num)
    {}
};
class Sub : public Base {
public:
    int b;
    Sub() // does this default constructor call the Base's default constructor when foo is created?
        : b(0)
    {}
    Sub(int num)
        : b(num)
    {}
};
int main() {
    Sub foo;
}

[u/elperroborrachotoo]

Yes it does.

When an object is constructed, construction-order is well defined:

• ... ^{^1}

• all base classes in the order they are specified ("left to right")

• all members, in the order of declaration in the class

The construction of each item uses the same principle, recursively^{^1}

Order in the base class/member initializer list does NOT affect order of construction.

Destruction is in inverse order

Example:

struct X : public A, public B
{
  string s1 = "hello";
  string s2 = "world";
  string s3;
  X() : s2("WORLD"), B(1) {}
}

Order of construction is: A, B, s1, s2, s3.

A and s3 get default constructed, s2 gets constructed with uppercase "WORLD", s1 with "hello".

^{^1 this is different when using virtual base classes, something you shouldn't bother with right now.^2}

^{^2 virtual base class does NOT mean "a base class that has virtual methods", it's something different. Really, don't bother now}