C++: Unable to reassign class, online the default constructor is used - c++

Question

I am trying to use a class that I have written in another class. The problem is I am having difficulty using anything but the default parameters/constructor from the original class.
I have simplified the classes below to show what I am trying to achieve (I have tried all the variations on this I could think of but I think fundamentally I do not understand what's going on).
When I call class_two through class_one I want to use my own values for the calculation. However when I try this I just get the default parameters written in class_two.
class class_one {
private:
double a;
class_two class2;
public:
class_one(double A = 0.5, double B = 0.2) {
a = A;
class2 = class_two(b);
}
double calculation(){
return A*class2.get_b();
}
}
Where class_two would be,
class class_two {
private:
double b;
public:
class_two(double B = 0.5){
b = B;
}
double get_b(){
return b;
}
}
Example of what I am trying to run,
class_one().calculation();
actual output: A*B = 0.5 * 0.5 (Where the B values comes from class_two)
desired output: A*B = 0.5 * 0.2 (Where the B values comes from class_one)
Apologies this seems like a super simple question but I cannot get it to work!

Answer 1

When the constructor of a class is called, then - before its body starts executing - all of its data members are initialized. That means that for `class_two, the default constructor is called for its creation, since you don't say anything different.
In order to achieve your goal, use an initialization list, like this:
class_one(double A = 0.5, double B = 0.2) : a(A), class_two(B) {
// leave it empty in this case
}
PS: I initialized a via the initializer list as well, which is unrelated to your question, but a good habit to have, since initialization of data members is usually done via the initialization list, and not in the body of the constructor itself.

Answer 2

My mistake was very dum and not very interesting.
Basically I was using class2 to assign variables but I was only reassigning class2 after I had done this. I hadn't realised this.
Thanks Holt for showing me that my problem was not where I thought it was and gsamaras and François Andrieux for explaining that it's best to use an initialization list for this scenario!
class class_one {
private:
double a;
class_two class2;
public:
class_one(double A = 0.5, double B = 0.2) {
a = A*set_a();
class2 = class_two(b);
}
double set_a(){
return class2.some_function();
}
double calculation(){
return A*class2.get_b();
}
}