C++ Make array/linked list with different class objects - c++

Question

I am wildly searching the net and this site in order to understand how to do what I want to do. And while there is tons of search results I am not sure if I really understand how to do this (I am fairly new to C++). So my hope is that I get some help here.
What I want to do:
I want to create a contour of geometric segments, e.g. a line segment, followed by a circular arc segment, followed by three lines, followed by whatever may come. The order and definition of each segment composing the contour can vary from execution to execution.
The geometric items have already been created. Now I want to put them together. I am entirely uncertain which way to adopt:
My first idea was to use a linked list, since later I want to go through each segment in the same order (so not jumping from 3rd segment to 9th segment back to 5th segment).
1a) How can I implement that provided that the classes for each segment are very different.
1b) How can I access a specific element? Would I need to define an id number for each list member and then walk thru the list until I reach that id number?
More comfy would be probably an array. But I still don't understand how to do this properly.
Would be awesome if somebody here could indicate how to approach this. Thanks!
Here are the simplified classes that I am dealing with (circle is here representing the circular arc):
struct point {
double x;
double y;}
class Line
{
public:
Line(const point pt1, const point pt2)
{
P1.x = pt1.x;
P1.y = pt1.y;
P2.x = pt2.x;
P2.y = pt2.y;
}
~Line() {};
double get_length() { return calc_length(); }
double get_angle() { return angle; }
private:
point P1;
point P2;
double calc_length()
{
// calculate length (here: dummy value)
length = 1;
}
double calc_angle()
{
// calculate angle (here: dummy value)
angle = 0.5;
}
double length = 0;
double angle = 0;
}
class circle
{
public:
circle(const double r, const point c)
{
radius = r;
center.x = c.x;
center.y = c.y;
}
~circle() {};
double get_radius() { return radius; };
point get_center() { return center; };
double get_circumference() { return 3.14 * radius; };
private:
double radius;
double circumference = 0;
point center;
}

Answer 1

C++ containers store elements of the same type, regardless of the type of the container - both vector and list behave the same way in this regard - they only hold elements of a specific type.
To overcome this restriction, you should use polymorphism - the type of the container's elements should be able to behave like Line or circle.
One way to do this is by using std::variant. Declare your container like this:
std::vector<std::variant<Line, circle>> my_list_of_shapes;
To add a shape to the list, use push_back:
Line my_line(...);
circle my_circle(...);
my_list_of_shapes.push_back(my_line);
my_list_of_shapes.push_back(my_circle);
To retrieve a shape from the list, access the list using a subscript (or an iterator), and use std::get to convert std::variant to your specific type:
circle your_circle = std::get<circle>(my_list_of_shapes[1]);
If you write some code which iterates through the container, and doesn't "know" what is stored at a specific place - Line or circle - you can use std::holds_alternative to check that:
for (auto shape: my_list_of_shapes)
{
if (std::holds_alternative<Line>(shape)
{
std::cout << std::get<Line>(shape).get_angle();
}
else if (std::holds_alternative<circle>(shape)
{
std::cout << std::get<circle>(shape).get_radius();
}
}