in lessons/extract.cpp [10:20]
int main()
{
std::vector<Circle> circles;
circles.emplace_back(Circle{2, 2});
circles.emplace_back(Circle{3, 5});
for (const auto& circle : circles) {
std::cout << "area = " << 3.1415926f * circle.radius * circle.radius << '\n';
std::cout << "circumference = " << 2 * 3.1415926f * circle.radius << '\n';
}
}