How to use c++ structure array like flexible? - c++

Question

What should I do if I want to use the structure array flexibly?
I made my code like below, and tried Triangle and Rectangular..
Triangle was success but when i tried Rectangular I got error messages.
struct C2D {
double x, y;
};
class Polygon {
int point;
std::vector<C2D> arr;
public:
Polygon(int point_, C2D arr_[]) : arr(point_) {
point = point_;
memcpy(arr.data(), arr_, sizeof(C2D) * point);
};
void print() const {
for (int i = 0; i < point; i++) {
cout << arr[i].x << " " << arr[i].y << endl;
}
};
};
int main() {
int point;
C2D c2d[3];
cout << "point : ";
cin >> point;
cout << endl;
vector<C2D>c2d(point);
for (int i = 0; i < point; i++) {
cout << i + 1 << "x : ";
cin >> c2d[i].x;
cout << i + 1 << "y : ";
cin >> c2d[i].y;
cout << endl;
}
cout << endl;
Polygon p(point, c2d);
p.print();
return 0;
}

Answer 1

The problem is that you use the same name for
C2D c2d[3];
and
vector<C2D>c2d(point);
However, your code has several other issues such as using C-arrays in C++. Consider using std::vector, e.g., like this:
#include <iostream>
#include <vector>
struct C2D {
double x, y;
};
class Polygon {
std::vector<C2D> coordinates;
public:
explicit Polygon(const std::vector<C2D> &coords) : coordinates(coords) {}
void print() const {
for (const auto &c : coordinates) {
std::cout << c.x << " " << c.y << "\n";
}
}
};
int main() {
int point;
std::cout << "point : ";
std::cin >> point;
std::cout << "\n";
std::vector<C2D> coordinates(point);
for (int i = 0; i < point; i++) {
std::cout << i + 1 << "x : ";
std::cin >> coordinates[i].x;
std::cout << i + 1 << "y : ";
std::cin >> coordinates[i].y;
std::cout << "\n";
}
std::cout << "\n";
Polygon p(coordinates);
p.print();
}

Answer 2

As per your code, you declared variable name c2d one is C2D array type and the other is a vector of C2D what is the purpose of two variable names are the same you declared. My suggestion is to allocate c2d variable based on point count and pass to the polygon. As per my understanding change your polygon constructor with parameter array or vector and passing the corresponding argument to the constructor.