Default argument ctor is bad

default_arg_ctor.cpp

#include <iostream>

/*
 * This program illustrates why using default arguments in a constructor is
 * bad. Without explicitly defining an operator+(const float k), a Vector2D
 * with y being 0 is created.
 */

struct Vector2D {
	/// default constructor
	inline Vector2D(const float x = 0, const float y = 0) { this->x = x; this->y = y; }
	// inline Vector2D operator + (const float k) const { return Vector2D(x + k, y + k); }
	// inline Vector2D operator - (const float k) const { return Vector2D(x - k, y - k); }
	inline Vector2D operator * (const float k) const { return Vector2D(x * k, y * k); }
	inline Vector2D operator / (const float k) const { return Vector2D(x / k, y / k); }
	inline Vector2D operator + (const Vector2D& b) const { return Vector2D(x + b.x, y + b.y); }
	inline Vector2D operator - (const Vector2D& b) const { return Vector2D(x - b.x, y - b.y); }
	inline Vector2D operator - () const  {return Vector2D(-x, -y);}
    friend std::ostream& operator<<(std::ostream& os, const Vector2D& b) {os << b.x << ", " << b.y; return os; }
	float x;
	float y;
};

int
main() {
    auto va = Vector2D(3, 2);
    // Intuitively an error, but valid because of default arguments in ctor
    auto vb = Vector2D(3);
    auto vc = Vector2D();

    std::cout << va << std::endl;
    std::cout << vb << std::endl;
    std::cout << vc << std::endl;

    // Expected: (5, 4), Actual: (5, 2)
    std::cout << (va + 2) << std::endl;
    // Doesn't make sense
    std::cout << (vb + 2) << std::endl;
    // Expected: (2, 2), Actual: (2, 0)
    std::cout << (vc + 2) << std::endl;

    return 0;
}