https://gist.github.com/yashihei/aad13278d4a1ae8d8800

main.cpp

#include <Siv3D.hpp>
#include <deque>

class Bullet
{
public:
	Bullet(Vec2 pos, Vec2 vec, Color color)
		: pos(pos), vec(vec), color(color) {}

	void move()
	{
		pos += vec;	
		enabled = Window::ClientRect().intersects(pos);
	}

	void draw() const
	{
		Circle(pos, 5.0).draw(color);
	}

	bool isEnabled() const
	{
		return enabled;
	}

private:
	Vec2 pos, vec;

	Color color;

	bool enabled = true;
};

void Main()
{
	std::deque<Bullet> bullets;
	Vec2 pos(320, 240);

	double sp = 2.0;
	double turnAngle = 3.0;
	int interval = 3;
	int sep = 5;
	double hue = 0.0;

	GUI gui(GUIStyle::Default);
	gui.addln(GUIText::Create(L"bulletSpeed"));
	gui.addln(L"speed", GUISlider::Create(1.0, 10.0, sp, 200));
	gui.addln(GUIText::Create(L"turnSpeed"));
	gui.addln(L"turnAngle", GUISlider::Create(-5.0, 5.0, turnAngle, 200));
	gui.addln(GUIText::Create(L"interval"));
	gui.addln(L"interval", GUISlider::Create(1.0, 10.0, 3.0, 200));
	gui.addln(GUIText::Create(L"separate"));
	gui.addln(L"separate", GUISlider::Create(3.0, 10.0, 5.0, 200));
	gui.addln(GUIText::Create(L"color"));
	gui.addln(L"color", GUISlider::Create(0.0, 360.0, hue, 200));

	Println(L"Spaceキーで各パラメーターを設定、右クリックで発射地点変更");

	while (System::Update())
	{
		const int cnt = System::FrameCount();

		if (cnt == 600)
			ClearPrint();

		sp = gui.slider(L"speed").value;
		turnAngle = gui.slider(L"turnAngle").value;
		interval = gui.slider(L"interval").valueInt;
		sep = gui.slider(L"separate").valueInt;
		hue = gui.slider(L"color").value;

		if (Input::KeySpace.clicked)
			gui.show(!gui.style.visible);

		if (Input::KeyC.clicked)
			bullets.clear();
		
		if (Input::MouseR.pressed)
			pos = Mouse::Pos();

		const double rad = Radians(cnt * turnAngle);

		if (cnt % interval == 0)
		{
			for (int i = 0; i < sep; ++i)
			{
				const double tRad = rad + (TwoPi / sep) * i;
				bullets.emplace_back(pos, Circular(sp, tRad), HSV(hue, 0.7, 1.0));
			}
		}

		for (auto& bullet : bullets)
			bullet.move();

		Erase_if(bullets, [](const Bullet& bullet) { return !bullet.isEnabled(); });

		for (const auto& bullet : bullets)
			bullet.draw();
	}
}