CODE:

#include 
#include 
#include 
#include 

using namespace std;
using namespace Linear;
double dt = 1.0 / 60.0;
double width = 640.0;
double height = 480.0;

class Circle
{
public:
	Circle( double r, Vector s, Vector v )
		:r_(r),m_(r*r),s_(s),v_(v)
	{
	}
	void Update()
	{
		s_ += v_ * dt;
	}
	void Draw( int color )
	{
		DxLib::DrawCircle( s_.x, s_.y, r_, color );
	}

	double			R() const { return r_; }
	double			M() const { return m_; }
	Vector	V() const { return v_; }
	Vector	S() const { return s_; }
	void S( Vector s ){ s_ = s; }
	void V( Vector v ){ v_ = v; }

private:
	double		   r_;	//半径
	double		   m_;	//質量
	Vector v_;	//速度
	Vector s_;	//位置
};

class DetectManager
{
public:
	void Add(const Circle& c)
	{
		circles_.push_back( c );
		hitflags_.push_back( false );
	}

	void Update()
	{
		for( int i = 0; i  v = circles_[i].V();
			Vector s = circles_[i].S();
			if( s.x  width )
			{
				s.x = width;
				v.x = -abs( v.x );
			}
			if( s.y  height )
			{
				s.y = height;
				v.y = -abs( v.y );
			}
			circles_[i].S(s);
			circles_[i].V(v);

			circles_[i].Update();
		}

	}
	void Draw()
	{
		for( int i = 0; i  d = rhs.S() - lhs.S(); 
		double rsum = lhs.R() + rhs.R();
		if( d*d > rsum*rsum )
			return false;

		//重心系を使って計算させる
		//まず重心の速度を求める
		Vector gv = ( lhs.M()*lhs.V() + rhs.M()*rhs.V() ) / ( lhs.M() + rhs.M() ); 

		//速度を重心系に変換する
		Vector lv = lhs.V() - gv;
		Vector rv = rhs.V() - gv;

		//速度の中心方向成分を求め、それらを逆転させる
		lv += -2.0*Linear::GetComponent( lv, d );
		rv += -2.0*Linear::GetComponent( rv, -d );

		//速度を絶対座標へ戻す
		lhs.V( lv + gv );
		rhs.V( rv + gv );

		//のめりこんでいる場合は退避させる
		Vector dn = d / d.Length();
		dn = dn*rsum - d;
		rhs.S( rhs.S() + dn * ( lhs.M() / ( lhs.M() + rhs.M() ) ) );
		lhs.S( lhs.S() - dn * ( rhs.M() / ( lhs.M() + rhs.M() ) ) );
		return true;
	}

	vector	   hitflags_;
	vector circles_;
};

int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance,LPSTR lpCmdLine, int nCmdShow ){
        if( ChangeWindowMode(TRUE) != DX_CHANGESCREEN_OK || DxLib_Init() == -1 ) return -1; //初期化処理
        SetDrawScreen( DX_SCREEN_BACK );        //裏画面に設定

		DetectManager manager;
		std::uniform_real_distribution rndr(1.0,15.0);
		std::uniform_real_distribution rndvs(0.0,1.0);
		std::mt19937	engine( 123 );

		for( int i = 0; i ( rndvs(engine)*width,  rndvs(engine)*height, 0.0 ),
					Vector( rndvs(engine)*160-80.0,  rndvs(engine)*160 - 80.0, 0.0 )
				)
			);
		}

 
        while(!ProcessMessage() && !ClearDrawScreen()){

						
				manager.Update();
				manager.Draw();
                ScreenFlip();//裏画面を表画面に反映

        }
 
        DxLib_End();
        return 0;
}