画线

#include <graphics.h>
#include <ctime>
using namespace std;
int main()
{
    initgraph(640, 480);
    ExMessage m;       
    int x=-1,y=-1;
    while(true){
    m=getmessage();
        if(m.message==WM_LBUTTONDOWN){
            if(x==-1){
                x=m.x;
                y=m.y;
            } 
            
        }
		if(m.message==WM_MOUSEMOVE&&x!=-1){
            	cleardevice();
	            line(x,y,m.x,m.y);
	    }
        if(m.message==WM_LBUTTONUP){
            line(x,y,m.x,m.y);
            x=-1;
        }
    }
    closegraph();
    return 0;
}