我从一个基础的Shape
类(由Stroustrup编写,我正在使用编程原理和实践)中创建了一个派生的Octagon
类。构造函数和其他方法似乎都没问题,只是调用修改类成员变量的函数没有任何改变。
这是我编写的派生类(遵循基类接口):
class Octagon : public Shape
{
public:
Octagon(Point centre, int side_length);
double cot(double x) const{return (cos(x) / sin(x));}
void draw_lines() const;
void set_side(int x) { s_l = x; } //am i missing something?
double rad(int side){return (0.5*side*cot(PI / 8));}
private:
int s_l;
};
Octagon::Octagon(Point centre, int side_length)
:s_l( side_length )
{
for (int ang = 22.5; ang <= 337.5; ang += 45)
{
add(Point{ int(centre.x + rad(s_l) * cos(ang*PI / 180)),
int(centre.y - rad(s_l) * sin(ang*PI / 180)) });
};
}
当我在主函数中调用set_side(int)
函数时,它实际上并没有执行任何操作...
int main()
{
Simple_window win{ Point{100,100}, 1200, 900, "window" }; //Simple_window class
//designed by Stroustrup to display Shape objects
Octagon oct{ Point(600,450), 100};
oct.set_color(Color::black);
oct.set_side(20); //supposed to change the side length from 100 to 20
//but it stays at 100 when i run the program
win.attach(oct); //attaches Shape object to Simple_window
win.wait_for_button();
}
我不知道这是否必要,但这是由Stroustrup使用FLTK GUI库设计的Shape
基类。
class Shape { // deals with color and style, and holds sequence of lines
public:
void draw() const; // deal with color and draw lines
virtual void move(int dx, int dy); // move the shape +=dx and +=dy
void set_color(Color col) { lcolor = col; }
Color color() const { return lcolor; }
void set_style(Line_style sty) { ls = sty; }
Line_style style() const { return ls; }
void set_fill_color(Color col) { fcolor = col; }
Color fill_color() const { return fcolor; }
Point point(int i) const { return points[i]; } // read only access to points
int number_of_points() const { return int(points.size()); }
virtual ~Shape() { }
protected:
Shape();
virtual void draw_lines() const; // draw the appropriate lines
void add(Point p); // add p to points
void set_point(int i,Point p); // points[i]=p;
private:
vector<Point> points; // not used by all shapes
Color lcolor; // color for lines and characters
Line_style ls;
Color fcolor; // fill color
Shape(const Shape&); // prevent copying
Shape& operator=(const Shape&);
};
顺便说一下,
draw_lines()
是由显示引擎调用的 draw()
函数调用的。(我按照书本逐章学习,所以还没有到他完全讨论这个问题的章节)。