Stroustrup 的 PPP 书中有关多边形的问题

我阅读了 Stroustrup 的《使用 C++ 进行编程原理和实践》一书。在第12章和第441页中有这样的代码：

//
// This is example code from Chapter 12.3 "A first example" of
// "Programming -- Principles and Practice Using C++" by Bjarne Stroustrup
//

#include "Simple_window.h"    // get access to our window library
#include "Graph.h"            // get access to our graphics library facilities

//------------------------------------------------------------------------------

int main()
{
    using namespace Graph_lib;   // our graphics facilities are in Graph_lib

    Point tl(100,100);           // to become top left  corner of window

    Simple_window win(tl,600,400,"Canvas");    // make a simple window

    Polygon poly;                // make a shape (a polygon)

    poly.add(Point(300,200));    // add a point
    poly.add(Point(350,100));    // add another point
    poly.add(Point(400,200));    // add a third point 

    poly.set_color(Color::red);  // adjust properties of poly

    win.attach (poly);           // connect poly to the window

    win.wait_for_button();       // give control to the display engine
}

//------------------------------------------------------------------------------

当我运行代码时，我收到 13 个错误，其中一定与Polygon标识符。例如第一个错误是：错误 C2872：“多边形”：不明确的符号

为什么我的编译器不知道这一点Polygon please?

如果符号不明确，则尝试使用其限定名称：

Graph_lib::Polygon poly;