按照派生类型打印对象

#include <iostream>
using namespace std;

class Shape {
 public:
  virtual void draw () {}
};

class Circle : public Shape {
 int radius;
 public:
  Circle () { radius = 1; }
  void draw () { cout << "drawing a circle...\n"; }
};

class Square : public Shape {
 int height;
 public:
  Square () { height = 2; }
  void draw () { cout << "drawing a square...\n"; }
};

void drawShape (class Shape &p)
{
  p.draw ();
}

int main (void)
{
  Circle a;
  Square b;
  drawShape (a);
  drawShape (b);
  return 0;
}

技巧

在gdb中，当打印一个对象时，缺省是按照声明的类型进行打印：

(gdb) frame
#0  drawShape (p=...) at object.cxx:25
25      p.draw ();
(gdb) p p
$1 = (Shape &) @0x7fffffffde90: {_vptr.Shape = 0x400a80 <vtable for Circle+16>}

在这个例子中，p虽然声明为class Shape，但它实际的派生类型可能为class Circle和Square。如果要缺省按照派生类型进行打印，则可以通过如下命令进行设置：

(gdb) set print object on

(gdb) p p
$2 = (Circle &) @0x7fffffffde90: {<Shape> = {_vptr.Shape = 0x400a80 <vtable for Circle+16>}, radius = 1}

当打印对象类型信息时，该设置也会起作用：

(gdb) whatis p
type = Shape &
(gdb) ptype p
type = class Shape {
  public:
    virtual void draw(void);
} &

(gdb) set print object on
(gdb) whatis p
type = /* real type = Circle & */
Shape &
(gdb) ptype p
type = /* real type = Circle & */
class Shape {
  public:
    virtual void draw(void);
} &

详情参见 gdb手册