按照派生類型打印對象

例子

#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，當打印一個對象時，缺省是按照聲明的類型進行打印：

(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>}

在這個例子中，p雖然聲明為class Shape，但它實際的派生類型可能為class Circle和Square。如果要缺省按照派生類型進行打印，則可以通過如下命令進行設置：

(gdb) set print object on

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

當打印對象類型信息時，該設置也會起作用：

(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);
} &

詳情參見gdb手冊

貢獻者

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