C ++对析构函数的未定义引用？

Question

我正在尝试使用Allegro 4进行游戏编码，而且我遇到了一个奇怪的碰撞。链接器在我的两个类中声明了对析构函数的未定义引用，但我没有做任何事情。可能是什么问题？这是我的代码：

Entity.h：

    #pragma once

#include <allegro.h>

struct Rectangle
{
       int x;
       int y;
       int w;
       int h;
};

typedef enum {
        FACE,
        POOP
} EntityType;

class Entity
{
      private:
              EntityType m_EntityType;
              BITMAP *m_Sprite;
              int m_Score;
              int m_X;
              int m_Y;
              Rectangle *m_Hitbox;

      public:
             Entity();
             virtual ~Entity() {destroy_bitmap(m_Sprite);}

             BITMAP *GetSprite() {return m_Sprite;}
             int GetScore() {return m_Score;}
             int GetX() {return m_X;}
             int GetY() {return m_Y;}
             Rectangle GetHitbox() {return *m_Hitbox;}

             void SetSprite(EntityType type);
             void SetScore(int value) {m_Score = value;}
             void SetX(int value) {m_X = value;}
             void SetY(int value) {m_Y = value;}
             void SetHitbox(EntityType type);
};

Entity.cpp：

#include "Entity.h"

void Entity::SetSprite(EntityType type)
{
 if (type == FACE)
    m_Sprite = load_bitmap("face.bmp", NULL);
 else if (type == POOP)
    m_Sprite = load_bitmap("poop.bmp", NULL);
}

void Entity::SetHitbox(EntityType type)
{
 if (type == FACE)
 {
  GetHitbox().x = m_X;
  GetHitbox().y = m_Y;
  GetHitbox().w = m_X + 32;
  GetHitbox().h = m_Y + 32;
 }
else if (type == POOP)
 {
  GetHitbox().x = m_X;
  GetHitbox().y = m_Y;
  GetHitbox().w = m_X + 16;
  GetHitbox().h = m_Y + 16;
 }
}