对vtable(C++）的未定义引用

我正在为我的sfml游戏制作一个Entity类，但我不明白为什么下面的代码不能编译。我收到一个undefined reference to vtable in Entity错误。

这是头文件：

#ifndef ENTITY_H
#define ENTITY_H
#include <string>
#include <SFML/System/Vector2.hpp>
#include <SFML/Graphics/Rect.hpp>
#include <SFML/System/Time.hpp>
class Entity
{
public:
        Entity(std::string id, sf::Vector2f position, float rotation );
        virtual ~Entity() = 0;
        virtual sf::FloatRect getBoundingRect() const;
        virtual float getRadius() const;
        void UpdateVelocity(sf::Time);
        void UpdatePosition(sf::Time);
        void Update(sf::Time);
        float getRotation() const;
        sf::Vector2f getPosition() const;
        sf::Vector2f getVelocity() const;
        sf::Vector2f getAcceleration() const;

private:
        std::string mId;
        sf::Vector2f mPosition;
        float mRotation;
        sf::Vector2f mVelocity;
        sf::Vector2f mAcceleration;
};
#endif // ENTITY_H

这是cpp文件：

#include "../include/Entity.h"
Entity::Entity(std::string id, sf::Vector2f position, float rotation)
        : mId(id)
        , mPosition(position)
        , mRotation(rotation)
{
}
Entity::~Entity()
{
}

void Entity::UpdateVelocity(sf::Time dt)
{
        mVelocity += mAcceleration;
}
void Entity::UpdatePosition(sf::Time dt)
{
        mPosition += mVelocity;
}
void Entity::Update(sf::Time dt)
{
        UpdateVelocity(dt);
        UpdatePosition(dt);
}
float Entity::getRotation() const
{
        return mRotation;
}
sf::Vector2f Entity::getPosition() const
{
        return mPosition;
}
sf::Vector2f Entity::getVelocity() const
{
        return mVelocity;
}
sf::Vector2f Entity::getAcceleration() const
{
        return mAcceleration;
}

非常感谢您的帮助！