Error on CC_SYNTHESIZE(cocos2d::CCSprite *, _drawings, Drawings) //('cocos2d::CCSprite* Ground::_drawings' is protected)

I have got the following class called ‘Ground’:

.h file

class Ground : public cocos2d::CCNode
{ 
protected:
//Some variables here...

public:

    Ground():_drawings(NULL) {};
    CC_SYNTHESIZE(cocos2d::CCSprite *, _drawings, Drawings); *// COMPILE ERROR HERE: 'cocos2d::CCSprite* Ground::_drawings' is protected*

    LAYER_NODE_FUNC(Ground);

   //Some methods here...   
};

And another class called Main:

.h file:

class Main : public cocos2d::CCLayerColor
{
protected:

    Ground * _ground;
//Some other variables here...


public:
    //Some methods here...

};

.m file:

CCSprite *drawings = ....
_ground->_drawings = drawings; *// COMPILE ERROR HERE: _drawings is a protected member of 'Ground'*

I don’t understand where I am wrong… Why does the compilator tells me that _drawings is protected ?

Thanks for your help !

Please refer the definition of CC_SYNTHESIZE, the declares the variable as a protected member variable.