Do I need a default constructor?

[lucky6969b]

class CObjects
{
public:
	
	// default constructor
	CObjects() : mesh(0) { }
	CObjects(Mesh *m) { mesh = m; }
	~CObjects() { }
};

I don't understand if I include the default constructor but calling the wrong constructor accidentally, the program will collapse. But it won't compile if I don't include the default one. Could anyone shed some lights on this?
Thanks
Jack