C++构造函数

#include<iostream>
using namespace std;

class Box{
     private:
         double length;
         double breadth;
         double height;
     public:
       Box(double len, double bre, double hei);//这是构造函数。
       Box(){};//请问，这是什么函数？
               //若没有这句，错误有两个，在下方已标出。
               //若没有{},也会出现错误。
       double GetVolume(void);
       Box operator+ (const Box&);
      
};
Box::Box(double len, double bre, double hei){
    length = len;
    breadth = bre;
    height = hei;
}
double Box::GetVolume(void){
    return length * breadth * height;
}
Box Box::operator+ (const Box& obj){
          Box box;//错误一：[Error] no matching function for call to 'Box::Box()'
          box.length = this->length + obj.length;
          box.breadth = this->breadth + obj.breadth;
          box.height = this->height + obj.height;
          return box;
}

int main(void)
{
        Box box1(2.0, 4.0, 6.0);
        Box box2(1.0, 2.0, 3.0);
        Box box3;//错误二：[Error] no matching function for call to 'Box::Box()'
        double Volume = 0.0;
        Volume = box1.GetVolume();
        cout << "BOX1'Volume is " << Volume << endl;
        Volume = box2.GetVolume();
        cout << "BOX2'Volume is " << Volume << endl;
        box3 = box1 + box2;
        Volume = box3.GetVolume();
        cout << "BOX3'Volume is " << Volume << endl;
    return 0;

 }