請教c++的建構子

我不太瞭解
   CBox(double lv = 1.0, double bv = 1.0, double hv = 1.0): m_Length(lv),
                                                            m_Breadth(bv),
                                                            m_Height(hv)
   {
      cout << endl << "Constructor called.";
   }
 建構子這裡的宣告是代表什麼?
 因為lv=1.0 後面又有m_Length(lv) 那我的lv值到哪裡去呢?

 附上原始碼，謝謝！

#include <iostream>
using namespace std;

class CBox
{
   public:

   CBox(double lv = 1.0, double bv = 1.0, double hv = 1.0): m_Length(lv),
                                                            m_Breadth(bv),
                                                            m_Height(hv)
   {
      cout << endl << "Constructor called.";
   }
      double Volume()
      {
         return m_Length*m_Breadth*m_Height;
      }

   private:
      double m_Length;
      double m_Breadth;
      double m_Height;
};

int main()
{
   CBox match(2.2, 1.1, 0.5);
   CBox box2;

   cout << endl
        << "Volume of match = "
        << match.Volume();

// box2.m_Length = 4.0;

   cout << endl
        << "Volume of box2 = "
        << box2.Volume();
   cout << endl;
system("PAUSE");
   return 0;
}

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