请教构造函数的用法

红字的冒号是什么用法
class Base
{
      public:
             Base(int i):a(i){};
             void Output();
             void Out();
      private:
              int a;
};

class Son : public Base
{
      public:
             Son(int i, int j):Base(i),b(j){};
             void Out();
      private:
             int b;
};