inheritance 和 c++ 中的初始化程序列表

Question

As seen in the picture above: class Triangle inherits from class Shape but why is there written Shape() in the initializer list and what happens if I leave it out?

Answer 1

if you don't put it it will still invoke the default parent class constructor anyway. Putting it explicitly like this can be necessary if the parent constructor has keyword "explicit" however, or if there is no default parent constructor so you need to pass parameters to it..

Answer 2

Your derived Triangle class looks like this:

class Triangle : public Shape {
  Point _x1, _x2, _x3;
public:
  Triangle(const Point& x1, const Point& x2, const Point& x3)
    : Shape(), _x1(x1), _x2(x2), _x3(x3)
  {}
};

The base class, Shape , has its own default constructor which "does stuff". It's not important what that is, really; you just want to make sure that Shape 's constructor does get called so it can do whatever it needs to do during construction/initialization. That might be setting some initial values, logging some debugging information to the console, or calling the constructor of its own parent class (if it has one).

Therefore, when Triangle is constructed, it needs to explicitly tell its base class Shape to construct, too.