Initializer List

Now let's go back to the Goods class we wrote before. Suppose we want to add a production date to the goods, with a Data class:

class Date {
 public:
  Date(int y, int m, int d) {
    _year = y;
    _month = m;
    _date = d;
  }
​
  void show() { 
      cout << _year << "/" << _month << "/" << _day << endl; 
  }
​
 private:
  int _yead;
  int _month;
  int _date;
}

Then we can make the data as one of the member attributes:

class Goods {
    ...
private:
      char _name[20];
      double _price;
      int _amount;
      Date _date;
}

But an error raises: Date does not have a default constructor. Remember that the declaration of an object includes both memory allocation and construction. We have already defined a customized constructor in Date, so there is no default constructor. But the constructor requires three parameters: year, month and date, which is provided when a Goods object is created. How can we pass them inside? A initializer list is used in initializing data members of a class. The list of members to be initialized is indicated with constructor as a comma-separated list followed by a colon:

Goods::Goods(const char *name, double price, int amount, int y, int m, int d) 
    : _date(y, m, d) {
  strcpy(_name, name);
  _price = price;
  _amount = amount;
}

In this way, y, m and d can be passed inside the constructor of Date. Not only can objects be initialized in this way, we can also use initializer list to initialize a normal member variable:

Goods::Goods(const char *name, double price, int amount, int y, int m, int d) 
    : _date(y, m, d), _price(price), _amount(amount) {
  strcpy(_name, name);
}

Notice that this initialization simply assigns values to variables, so _name has to be initialized inside the constructor with strcpy().

What's the initialization sequence of the initializer list? Does it follows the order from left to right? Let's look at this case.

class Test {
public:
    Test(int data = 10) : mb(_data), ma(mb) {}
    void show() {cout << "ma: " << ma << " mb " << mb << endl;}
private:
    int ma;
    int mb;
};
​
int main() {
    Test t;
    t.show();   // ma: -858993460 mb: 10
    return 0;
}

When t.show() is called, we find that mb is 10, but ma is a weird number. This is actually the default initial value in Visual Studio, which means that ma is not initialized correctly with mb. In fact, the initialization order only depends on the order we declared the variables, and has nothing to do with the initializer list. In the above case, we declared ma before mb, so ma is initialized first.