C ++ classes and objects (six) --- details

Details

1. Constructors aspects

Initialization list
in the constructor at the beginning with a colon followed by a comma-separated list of data members, a member variable after each parentheses, brackets internal use similar copy configured to initialize member variables.

class Date
{
public:
	Date(int year = 2019, int month = 10, int day = 19)//构造函数
		:_year(year)//初始化列表
		,_month(month)
		, _day(day)
	{
	}
	Date(Date& d)//拷贝构造函数
		:_year(d._year)//初始化列表
		,_month(d._month)
		,_day(d._day)
	{
	}
private:
	int _year;
	int _month;
	int _day;
};

note:

1. Each member variable initialization list can only be initiated once .
2. Class contains about members must be initialized in the initialization list.
   const member variables modified
   reference type member variables of
   a class type (custom type) member variable (custom class and no default constructor)
   Example 1

class B
{
public:
private:
	int& _ref;
	const int _b;
};
int main()
{
	B b;
	return 0;
}

When the direct object is instantiated b, it reported a mistake?
When const member variables when B & types exist or directly report an error.
Example 2

class A
{
public:
	A(int a)
		:_a(a)
	{}
private:
	int _a;
};

class B
{
public:
private:
	A _ca;
};

int main()
{
	B b;
	return 0;
}

It also appeared on the error:

When the object class B Example B, since the object B exists in class A, so that when the object is instantiated, if no appropriate constructor for object B in _a call can also occur this type of error.
Solution
use a list initialized to these can not be resolved correctly instantiated three kinds that occur when an object is instantiated.
const: non-modifiable variables, so when you create must be initialized;
& Type: When you create a reference type must be referenced to a variable, otherwise no way to be created.
Custom type: If no custom types callable appropriate constructor call of a custom type object can not be instantiated.

class A
{
public:
	A(int a)
		:_a(a)
	{}
private:
	int _a;
};

class B
{
public:
	B(A ca, int ref, int b)//使用初始化列表
		:_ca(ca)
		,_ref(ref)
		,_b(b)
	{}
private:
	A _ca;
	int& _ref;
	const int _b;
};

int main()
{
	B b(1, 2, 3);//传递列表中的参数
	return 0;
}

First custom class _ca be constructed, in the call initialization list const, int & initialization together.

note

1. Try to use initialization list to initialize , because the member variables to customize the type, use initialization list to initialize.
2. In the member variable is initialized in the initialization list in the order of the classes in the order of declaration , irrespective of their order in the initialization list.

class B
{
public:
	B(int a, int b)
		: _b(b)//先初始化成员变量_a
		, _a(a)//与所放置位置无关
	{
	}
private:
	int _a;
	int _b;
};

Construction unnamed objects

When instantiate an object that only one parameter can be assigned directly by the symbol =.

class B
{
public:
	B(int a) :_a(a)
	{
	}
private:
	int _a;
};

int main()
{
	B b1(1);
	b1 = 20;
	return 0;
}

In b1 (1); time, b1 _a first member variables to 1;
although b1 = 20; direct type B 20 is configured to a nameless objects , to perform the assignment of b1.

2. Static member

Static member variables

1. Static member variables at initialization, initialization can not be placed in position in the list. Outside the class must be initialized .
   
2. Static member variables can be thought of as a member variable outside the class, all objects share the static member variables .

class B
{
public:
	B()
	{
	}
private:
	static int _s;
};

int B::_s = 10;// 在类外进行初始化静态成员变量

int main()
{
	B b;
	return 0;
}

Static member function

1. Static member function declared in class, outside of class can be achieved, can also be achieved in the class.

class B
{
public:
	static void Func1();
	static void Func2()
	{
		cout<<"static Func2"<<endl;
	}
private:
	int _a;
};

void B::Fun1c1()//在类外实现也和静态成员变量相同，
{			  //必须的加上作用域限定符B
	cout << "static Func1" << endl;
}

int main()
{
	B b;
	b.Func1();
	b.Func2();
}

2. Static member function does not contain this pointer

class B
{
public:
	static void Func()
	{
		_a = 10;//在类内静态成员函数调用this指针中的变量_a
	}
private:
	int _a;
};

int main()
{
	B b;
	b.Func();
}