Classes and objects (4)

Catalog

1. Supplement to constructor

1.1 Constructor assignment

1.2 Use of initialization list

1.3 explicit keyword

2. static

3. Friends

3.1  The concept of friends

3.2 Friend function

3.3  Friend class

3.4 Friend characteristics

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1. Supplement to constructor

1.1 Constructor assignment

stay Classes and objects （2） In, we talked about that when an object is created, the constructor will be called to assign values to member variables , So is this assignment initialization ？

class Date
{
public:
 Date(int year=1, int month=1, int day=1)
 {
 _year = year;
 _month = month;
 _day = day;
 _year=2;
 _month=1;
 _day=3;
 }
 
private:
 int _year;
 int _month;
 int _day;
};

Although after the above constructor call , Object already has an initial value , But you can't call it initialization of class object members , The statement in the constructor body can only call it as an initial value , It can't be called initialization . Because initialization can only be initialized once , The constructor body can be assigned multiple times .

Then we have to think about a problem ： What if there are certain members in our member variables that need to be initialized at the time of creation ？

such as int&, Previously, we learned that when using references , We have to initialize it , The constructor here cannot provide , that C++ Just added a new thing to the constructor -- Initialization list .

1.2 Use of initialization list

Start with a colon , Then there is a comma separated list of data members , Each member variable is followed by an initial value or expression in parentheses .

class Date
{
public:
 Date(int year, int month, int day)
  :_year(year)
  ,_month(month)
  ,_day(day)
 {}
 
private:
 int _year;
 int _month;
 int _day;
};

Be careful ：

1. Each variable in the initialization list can only be initialized once , Because initialization can only be initiated once

2. If the class contains reference member variables 、const Member variables 、 Custom type members ( This class has no default constructor ）, It must be placed in the initialization list for initialization , Because it cannot be initialized in the constructor .

3. After learning the initialization list , When no matter what kind of member variable you encounter , We all suggest to put it in the initialization list for initialization .

4. The initialization order in the initialization list is based on the order in which member functions are declared

class A
{
public:
 A(int a)
 :_a1(a)
 ,_a2(_a1)
 {}
 
 void Print() {
 cout<<_a1<<" "<<_a2<<endl;
 }
private:
 int _a2;
 int _a1;
}
int main() {
 A aa(1);
 aa.Print();
}

  The current code will output 1 And random values , This is because _a2 and _a1 The statement is _a2 First , Therefore, in the initialization list, first carry out the following statement , Then execute the above statement .

1.3 explicit keyword

explicit The moral is to show the meaning of , So what's its function ？

class Date
{
public:
 Date(int year)
 :_year(year)
 {}
private:
 int _year;
 int _month:
 int _day;
};
void TestDate()
{
 Date d1(2018);
 d1 = 2019;
}

When the constructor is a single argument , Let's assign a value to the object. Will this cause a problem ？

We found that there would be no problem here , Because there are implicit transformations in the compiler , Will use 2019 This value is used to create an unnamed object , Use this object to create a for d1 assignment .

But let's look at this code , If you don't know the implicit conversion , Is it difficult to understand whether what is written here is correct , So we used explicit Keyword to decorate the constructor , Make this poorly readable code disappear .

2. static

2.1 Concept

Declare as static A class member of a class is called a static member of a class , use static Modified member variables , Call it static member variable ,

By static Decorated member function , Called static member function , Static member variables must be initialized outside the class .

2.2 features

1. Static members are shared by all class objects , It doesn't belong to a specific instance

2. Static member variables must be defined outside the class , Do not add... When defining static keyword

3. Class static members are available class names :: Static members or objects . Static members to access

4. Static member functions do not belong to any object , No hidden this The pointer , Cannot access any non static members

5. Static members are like ordinary members of a class , Also have public、protected、private3 Access levels , You can also have a return value

3. Friends

3.1  The concept of friends

1. Friends are divided into friend functions and friend classes

2. Friends provide a way to break through encapsulation , But increased coupling , So try to avoid using

3.2 Friend function

If we want to achieve <<、>> An object , Then we have to overload these two operators

But we can't overload these two operands in a class , because cout and cin And hidden in member functions this The pointer competes for the position of the first parameter , Then we can only define it outside the class . But how do we access private member variables in a class ？

At this time, our friends play a corresponding role ：

  We define functions outside the class , Use... In classes friend Keyword to declare that this function is mine “ friend ”, But remember that this friend is one-way .

Be careful ：

1. Friend functions can access all member variables in the class , But it is not a member function .

2. Friend function cannot be used const To modify

3. Friend functions can be declared anywhere in the class definition , No access restrictions

4. A function can be a friend function of multiple classes

3.3  Friend class

If I want to access member variables of another class in one class , But I have no authority , So what should I do ？

This is when we think of friends , We can define access classes in classes , Make it my “ friend ”, So I can access its member variables .

  When added friend Statement of , We can use Time Private member variables in the class .

3.4 Friend characteristics

1. Friendship is one-way , Not exchangeable .

2. Friendship cannot be transmitted If B yes A Friends ,C yes B Friends , Can't explain C when A Friends .