Explain initialization list

Catalog

One ： Definition

Two ： characteristic

Two ： Initialization phase ：

3、 ... and ： The calculation phase

Four ： Initialization order of member variables

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One ： Definition

Let's start with an example

Date(int year, int month, int day)// Constructor with parameters 
        :_year(year)
        , _month(month)
        , _day(day)
    {}

This is a simple initialization list case , Start with a colon , Then there is a comma separated list of data members , Every " Member variables " Followed by one in the enclosure The initial value or expression in the number .

Two ： characteristic

A. The list can only be initialized once , Multiple initialization will result in an error

Date(int year = 2022, int month = 4, int day = 19)// This is a general constructor 
	{
		_year = year;
		_month = month;
		_day = day;
        _day = day;// here _day Initialized twice , The compiler will report an error , Each member can only be initialized once 
	}

B. The compiler also allows constructor initialization and initialization list initialization to be mixed  

Date(int year = 2022, int month = 4, int day = 19)// There is no conflict between the two 
		: _year(year) // initialization 
		, _month(month)
	{
		_day = day;// Note that neither here nor above can be repeated twice , Such as _year(year). So it's repeated , Initialization can only occur once 
	}

C.const Member variables 、 Reference member variables 、 Custom type members without a default constructor can only be initialized in the initialization list .

Date(int year = 2022, int month = 4, int day = 19)
		: _year(year) 
		, _month(month)
		, _n(2) //const Member variables must be initialized with an initialization list 
	{
		_day = day;
		//_n = 2; //const Member variables cannot be initialized in a function body 
	}

Date(int year = 2022, int month = 4, int day = 19)
		: _year(year) 
		, _month(month)
		,_ref(year)// Reference member variables should be initialized in the initialization list 
	{
		_day = day;
		//_ref = year; // A reference member variable cannot be initialized in a function body 
	}

Date(int year = 2022, int month = 4, int day = 19)
		: _year(year) 
		, _month(month)
		, _a(20)// Custom type member variables without a default constructor must be initialized in the initialization list 
	{
		_day = day;
	}

 

const Member variables 、 Reference member variables 、 The reason why a custom type member variable without a default constructor must be initialized in the initialization list ：

1. The initialization list is where the object's member variables are defined .

2. The built-in type member variable of the object does not require initialization when initializing the list definition , Therefore, you can initialize in the initialization list , You can also initialize in the constructor body .

3.const Member variables 、 Reference member variables 、 A custom type member variable without a default constructor cannot be defined first and then initialized , They are defined in the initialization list , And must be initialized at definition time , Therefore, you must initialize... In the initialization list .
 

At the same time, the execution of constructor can be divided into two stages , Initialization phase and calculation phase , The initialization phase precedes the calculation phase .

Two ： Initialization phase ：

All class types （class type） All members of will be initialized in the initialization phase , Even if the member does not appear in the constructor's initialization list .

3、 ... and ： The calculation phase

It is generally used to perform the assignment operation in the constructor body .

Four ： Initialization order of member variables

class Date
{
public:
    Date(int year, int month, int day)// Initialization is based on the order of definition , Such as year The first definition , Just the first initialization 
        :_month(month)// The second initialization 
        , _year(year) // First initialization 
        , _day(day)   // The third initialization 
    {}

private:
    int _year; // The first definition 
    int _month;// The second definition 
    int _day;  // The third definition 
};