我想在组合/整体 class 的成员初始化程序列表中初始化一个“部分”数组 class

Question

Please help me on this. I want to initialize an array of "part" class in member initializer list of composed/whole class. Here Class B is composed from class A. Now in class B constructor how I will initialze the array in member initialzer list. I know how to initalize single object of "part" class in constructor of whole class but how an array of part class can be initialized in constructor of whole class. Also if i dont initialize array of part class in member initializer list will the code work? Thanks in advance.

#include <iostream>
using namespace std ;

class A  //part class
{
  public:
    A( int value = 0)
    { a = value ; }
    
    void printA()
    {
        cout << "\nPrinting A members : " << a << endl ;
    }
 
    void setA( int value)
    { a = value ; }

    protected:
      int a ;

 };


 class B  //whole class
 {
   public:
     B( int value = 5 ) : aM(0) //member initializer list
     { b = value ; }
  
     void printB()
     {
       cout << "\nPrinting B members : " << b << endl ;
       aM.printA() ;
       for (int i = 0 ; i < 5 ; i++)
         cout << arr[i] << " , " ;
     }

   private:
     int b ;
     A aM ; //composition
     A arr[5] ;

};


int main()
{
  B objB ;
  objB.printB() ;


  return 0 ;
}

Answer 1

You initialize the array of a member almost the same as you initialize an array otherwise, using a curly-brace enclosed list of elements. Or nothing if you want all elements default constructed:

B( int value = 5 ) : b{ value }, aM{ }, arr{ }
{ }

---

On another note,

cout << arr[i] << " , " ;

is invalid since you don't have an overloaded << operator for A . Perhaps you want this instead:

arr[i].printA();

Answer 2

If I have understood correctly what you need is the following

B( int value = 5 ) : b( value ), aM(0), arr{}
{
}

However it is enough to write

B( int value = 5 ) : b( value )
{
}

because the default constructor of the class A will initialize data members aM and arr with zeroes.

If you want to use initializers of the array other than zero then you can write for example

B( int value = 5 ) : b( value ), aM(0), arr{ 1, 2, 3, 4, 5}
{
}

And the member function

 void printB()
 {
   cout << "\nPrinting B members : " << b << endl ;
   aM.printA() ;
   for (int i = 0 ; i < 5 ; i++)
     cout << arr[i] << " , " ;
 }

is incorrect because there is no defined operator << for objects of the type A.

To make this statement valid

     cout << arr[i] << " , " ;

define the operator << as a friend function of the class A. For example

class A  //part class
{
  public:
    A( int value = 0)
    { a = value ; }
    
    void printA()
    {
        cout << "\nPrinting A members : " << a << endl ;
    }
 
    void setA( int value)
    { a = value ; }

    friend std::ostream & operator <<( std::ostream &os, const A &a )
    {
        return os << a.a;
    }
    protected:
      int a ;

 };