通过复制派生类的另一个对象来创建派生类的对象时，调用基类的副本构造函数

Question

class base {};
class der : public base{};


der d1;
der d2(d1);

This statement invokes default constructor of class base then copy constructor of claas der. My question is why C++ has not provided the feature of calling copy constructor of base class while creating object of derive class by copying another object of derive class

Answer 1

Short version

This statement invokes default constructor of class base then copy constructor of claas der.

No, it doesn't.

My question is why C++ has not provided the feature of calling copy constructor of base class while creating object of derive class by copying another object of derive class

It does.

---

Long(er) version

I don't know how you came to the conclusion that the base default constructor is invoked during the construction of d2 , but it is not. The synthesised base copy constructor is invoked, as you expect.

This is really easy to test :

struct base {
   base() { cout << "*B"; }
   base(base const& b) { cout << "!B"; }
  ~base() { cout << "~B"; }
};

struct der : base {};

int main() {
   der d1;
   der d2(d1);
}

// Output: *B!B~B~B

Answer 2

Copy constructor of the derived class call the default constructor of the base class.

Below sample programs demonstrates the same.

#include <iostream>

using namespace std;

class Base
{
public:
    Base(){ cout<<"Base"<<endl; }
    Base(int i){ cout<<"Base int "<<endl; }
    Base(const Base& obj){ cout<<"Base CC"<<endl; }
};

class Derv : public Base
{
public:
    Derv(){ cout<<"Derv"<<endl; }
    Derv(int i){ cout<<"Derv int "<<endl; }
    Derv(const Derv& obj){ cout<<"Derv CC"<<endl; }
};

int main()
{
    Derv d1 = 2;
    Derv d2 = d1; // Calls the copy constructor

    return 0;
}

Answer 3

This statement invokes default constructor of class base then copy constructor of claas der.

No it does not.

The first line invokes the default construct of class der , which invokes the default constructor of class base . The second line invokes the copy constructor of class der , because you're copying one der instance to another.

Answer 4

The compiler-generated copy-constructor will invoke the copy constructor of the base class.

You have probably added a user-defined copy constructor for der . In such a case you must explicitly invoke the copy constructor of the base class.

der::der(const der& other): base(other), ... {}