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c++constructorbase-class

Calling a constructor of the base class from a subclass' constructor body


I was under impression that it's impossible, see for example: Calling the constructor of the base class after some other instructions in C++
But the following program runs and produces two lines of "Constructor Person":

#include <iostream>

class Person
{
public:
    Person() 
    { 
        std::cout << "Constructor Person" << std::endl; }
    };

class Child : public Person
{
public:
    Child() 
    { 
        c = 1; 
        Person(); 
    }
    int c;
};

int main() 
{
    Child child;
    return 0;
}

The first one is implicit call of the default constructor, that's clear. What about the 2nd one - does it mean that the action described in the title is legitimate? I use Visual C++ 2010.


Solution

  • The following is an excerpt from "Accelerated C++": "Derived objects are constructed by:
    1. Allocating space for the entire object (base class members as well as derived class members);
    2. Calling the base-class constructor to initialize the base-class part of the object;
    3. Initializing the members of the derived class as directed by the constructor initializer;
    4. Executing the body of the derived-class constructor, if any."

    Summarizing the answers and comments: Calling a constructor of the base class from a subclass' constructor body is impossible in the sense that #2 above must precede #4. But we still can create a base object in the derived constructor body thus calling a base constructor. It will be an object different from the object being constructed with the currently executed derived constructor.