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

You can't call it from the body of the child constructor, but you can put it into the initializer list:

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

Of course it's not helpful to call the default constructor of the parent because that will happen automatically. It's more useful if you need to pass a parameter to the constructor.

The reason you can't call the constructor from the body is because C++ guarantees the parent will be finished constructing before the child constructor starts.

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.

The call inside the child class constructor is not calling the base class constructor, it is creating a temporary, unnamed and new object of type Person. It will be destroyed as the constructor exits. To clarify, your example is the same as doing this:

Child() { c = 1; Person tempPerson; }

Except in this case, the temporary object has a name.

You can see what I mean if you modify your example a little:

class Person
{
public:
    Person(int id):id(id) { std::cout << "Constructor Person " << id << std::endl; }
    ~Person(){ std::cout << "Destroying Person " << id << std::endl; }
    int id;
};

class Child : public Person
{
public:
    Child():Person(1) { c = 1; Person(2); }
int c;
};

int main() {
Child child;

Person(3);
return 0;
}

This produces the output:

Constructor Person 1
Constructor Person 2
Destroying Person 2
Constructor Person 3
Destroying Person 3
Destroying Person 1