CODEWITHSUNDEEP
c++oopinheritance
quant
quant

Reputation: 23172

Using constructor initialiser list to initialise elements of the base class

As I understand it, when I initialise the base class in the derived class initialiser list, the base class is immediately constructed and base class elements should then be available. If this is right, why doesn't this work?

class Base
{
public:
  int elem;
}

class Derived : public Base
{
  Derived() : Base(), elem(1) {}
  // error: class 'Derived' does not have any field named 'elem'
}

NOTE: In my case I cannot make any changes to Base (it is a fixed interface class).

Upvotes: 2

Views: 175

Answers (5)

seanmcl
seanmcl

Reputation: 9944

In that case, you can not initialize elem in the initializer list, and must instead initialize it in the function body. 

class Base {
 public:
  int elem;
};

class Derived : public Base {
 public:
  Derived() : Base() {
    elem = 1;
  }
};

Upvotes: 0

Mike Seymour
Mike Seymour

Reputation: 254771

The initialiser list is used to initialise the direct members and base sub-objects of that class, not members of base classes. Base class members are initialised by the base class constructor, and you can't initialise anything twice.

If you want to reassign the base-class member after it's been initialised, then you can:

Derived() : Base() {elem = 1;}

or you could allow the value to be passed through to the base class's constructor:

explicit Base(int e) : elem(e) {}
Derived() : Base(1) {}

although the note you've added to the question indicates that that's not an option in your particular circumstances.

Upvotes: 2

Motti
Motti

Reputation: 114845

You can only initialize members of the class and base classes in initialization list NOT base class members.

If you want to override the value of a base class member do it in the constructor's body.

Upvotes: 0

ergysdo
ergysdo

Reputation: 1159

elem belongs to class Base, so Base is responsible for initializing it.

The correct code looks like this:

class Base
{
public:
  int elem;
  Base(int n) : elem(n) {}
};

class Derived : public Base
{
  Derived() : Base(1) {}
};

Upvotes: 0

Kerrek SB
Kerrek SB

Reputation: 477710

A constructor can only construct its own class's members. You need to give Base a suitable constructor:

class Base
{
public:
  Base() : elem(23) { }
  int elem;
protected:
  Base(int n) : elem(n) { }
};

class Derived : public Base
{
  Derived() : Base(1) {}
};

Upvotes: 2

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