Initialize parent's protected members with initialization list (C++)

Question

It is not possible in the way you describe. You'll have to add a constructor (could be protected) to the base class to forward it along. Something like:

class Parent
{
protected:
    Parent( const std::string& something ) : something( something )
    {}

    std::string something;
}

class Child : public Parent
{
private:
    Child() : Parent("Hello, World!")
    {
    }
}

When the compiler comes across the initializer list, the derived class object is yet to be formed. The base class constructor has not been called till then. Only after the base class constructor has been called, something comes to being. Hence the problem. When you do not call the base class constructor explicitly, the compiler does that for you (by generating the appropriate trivial constructor for the base class). This causes the something member to be default initialized.

From C++0x draft:

12.6.2 Initializing bases and members

2 Names in a mem-initializer-id are looked up in the scope of the constructor’s class and, if not found in that scope, are looked up in the scope containing the constructor’s definition. [ Note: if the constructor’s class contains a member with the same name as a direct or virtual base class of the class, a mem-initializer-id naming the member or base class and composed of a single identifier refers to the class member. A meminitializer- id for the hidden base class may be specified using a qualified name. —end note ] Unless the mem-initializer-id names the constructor’s class, a non-static data member of the constructor’s class or a direct or virtual base of that class, the mem-initializer is ill-formed.

Note: Emphasis mine.

You can't initialize members of the parent class in the derived class constructor initialization list. It doesn't matter whether they are protected, public or anything else.

In your example, member something is member of Parent class, which means that it can only be initialized in the constructor initializer list of Parent class.