Inherited class initializing a custom made class using non-default constructor

Question

So I have searched all over the place and I can not seem to find the answer to this specific question. I am using a winXP with cygwin and gcc 3.4.4 cygming special.

Problem: I have a class that works as an interface with some abstract methods and protected variables that should be in every class that inherits from this class. Now I also have another class that is a member variable of this interface.

class Bar {
private:
    int y;
public:
    Bar(int why);
};

Bar::Bar(int why) : y(why) {}

class Foo {
protected:
    Bar b;
public:
    Foo(int x);
    virtual void print_base();
};

Foo::Foo(int x) : b(x+3)  // Have to use initializer list here.
{
    //this->b(x+3); // doesn't work
}

class DerFoo : public Foo {
protected:
    Bar db;
public:
    DerFoo(int x);
};

DerFoo::DerFoo(int x) : Foo(x), 
    db(x+3) // (Bar)(int) being called, works fine
    // db(4.0, 30) // no matching function for call to Bar::Bar(double, int)
    // note: candidates are Bar::Bar(const Bar&), Bar::Bar(int)
    // b(x-3) // doesn't work class DerFoo does not have any field named 'b'
{
    //this->b(x - 3); //  Doesn't work, error no match for call to (Bar)(int)
    //this->db(x + 3); // Doesn't work, error no match for call to (Bar)(int)
}

So the problem as you can see is inside the derived foo class, DerFoo how to initialize b. I have tried member initialization method, but then the compiler doesn't realize about protected variables. So then for some bizarre reason unbeknownst to me, it can not find the constructor in this class. Even though if were to include a "wrong" call to the constructor of a protected member variable (non inherited) it would suggest the correct version of the constructor.

I have no clue still how to do this. Any help is greatly appreciated.

Answer 1

DerFoo's constructors must not (and cannot) initialize b, that's Foo job. DerFoo's constructors are responsible for initializing only DerFoo's immediate subobjects, namely db and the Foo which is a base class of DerFoo. Foo's constructor, in turn, is responsible for initializing b.

The sequence of events is like:

• DerFoo's constructor invokes Foo's constructor
• Foo's constructor invokes b's constructor
• Foo's constructor runs its body, leaving the Foo object completely constructed
• DerFoo's constructor invoke's db's constructor
• DerFoo's constructor runs its body, leaving the DerFoo object completely constructed.

If, in the DerFoo constructor, you don't like the value that the Foo constructor left in b, you can assign a new value to b, using any one of these syntaxes:

b = Bar(47);
this->b = Bar(47);
this->Foo::b = Bar(47);
Foo::b = Bar(47);

Answer 2

After you declare a variable you have to set it otherwise you will be calling it like a function.

this->b = Bar(x+3);

The preferred way is to use the initializer list to avoid unnecessary copies of Bar. If you do, however, need to set b outside of the constructor the above example is how to do so.

Answer 3

I don't find the question very clear, but let's see if I understood what you are trying to do and how.

DerFoo::DerFoo(int x) : Foo(x), [a]
    db(x+3) 
    // db(4.0,30)          [1]
    // note: candidates are Bar::Bar(const Bar&), Bar::Bar(int)

    // b(x-3)              [2]
{
    //this->b(x - 3);      [3]
    //this->db(x + 3);     [4]
}

The first error is [1], where the compiler is telling you that there is no constructor of Bar that takes both a double and an int. The error also lists the two possible constructors that you can use: Bar(int), Bar(Bar const &). I am unsure as of what you intended with this line, but you have already figured out (previous line) that by just providing an int the call will work.

[2] b is not a member of DerFoo, and thus cannot be initialized in the initializer list of DerFoo. It is the responsibility of Foo to initialize it's own member and that will happen through the call to the Foo constructor in [a].

[3],[4], both expressions take the form this->member(i). During initialization the syntax member(i) will well, initialize member with the value of i. Outside of initialization the syntax means call operator()( int ) passing the value of i. Those members have already been initialized, but if you want to reset them you need to assign rather than initialize them.

Answer 4

b is inside the Foo class. to access it (for sure) use

Foo::b = Bar(x-3);

Answer 5

You don't have to use an initializer list, you definitely also should use an initializer list at this point.

At construction of an object, before the code of the constructor is entered all member variable are already constructed. If you don't give initializers, they will be default constructed.

Also you cannot construct a variable again, after it already has been constructed. Your

this->b(x+3)

is not telling the compile to construct b it is telling it to call a function named b on your object. Such a function does not exist in your class, hence the error. Note that once an object has been constructed into a variable, there is no way, to ever call the constructor for that variable again (only to change the value).

A value can be changed using = as in most languages. Hence you could do:

Foo::Foo(int x)
{
   this->b = Bar(x+3);
}

This means you are creating another nameless Bar object and assigning it's value to this->b. You should be aware, that this means, you will create two Bar objects, when creating a Foo. First the default constructed one, before. The constructor code is entered, then the new nameless one. Then you will finally assign the value to the already constructed object, so this code is much more inefficient than the one, which uses initializer lists.

EDIT:

Since I missed the second doesn't work in the code above here some additional information:

You are also trying to initialize b directly in the constructor of the derived DerFoo object. However once this part of code is reached, it has already been constructed. Hence any attempt to construct it in the derived constructor comes to late.

Hence you either have to add another constructor to Foo which takes the value and use that one in your constructor of DerFoo. This solution is preferable, since it will only construct the Bar object in b once. If you cannot add such a constructor, then you need to use an assignment in the constructor code for DerFoo.

Trying to initialize b directly in the DerFoo constructor will not work even if the scoping operator is used.

DerFoo::DerFoo() : Foo::b(x-3) {}

will still produce an error: http://ideone.com/6H8ZD