CODEWITHSUNDEEP
c++constructor
Veritas
Veritas

Reputation: 2210

Hiding implementation details of base class from derived class

Say that we have a base A class and a derived B class:

struct A
{
    A(int a) : a(a) {}
    int a;
};

struct B : public A
{
    B(int a, int b) : A(a), b(b) {}
    int b;
};

Is there any way to define the B class without knowing the A class' constructor? For instance a factory class may be responsible for providing the data that the A class needs but the writter of the B class doesn't have to know anything about it. Basically assuming that we don't delegate the constructors is it possible to first initialize the A class part of an object and then initialize the B part using another set of constructor arguments?

Upvotes: 1

Views: 119

Answers (3)

RostakaGmfun
RostakaGmfun

Reputation: 507

You can't inherit from incomplete type. If you just forward declare the base class you will get a compile-time error. For example, in gcc (I have 4.8.1) you will get a invalid use of incomplete type error. So, you can't hide class declaration if that class is going to be the base class.

Upvotes: 0

Nikos Athanasiou
Nikos Athanasiou

Reputation: 31519

You could use a variadic templated constructor that can forward its arguments to the base class [1]

struct B : public A
{
    template<typename...Ts>
    B(int b, Ts&&...args) : A(std::forward<Ts>(args)...), b(b) 
    {
    }
    int b;
};

But the above would not be any of any practical use unless [2] the base class was itself a generic aspect, eg

template<typename Base>
struct B : public Base
{
    template<typename...Ts>
    B(int b, Ts&&...args) : Base(std::forward<Ts>(args)...), b(b) 
    { 
        // above the initialization of b (its related argument)
        // are not part of the variadic pack
    }
    int b;
};

Example

1. The above would imply that a related factory method also uses variadic templates

2. Considering just a class that wants to construct its base without knowing how many or what type of variables it's constructed from. Uses may vary depending on context

Upvotes: 2

Zsolt
Zsolt

Reputation: 582

A derived class needs to see the full definition of it's base class, so no, you can't define B without A's definition (which involves it's constructors).

You need to check, if you really need inheritance here. If you don't, then A and B can be separate classes (so B isn't a derived class of A), and you can construct them separately. Then e.g. you can create a third class which has members of A and B.

Upvotes: 0

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