Automatic c++ constructor creation?

Question

I just want to check this:

If I have a derived class that has no new members. In case that the base class has already a long constructor (many parameters), the creation of the constructor in the derived class that just throws all the parameters to the base constructor is somewhat a boring (and obviously not necessary) process. Is there a way to tell the compiler to do it automatically? I suppose not, but just needed to check. I haven't found anything on this topic but still sorry for a duplicate if this is the case...

//I'm not interested in IDE automatic generation features (although I would welcome an advice of that type either), just a compiler...

maybe you should reconsider the derived class.. if it doesn't (seemingly) add value? — AndersK, Aug 02 '13 at 17:48
I don't think so... It overrides the logic of inherited methods. I don't think that that's a bad thing... — Aros, Aug 02 '13 at 22:10

score 3 · Answer 1 · answered Aug 02 '13 at 17:37

3

You could use constructor inheritance (available since C++11)

In the definition of the derived class write:

public:

  using base_class::base_class;

where base_class is your base class.

answered Aug 02 '13 at 17:37

dieram3

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3
7

Note that this is not available in Visual Studio. – Alexandre C. Aug 02 '13 at 17:44
Because it is not C++11 feature complete – dieram3 Aug 02 '13 at 21:07
Hm, that's unfortunate, because it is VS that I use, as you somehow correctly guessed... – Aros Aug 02 '13 at 22:15

score 2 · Answer 2 · answered Aug 02 '13 at 17:42

It really depends on the version of C++ you are targetting.

The first answer is that in C++11 it is easy:

you can either use inheriting constructors
or perfect forwarding

Implementation:

// Inheriting
struct D: B { using B::B; };

// Forwarding
struct D: B {
    template <typename... T>
    D(int a, T&&... args): B(std::forward<T>(args)...), _a(a) {}
    int _a;
};

As demonstrated, while more verbose perfect forward is also more versatile.

If you are stuck in C++03, though, all is not lost: just refactor the base constructor so it takes less parameters (ideally one):

// Before
struct B { B(int a, int b, int c, ...); };

// After
struct Pack { Pack(int a, int b, int c, ...); };
struct B { explicit B(Pack p); };

Easy Peasy.

OK, well... I would come out with the "Pack" option but the reason I asked the question is also for the template use... I wanted to have a template that would override certain method in the class specified by the template argument but there's a problem that all of the classes have a different constructor and therefore i would not be able to make an instance of such it... — Aros, Aug 02 '13 at 22:23

score 0 · Answer 3 · answered Aug 02 '13 at 17:40

0

If you are initializing several parameters in the base constructor and their values are being passed to the constructor, you can overcome this problem without C++11 by implementing setX() functions instead of initializing the values in the constructor.

BaseClass::BaseClass(int arg1, int arg2, int arg3, ...) :
    _member1(arg1),
    _member2(arg2),
    _member3(arg3),
    ...
{}

Change it to:

BaseClass::BaseClass() :
    _member1(0),
    _member2(0),
    _member3(0),
    ...
{}

void BaseClass::setMember1(int value) { _member1 = value; }

answered Aug 02 '13 at 17:40

Cory Klein

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That is arguably worse, though. 1/ Maintaining invariants is more and more difficult as you add mutation possibilities, also the class may not have a natural default initialization 2/ All call sites now have to call a bunch of `setX` methods 3/ Whereas before you would get a compiler error when adding a new parameter, giving you a chance to find all calling sites; now you won't get anything. Arguably, a single `set` method (multiple arguments) would solve 2/ and 3/, however 1/ would not be solved. – Matthieu M. Aug 02 '13 at 18:06
Arguably, yes. It depends on whether you actually *need* the arguments upon construction or not. I think the solutions above are all better, but this has definitely been a cleaner solution for *some* cases I have encountered. – Cory Klein Aug 02 '13 at 18:10

Automatic c++ constructor creation?

3 Answers3