On would–be const constructors

In this post I would like to discuss the idea of having constructors, or some equivalent, designed to build only objects that are const. My motivating example is an attempt to implement matrices and matrix views. I believe this is a canonical example where temptation to have const constructors appears in const methods of the principal class, the chief context in which one has limited data that suffice to only build a const–restricted object. I will try to be clever implementing them, and then show why it does not really work, and what lessons should be learnt from that exercise.
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Polymorphic objects with value semantics – custom interfaces

polymorphic values

In my previous post I showed how we could provide object handlers, called polymorhic, that were the cure to most of the troubles that follow from using pointers, even smart pointers. Given class b with base class a we could write

polymorphic<a> p1(create<b>(args_to_constructor_of_b));
p1->foo();              // properly calls foo virtual or not
polymorphic<a> p2 = p1; // real deep copy of contained b
const auto p3 = std::move(p1); // real move from p1
p3->mutate();           // doesn't compile, implements ordinary const semantics
b bobj;
p2 = bobj;                 // properly copies bobj of type b into p2
std::cout << p2;        // calls operator<< suitable for b
}              // p2,p3 correctly delete b even with no virtual destructor in a

All this boon has been achieved by keeping and leveraging a second virtual dispatch table inside polymorphic. The remaining question was how to open the interface of polymorphic to using additional configurable interfaces–vtables. I will try to present two solutions to this problem in this post.
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Polymorphic objects with value semantics

(Updated on 27.11.2016 to correct the forwarding through tuples infrastructure, corrected code shows in the post, old version is available in collapsed sections. Update to the follow-up post will be posted in a few days.)

My last article got posted on reddit, quite unexpectedly for me, and in the comments there some people mentioned things they find abominable in c++ or in my coding. That’s great to know someone cares, so I decided to share one thing I personally find abominable, and some ideas how to cope. Please don’t hesitate to express your opinion, as usual, there’s a comment section below…

Too low level unique_ptr

C++11 introduced unique_ptr, which, together with move semantics, finally allowed for a reasonable management of heap-allocated objects. Unfortunately, one was still forced to write std::unique_ptr<A> ua(new A{}), keeping new on the client’s side of things. This was somewhat corrected in C++14, where you can write std::unique_ptr<A> pa = make_unique<A>{}. However, while unique_ptr has its merits in low-level implementations, I think that it is an abomination as far as dealing with objects belonging to class hierarchies is concerned. I say this, because there is no way to make deep copies (unless you hand-craft zilions of boilerplate clone methods all over the hierarchy), you can shoot yourself in the foot if you forget to make your base destructor virtual, the pointer objects have pointer const semantics, there is no immediate way to convert an existing object of one of the classes into the pointer, make_unique is the preferred way of creating the pointer but it is impossible to create a nullptr pointer with it so this case needs special treatment, this has no chance of working: std::cout << *pa, and, finally, you can call .get() on the smart pointer and leak the result (see for instance http://bartoszmilewski.com/2009/05/21/unique_ptr-how-unique-is-it/ for possible problems). I will try to address all these issues in this article. I am not sure, as usual, if somebody hasn’t already invented what I present here, if you know any references, please let me know. For a discussion on OO (aka class hierarchies) vs. value sematics see http://akrzemi1.wordpress.com/2012/02/03/value-semantics/.

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