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From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2021-02-22 09:08:04

niedz., 21 lut 2021 o 16:18 Niall Douglas via Boost <boost_at_[hidden]>

> On 20/02/2021 22:15, Andrzej Krzemienski via Boost wrote:
> > Now, I choose to use the DI-library (this is where I have troubles with
> > understanding: why would I want to do that?). I get the following result:
> >
> > ```
> > int main()
> > {
> > State s = di::make_injector().create<State>(0, 0, 2, 2, 1, 1);
> > }
> > ```
> > And now I get back to the situation where I am passing six ints and can
> > easily confuse which int represents what.
> >
> > I am pretty sure I am now unfairly mischaracterizing the library. But
> this
> > is what I get from the motivation and tutorial sections, and the
> > explanations I have seen so far. You cannot see this behavior in the
> > tutorial example that is using class `app`, because most of the types in
> > there are default-constructed or constructed from values that were
> > themselves (perhaps recursively) default-constructed. So it looks to me
> > that in order to appreciate this library, I have to make most of my types
> > default-constructible.
> >
> > At which point am I missing something?
> I think the other answers answered most of your points, so I'll add to
> those only two:
> 1. It is hard to see the wood from the trees for C++ devs because C++
> uses dependency injection all over the place in its standard library,
> its standard idioms, and commonplace practice. The classic example is
> Allocators:
> std::vector<int, MyAllocator<int>>
> std::vector delegates the responsibility of allocating, creating,
> destroying and deallocating arrays of int to the user supplied service
> MyAllocator. This is literally Dependency Injection, and it is so
> commonplace in C++ as a design pattern that we don't call it that.
> Purists from other languages will point out that std::vector knows the
> *concrete type* of the delegated service. But other than that it has no
> idea what the implementation is, other than it promises the side effects
> guaranteed by the Allocator Concept.
> If you remove knowledge of _concrete_ type, and replace that with
> _abstract_ type, you get what we would call a visitor class in C++ -
> basically a bunch of pure virtual functions. This corresponds to
> std::pmr::vector<int> whereby the concrete implementation type for
> allocating, creating, destroying and deallocating arrays of int is no
> longer known to vector, only that there is an abstract API which
> promises the side effects guaranteed by the Allocator Concept.
> Now, imagine that your program has some memory problem, and it only ever
> uses std::vector<int>. Thanks to the Dependency Injection, you get two
> degrees of freedom:
> a) If you chose the concrete type MyAllocator, through a recompile you
> can inject a mock MyAllocator for testing and debug.
> b) If you chose the abstract type MyPmrAllocator, you don't need to
> recompile your code, you simply swap the MyPmrAllocator instance you
> construct at the beginning which is injected into all your classes with
> a mock MyPmrAllocator for testing and debug.
> Option a) is tractable in codebases < 100M lines of code. Option b)
> becomes worth it in codebases > 100M lines of code. Note that as my
> codebase grows, I can proactively take a decision to move from degree of
> freedom a) to b) without breaking all my source code i.e. I can choose
> for my runtime to be slower in exchange for radically reduced recompile
> times.

Thanks Niall. Maybe this explains the bad reception of the DI-library. If
injecting dependencies is so natural to C++ and the library documentation
starts from convincing me that I do not know what it is and I am STUPID
(rather than SOLID), then this builds a confusion that makes it more
difficult to conume the rest.

> 2. I just gave a specific example of the value of the two typical forms
> of Dependency Injection in C++, and I'm going to assume it's
> uncontroversial (I actually think it's an exemplar of all that's wrong
> with Allocators, but that is off topic for here).
> Something peculiar about how we typically do Dependency Injection in C++
> is that it's always *specific* and not *generalised*. If we have a
> problem e.g. delegation of memory allocation, we design a _specific_
> dependency injected solution. What we don't do in C++ is design a
> _generalised_ dependency injection solution which is universal (unlike
> say in Java).
> The advantage of a universal DI which exists everywhere is very much
> like why choosing Outcome is better than rolling your own result<T>
> type. Yes anybody can roll their own result<T> type, indeed probably
> most people do. But when library A has resultA<T>, and library B has
> resultB<T>, and library C has resultC<T>, how is a codebase dependent on
> all three libraries supposed to interoperate between those three
> libraries easily?
> Most of Outcome's complexity stems from being friendly to third party
> resultX<T> types. I myself I have deployed Outcome in foreign codebases
> each using their own Result types, and Outcome can (usually) capture all
> of those seamlessly without loss of original information fidelity. Thus
> Outcome becomes "the ring to rule them all", which is its exact value
> proposition and why I would suppose Outcome was accepted into Boost.
> What I would like to see of any Boost.DependencyInjection is the exact
> same "one ring to rule them all" in that it should be easy to integrate
> *any* bespoke third party Dependency Injection mechanism or framework
> into Boost.DependencyInjection such that one can *seamlessly* compose
> library A, library B and library C in a single application, and it all
> "just works".

I am having difficulties with mapping the library-interop offered by
Outcome onto the similar library-interop that would be gained from such a
hypothetical dependency injection library. The only way I can interpret
your words is a situation where one library creates a recipe for injecting
parameters to my class widget and another library uses this recipe to
actually create objects of my type:

// library one:

std::function<app> make_app = [] {
  logger log1{"app.logger.lo"_l};
  logger log2{"app.logger.hi"_l};
  renderer renderer_{"main_renderer"_l};
  view view_{"main_view"_l, renderer_, log1};
  model model_{"main_model"_l, log2}; // note: the other logger
  controller controller_{"main_controller"_l, model_, view_, log2}; //
note: the other logger
  user user_{"main_user"_l, log1};
  return app {"main_app"_l, controller_, user_};

// library two:

void some_fun(std::function<app> make_app)
  app app1 = make_app();;

  app app2 = make_app();;

But did you have something like this in mind when you wrote the above
description? And if so, what is the value added by a dedicated library, if
one can achieve the same foal with std::function? Is the only motivation
that in some subset of cases the library can try to deduce (hopefully
correctly) from the number of arguments, how I wanted to initialize the

It looks like a more adequate name for such a library is "factory", because
what it does is create objects.

Or maybe I am still missing something?


> I'll be frank in saying that I don't believe the current proposed
> Boost.DI does this. Unless I and most other people here can be convinced
> otherwise, my personal current expectation is that the proposed Boost.DI
> will be rejected, but hopefully with ample feedback on what to do for a
> Boost.DI v2, assuming Kris has the stamina and will.
> In my personal opinion, if Boost.DI _can_ do things like seamlessly
> compose arbitrary Allocators, both concrete and abstract, and arbitrary
> other custom bespoke Dependency Injection designs from across the C++
> standard library and the Boost libraries, then its tutorial ought to
> describe that integration just like the end of the Outcome tutorial
> shows three separate, independent, different error handling strategies
> in three separate library dependencies being seamlessly integrated into
> one application with Outcome doing all the donkey work between those
> dependencies.
> I think that if the tutorial demonstrated that seamless composure in
> action, that would be compelling.
> Niall
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