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Subject: Re: [boost] New libraries implementing C++11 features in C++03
From: Nathan Ridge (zeratul976_at_[hidden])
Date: 2011-11-25 02:06:39

> From: mikhailberis_at_[hidden]
> Subject: Re: [boost] New libraries implementing C++11 features in C++03
> On Fri, Nov 25, 2011 at 3:38 PM, Nathan Ridge <zeratul976_at_[hidden]> wrote:
> >> Stuff I (mikhailberis_at_[hidden]) said...
> >>
> >> Huh? This is a matter of the rules of the programming language and
> >> (dare I say) sane engineering practices.
> >
> > Are you saying it's sane engineering practice to avoid local functions?
> > If so, why don't you avoid C++11 lambdas equally?
> >
> Because C++11 lambdas are function objects I can define in-line where
> I need function objects -- which is what modern C++ prefers over
> function pointers. The STL and every other sanely engineered C++
> library that requires function object callbacks will support function
> objects whether they're defined in-line with C++11 lambdas or as
> hand-rolled function objects using operator() overloading.

And they won't support Boost.Local functions?

> Also, I personally do not see a need for a construct like this:
> auto f = []() { /* do something here */ }
> I say this because there's absolutely 0 point in doing this if I can
> drop the lambda in where I need f *anyway*.

I use this construct frequently in my C++11 projects. It can serve to
make the code more readable. Here's an example of a line of code I
wrote recently. It makes use of Boost.Range in addition to C++11 lambdas:

vector<message> messages;
push_back(messages, message_ids | transformed(lookup_message_by_id)
                                | filtered(newer_than_one_year)
                                | filtered(not_hidden)
                                | filtered(satisfies_user_preference));

The arguments to the transformed and filtered adaptors are all C++11
lambdas defined immediately above these lines. How do you think this
code would look if I instead dropped the lambda definitions inline?
Or would you have wasted extra lines and time defining these little
one-or-two-liner functions that are not used anywhere else, out of line?

> >> Did you really mean that C++ shouldn't stand in your way when you want
> >> to write Pascal code?
> >
> > I don't know Pascal so I can't comment on that. I am saying that C++
> > shouldn't stand in my way when I want to make design choices like making
> > one function local to another versus making them "equals" by putting them
> > in the same scope.
> >
> What's the problem with making them equals? They're just functions.

Suppose you have a namespace N, with two classes N::A and N::B. Suppose you
have a function f() that is used only in the implementation of A, and has
nothing to do with B. Is it better to put this function inside namespace N,
where it is visible by both A and B, or inside the class A, where it is
only visible by A?

Now repeat this argument, substituting a class for namespace N, and two
methods for A and B, and you have a use case for local functions.

> >> I'm sorry but this logic is just broken. You're saying "I want local
> >> functions in C++ but I don't want it as a language extension". This
> >> makes no sense at all.
> >
> > I do want it as a language extension. We have it as a language extension,
> > it's called C++11 lambdas. But let's let the poor guys who can't use C++11
> > yet have an approximation of it, too!
> Then propose/implement it as a language extension, or keep the current
> implementation and use it -- nobody's stopping you or anybody from
> doing that. Putting it in Boost is what I object to "just because".

Perhaps I'm not understanding what you mean by a "language extension".

Are you suggesting I propose it for the next standard of C++? That would
be silly, as C++11 already has a solution (lambdas), and the whole point
is to make the feature accessible to people who can't use C++11 yet.

Or are you suggesting that I propose a compiler-specific language
extension for my compiler? That will not fly in an environment where
one's code has to run of many different compilers, and in any case
it goes completely against the spirit of having a language standard
in the first place.

> >> > Otherwise, you have to accept that library writers face a trade-off
> >> > between the user-friendliness of error messages, and the expressiveness,
> >> > terseness, and power obtained by extensive use of advanced techniques
> >> > such as template metaprogramming. There is no one right answer to
> >> > this tradeoff, and it is good for users to have different alternatives
> >> > available to them.
> >>
> >> Sure. So what's the point again? I don't see how this paragraph is
> >> relevant to the discussion.
> >
> > You asked:
> >
> >> So again, how does broken code using any library become a basis for
> >> whether the library is a good library for inclusion in Boost? It just
> >> seems silly to me.
> >
> > Since dealing frequently with errors is a reality of programming, and we
> > cannot rely on compilers to substantially improve the readability of TMP
> > error messages in the short term, a library that solves a similar problem
> > to an existing library, but using different techniques that give rise to
> > more user-friendly error messages, adds value, and therefore can reasonably
> > be considered for inclusion in Boost.
> >
> No, sorry -- just having "prettier error messages" isn't a good
> measure for inclusion in Boost. You're going to have to try harder
> than that to make a case for a library.

So you admit that bad errors messages are a problem that users run into
frequently, and that we're not likely to see a solution from the compiler side
any time soon... but you say a library that solves this problem is not
important enough for Boost?


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