Why std::bind can’t be formally deprecated

http://www.sdowney.org/2017/06/why-stdbind-cant-be-formally-deprecated/

by ingve • 9 years ago

61 31 9 years ago

cjhanks• 9 years ago

I have never seen that idiom used intentionally, I would almost always suspect it to be an accident... but perhaps there are valid use cases in function composition.

The more likely reason it cannot be deprecated... people actively use it and like it.

In either case, nice find.

saurik• 9 years ago

The point being made here is heavily obfuscated to my eye due to the seemingly needless use of std::invoke when working with the lambda cases (which, even if useful for some reason, is not needed to make the point of this article), but when I finally figured out what is being requested all I could think of is "that functionality of std::bind is horrific and surprising and despite going into this article thinking that of course bind solves a slightly different problem to lambdas in a more readable way, you have now convinced me that std::bind should be deprecated ASAP" (though as it came into existence in the same version of C++ as lambda functions, I mostly end up questioning why it ever existed).

Essentially, in order to provide something that sort of feels like lambda functions, but entirely in templates, the placeholder arguments do not end up locked to a single expression--instead of returning something semantically like a function, std::bind returns something that might be better described as a partial syntax tree that happens to be something you can call as a function, at which point it is only then wrapped up as a function and all subexpression parameters are bound to arguments--and so if you bind something and include in it a bound function, that function is not passed as a value like you would expect: its parameter is hoisted and it becomes a call expression in the new bound composite. The author likes this behavior for some reason, but I can't figure out why it would ever be considered to be a feature and it is trivially simulated with lambdas (while the opposite is not true).

detaro• 9 years ago

quotes from the article:

which is likely working by accident, rather than design.
At the time, it made some sense. But it makes reasoning about bind expressions difficult. Don’t do this.
Example of fail code

Where do you get the idea the author likes it?! To me it reads more "it's stupid, but this edge case can't be trivially replaced and thus we can't just rip it out"

comex• 9 years ago

I don’t think it’s that the author likes the behavior, but that the author has in mind some rule where features can’t be deprecated unless it’s possible to write a purely mechanical migration tool. Not sure where that rule comes from or why it exists.

Karellen2• 9 years ago

Yeah, that's the biggest question that came to my mind too. Why can't you deprecate something that needs fixing up manually?

slezyr• 9 years ago

you have now convinced me that std::bind should be deprecated ASAP

Once made a mistake using std::bind and got 500 lines of errors.

prance• 9 years ago

instead of returning something semantically like a function, std::bind returns something that might be better described as a partial syntax tree that happens to be something you can call as a function

Indeed. std::bind seems to work more like a Lisp-ish macro than a function in this regard.

rileymat2• 9 years ago

An example from the article:

auto f1 = std::bind(f, 2, n, _1);

f1(10); // calls f(2, 5, 10);

auto l1 = [ p1 = 2, p2 = n ](auto _1) { return std::invoke(f, p1, p2, _1); };

l1(10);

It seems to me std::bind is a lot cleaner syntactically.

plorkyeran• 9 years ago

That's a literal translation of what std::bind() is doing that no one would actually write. `[n](auto a) { return f(2, n, a); }` is a more realistic way to write that lambda.

jlarocco• 9 years ago

bind() is still the cleaner way to write it.

Seeing std::bind tells me that the intention is to create a function that does nothing but call a second function with certain parameters fixed.

A lambda expression is far more general, so when I see it I have to think about what it's doing. There's also a subtle implication that the author chose the more general technique for a reason. It'll raise the questions, "Why isn't this using std::bind()?", "Is there something else going on here?", etc.

Off hand, I can't think of a reason why the bind would be any less efficient than the lambda.

makecheck• 9 years ago

And cleaner still is to not obfuscate things to this degree, no matter what syntax is involved.

It certainly violates the “write once, read many times” principle: no matter how many times I look at something like this, I have to study the whole thing just to figure out what is even happening! Whereas, seeing "void g(x) { f(2, 5, x); }" is perfectly clear.

It certainly violates the “premature optimization” principle: in exchange for some unproven benefit, the compiler does gymnastics to bind arguments and adds compile time instead of just paying a little more in space and runtime to define a 2nd function that calls a 1st function in a completely obvious way.

It also qualifies as over-design. Even if a single binding is considered OK (and that’s a big “if”), there is every reason to expect that during normal maintenance someone will need to keep adding more. Then you have several lines of template-ese. A simple function is something that anyone can extend, whereas it’s not clear most engineers will even know how to maintain the binding version.

solidsnack9000• 9 years ago

I'm not sure how to read your comment. Are you saying not to use lambdas or closures, and just to define new functions?

gpderetta2• 9 years ago

It’s sub-optimal because it has to type-erase, where a lambda allows the compiler to do much better

Bind doesn't have to type erase nor in fact does on any of the implementations I'm familiar of.

kevingadd• 9 years ago

Am I misunderstanding or does this example from the post:

    auto g1 = std::bind(g, _1);
    auto f2 = std::bind(f, _1, g1, 4);
    f2(10); // calls f(10, g(10), 4);

Imply that function values in a bind's argument list are silently converted into function invocations? Doesn't this mean you can't pass functions around? Why would you break fundamental assumptions about the type system like this?

The way placeholder values seem to silently propagate across nested binds is also super gross. This stuff doesn't seem composable at all and it seems like it shouldn't have made it into any standard.

evincarofautumn• 9 years ago

No, if I understand your question. You can pass a function pointer or function object via bind, it’s just that the behaviour may be surprising if it’s the result of a bind.

gpderetta2• 9 years ago

Bind is a stripped down version of boost.lambda [1] which is a generalized DSL for arbitrary expressions. Boost.lambda had, among other things, ways to protected nested binds calls and introduce new scopes for placeholders.

Adding special magic for nested binds (which is used about 0% of the time) was, in hindsight, a big mistake, but when bind was designed and added to the standard, lambda expressions were many years away, so any syntactic sugar that would ease the pain of higher order programming was very welcome.

[1] Although I don't recall who came first. Likely boost.bind was first, then boost.lambda improved on it and some of the improvements were fed back into bind. In fact boost.bind still has some features that std::bind doesn't have (and viceversa).

eej71• 9 years ago

The link does not work...

Rexxar• 9 years ago

Web archive link: https://web.archive.org/web/20170703000145/http://www.sdowne...

gpvos2• 9 years ago

Wow, I thought that the Wayback machine had a half-year delay in making pages available.

Avery3R• 9 years ago

I'm getting a 404

gpvos2• 9 years ago

Yes, it looks like the article has been deleted. Google cache still has it, for now: http://webcache.googleusercontent.com/search?q=cache:JSFVLgB...

Why std::bind can’t be formally deprecated

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