What's the difference between "STL" and "C++ Standard Library"?

Question

Someone brought this article to my attention that claims (I'm paraphrasing) the STL term is misused to refer to the entire C++ Standard Library instead of the parts that were taken from SGI STL.

(...) it refers to the "STL", despite the fact that very few people still use the STL (which was designed at SGI).

Parts of the C++ Standard Library were based on parts of the STL, and it is these parts that many people (including several authors and the notoriously error-ridden cplusplus.com) still refer to as "the STL". However, this is inaccurate; indeed, the C++ standard never mentions "STL", and there are content differences between the two.

(...) "STL" is rarely used to refer to the bits of the stdlib that happen to be based on the SGI STL. People think it's the entire standard library. It gets put on CVs. And it is misleading.

I hardly know anything about C++'s history so I can't judge the article's correctness. Should I refrain from using the term STL? Or is this an isolated opinion?

Solution 1

The "STL" was written by Alexander Stepanov in the days long before C++ was standardised. C++ existed through the 80s, but what we now call "C++" is the language standardised in ISO/IEC 14882:2014 (and earlier versions, such as ISO/IEC 14882:2011).

The STL was already widely used as a library for C++, giving programmers access to containers, iterators and algorithms. When the standardisation happened, the language committee designed parts of the C++ Standard Library (which is part of the language standard) to very closely match the STL.

Over the years, many people — including prominent book authors, and various websites — have continued to refer to the C++ Standard Library as "the STL", despite the fact that the two entities are separate and that there are some differences. These differences are even more pronounced in the upcoming new C++ standard, which includes various features and significantly alters some classes.

The original STL is now often called "an implementation of the C++ Standard Template Library" (rather backwards to actual history!), in the same way that your Microsoft Visual Studio or GCC ships an implementation of the C++ Standard Library. But the "Standard Template Library" and the "Standard Library" are not the same thing.

The battle is about whether the current Standard Library should be called "the STL" in whole or in part, and/or whether it matters what it's called.

For "STL"

There is a school of thought that says that everybody knows now that "STL" means the standard library, just as everybody now knows that "C++" is the ISO-standardised language.

It also includes those who believe that it doesn't really matter as long as all parties understand what is being talked about.

It's a term made even more prevalent by the nature of the beast, much of which makes heavy use of the C++ feature known as "templates".

For "C++ Standard Library" (or stdlib)

However, there is another school of thought — to which I subscribe — that says that this is confusing. People learning C++ for the first time do not know this distinction, and may not notice small language differences.

The author of that article has numerous times encountered people who believe that the entire C++ Standard Library is the STL, including features that were never part of the STL itself. Most vocal proponents of "the STL", in contrast, know exactly what they mean by it and refuse to believe that not everybody "gets it". Clearly, the term's usage is not uniform.

In addition, there are some STL-like libraries that are in fact implementations of the original STL, not the C++ Standard Library. Until recently, STLPort was one of them (and even there, the confusion abounds!).

Further, the C++ Standard does not contain the text "STL" anywhere, and some people habitually employ phrases like "the STL is included in the C++ Standard Library", which is plain incorrect.

It's my belief that continuing to propagate the usage of the term in this way will just lead to the misunderstanding going on forever. Alas, it may be entirely counter-productive to attempt to change things, even if it's supposed to be for the better. We may just be stuck with double-meanings forever.

Conclusion

I appreciate that this post has been a little biased: I wrote the article you linked to. :) Anyway, I hope this helps to explain the battle a bit better.

Update 13/04/2011

Here are three perfect examples of someone who is using "the STL" to refer to the entire C++ Standard Library. It continues to baffle me that so many people swear blind that nobody ever does this, when it's plain to see almost on a daily basis.

Solution 2

There is no one answer that's really correct. Alexander Stepanov developed a library he called STL (working for HP at the time). That library was then proposed for inclusion in the C++ standard.

That basically "forked" development. The committee included some parts, rejected others completely, and redesigned a few (with Alexander's participation). Development of the original library was later moved to Silicon Graphics, but continued separately from the C++ standard library.

After those pieces were added to the standard library, some other parts of the standard library were modified to fit better with what was added (e.g., begin, end, rbegin and rend were added to std::string so it could be used like a container). Around the same time, most of the library (even pieces that were completely unrelated) were made into templates to accommodate different types (e.g., standard streams).

Some people also use STL as just a short form of "STandard Library".

That means when somebody uses the term "STL" they could be referring to any of about half a dozen different things. For better or worse, most people who use it seem to ignore the multiplicity of meanings, and assume that everybody else will recognize what they're referring to. This leads to many misunderstandings, and at least a few serious flame-wars that made most of the participants look foolish because they were simply talking about entirely different things.

Unfortunately, the confusion is likely to continue unabated. It's much more convenient to refer to "STL" than something like "the containers, iterators, and algorithms in the C++ standard library, but not including std::string, even though it can act like a container." Even though "C++ standard library" isn't quite as long and clumsy as that, "STL" is still a lot shorter and simpler still. Until or unless somebody invents terms that are more precise (when necessary), and just as convenient, "STL" will continue to be used and confusion will continue to result.

Solution 3

The term "STL" or "Standard Template Library" does not show up anywhere in the ISO 14882 C++ standard. So referring to the C++ standard library as STL is wrong. The term "C++ Standard Library" or "standard library" is what's officially used by ISO 14882:

ISO 14882 C++ Standard:

17 - Library introduction [lib.library]:

1. This clauses describes the contents of the C++ Standard Library, how a well-formed C++ program makes use of the library, and how a conforming implementation may provide the entities in the library.

...

STL is a library originally designed by Alexander Stepanov, independent of the C++ standard. However, some components of the C++ standard library include STL components like vector, list and algorithms like copy and swap.

But of course the C++ standard includes much more things outside the STL, so the term "C++ standard library" is more correct (and is what's actually used by the standards documents).

Solution 4

I've made this same argument recently, but I believe a little tolerance can be allowed. If Scott Meyers makes the same mistake, you're in good company.

Solution 5

From the GNU Standard C++ Library (libstdc++) FAQ:

---

The STL (Standard Template Library) was the inspiration for large chunks of the C++ Standard Library, but the terms are not interchangeable and they don't mean the same thing. The C++ Standard Library includes lots of things that didn't come from the STL, and some of them aren't even templates, such as std::locale and std::thread.

Libstdc++-v3 incorporates a lot of code from the SGI STL (the final merge was from release 3.3). The code in libstdc++ contains many fixes and changes compared to the original SGI code.

In particular, string is not from SGI and makes no use of their "rope" class (although that is included as an optional extension), neither is valarray nor some others. Classes like vector<> were from SGI, but have been extensively modified.

More information on the evolution of libstdc++ can be found at the API evolution and backwards compatibility documentation.

The FAQ for SGI's STL is still recommended reading.

---

FYI, as of March 2018 even the official STL web site www.sgi.com/tech/stl/ is gone.

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Comments

[Lightness Races in Orbit]

+1. Although, in the STL, there was no std namespace (IIRC).

[Lightness Races in Orbit]

If there are any opposing views that I have not mentioned, please comment and I will be more than happy to add them to the answer (even though I would disagree with them).

[Jerry Coffin]

@Tomalak Geret'kal: That sort of works for the whole standard library, but even there you run into a problem: when I say (for example): "X is in stdlib", am I referring to what the standard requires all conforming compilers to supply, or what a particular compiler happens to supply?

[Lightness Races in Orbit]

@Jerry: The former; that's what "std" stands for. :)

[Jerry Coffin]

@Tomalak Geret'kal: The point wasn't to ask how you'd use it, but to point to the fact that different people use the term in different ways (and one of the points of adding namespaces was to allow vendors to define extra names safely...)

[Lightness Races in Orbit]

@Jerry: Not sure where namespaces come into it. Compliant implementations do not add anything to namespace std. I was talking about the "std" in "stdlib", which stands for "standard". I think it's pretty clear what that means!

[Jerry Coffin]

@Tomalak Geret'kal: Of course, an implementation allowed to add things to std (§17 lists several classes of reserved identifiers). It's not at clear what "standard" means though -- Microsoft has (recently) worked at clarifying what parts of the "standard" library are in the standard, but gcc generally does a lot less to differentiate between "mandated by the standard", and "provided in our library."

[Lightness Races in Orbit]

@Jerry: I really don't think that it's a stretch to expect someone to read the adjective "standard" as "this is in the standard". Meanwhile, 17.4.3.1/1 is quite clear that adding things to namespace std is UB except in some specific named cases: these cases of addition are named in the standard, and thus are still perfectly compliant; "standard" would still apply.

[sbi]

@Tomalak & @Mark: Actually Scott doesn't, -1 from me. The book is indeed about the STL in the meaning of "the parts of the std lib stemming from Stepanov's library". Take the time to look through the books TOC. The only thing outside the original STL I could find was std::string, and that had been equipped to be a full-blown STL container.

[Lightness Races in Orbit]

@sbi: You've evidently misinterpreted my position. I would not use "STL" in the manner that Scott does. Please read my answer.

[Mark Ransom]

@Tomalak, I would not use "STL" in that manner either, although I've probably been guilty of it in the past. I just don't think it's worth beating people up over.

[Lightness Races in Orbit]

That I can respect. I get most arsy about people who refuse to acknowledge the potential ambiguity at all. :)

[Kragen Javier Sitaker]

The Standard Template Library was not only created by Stepanov and Lee, but they were also working at the Software Technology Laboratory at the time.

[Kragen Javier Sitaker]

At the time, most C++ compilers didn't implement namespaces. In fact, I don't know if they were even in the standard.

[Kragen Javier Sitaker]

The history of SGI and HP here is backwards. Stepanov was at HP before he was at SGI.

[Lightness Races in Orbit]

Had someone today asking what header std::iota is in, because he couldn't get it to work. It's an SGI non-standard extension, that they call "standard" when it suits them because it's "the STL", and everybody knows that "the STL" is part of the C++ Standard Library, right? And it was introduced in C++0x, but is not available in C++03. Grrr.

[Lightness Races in Orbit]

Another example of someone talking about the STL as if it were a synonym for the whole stdlib.

[Lightness Races in Orbit]

@Kragen: Well, there was no standard.

[Kragen Javier Sitaker]

Oh. Well, there were some books by Stroustrup, but I suppose that's not quite the same thing, is it?

[Mihaela]

And MS STL is maintained by Stephan T. Lavavej, aka STL.

[Lightness Races in Orbit]

Apparently even STL himself uses "STL" to mean the whole damn lot

[Jerry Coffin]

I just reread the comments here and thought it was worth adding one more (admittedly minor) point: I think it is a bit of a stretch to expect "standard" to always mean "this is in the standard". In particular, many C++ programmers used the phrase "standard library" long before there was a standard. Lest one think they were referring to the library in the C standard, I'll point out that the same was true of C programmers well before there was (even a draft of) a C standard.

[Sander De Dycker]

You'd also be in the company of Bjarne Stroustrup - ref. eg. stroustrup.com/DnE2005.pdf : "the STL (the "Standard Template Library"; that is, the containers and algorithm framework of the ISO C++ standard library)"

[codenheim]

Bjarne Stroustrup specifically differentiates the STL from other parts of the Standard Library in the "canon", The C++ Programming Language 4th Edition.

[Peter K]

Interestingly, as noted by Stepanov himself, the very first version of STL was written in Scheme (a Lisp dialect), the next was in Ada iirc and in C++ it is actually version 3.

[Kuba hasn't forgotten Monica]

Would you then be a proponent of burninating the stl tag and establishing a tag like cppstdlib or somesuch?

[Lightness Races in Orbit]

@KubaOber: In my perfect hypothetical reality? Yes, absolutely. stdlib would do nicely. In the real world? Nah, probably not. More trouble than its worth as long as this misconception is so horribly widespread. Or perhaps it would help...? Hmm, dunno. I do know that I wouldn't get much support for it, so whatever. :)

[thor]

@mrjoltcola 'Bjarne Stroustrup specifically differentiates the STL from other parts of the Standard Library in the "canon"' -- Stroustrup is a /was a professor somewhere, and I guess there is an obligation in that profession to acknowledge the use of others' work, ie the inclusion of STL into the C++ standard. Though he allegedly removed half of the content so that it can be accepted by the conservative standard committee.

[Curious]

@LightnessRacesinOrbit I really like your answer!

[Galik]

Every prominent author and every important text on C++ refers only to the containers and algorithms parts of the standard library as the STL which gives the usage proper legitimacy. The reason being that the STL and its philosophy are revolutionary and need to be referred to as a whole for purposes of Computer Science and Education. Other parts of the Standard Library are not properly referred to as the STL and surely it should be our job to correct people rather than add to the confusion?

[Lightness Races in Orbit]

@Galik: Correcting people is what I am trying to do. Mixing terms is what confuses people.

[Galik]

@LightnessRacesinOrbit I did spend some time looking for a book that used the term STL to refer to something other than the containers, iterators and algorithms portion of the Standard Library and I was unable to find one. We all know that Straustrup, Mayer, Sutter and others use the term as I have described. I have yet to find an author from the Recommended Books list that doesn't.

[Alex]

@LightnessRacesinOrbit I think you contradict yourself in your answer. First you say "there is a school of thought that says that everybody knows now that "STL" means the standard library". And then "The author of that article has numerous times encountered people who believe that the entire C++ Standard Library is the STL, including features that were never part of the STL itself". That's the same thing. My point is, the origin of the term STL has only historical significance. People learning C++ now don't need to know what the STL was back then. It's just an alias for the stdlib now.

[Lightness Races in Orbit]

@Alex: How is that a contradiction? I know it's the same thing, and never claimed otherwise. I simply referred to the same group of people at difference places in my answer.

[Alex]

The contradiction I see is that if we simplify your statements we get this: "Some people believe X to be true. I do not agree because I have encountered many people who believe X to be true", and then some arguments follow.

[Alex]

Though you are probably right that there is still confusion. A better way to put what I wanted to say is, there could be no confusion if we just agree to call the standard library STL :)

[Lightness Races in Orbit]

@Alex: Indeed, if every C++ developer sat down and came to an agreement, then there would no longer be disagreement. But that hasn't happened, and likely never will :)

[Justin Time - Reinstate Monica]

Compounding the issue is that the C++ standard library is a conglomeration of modified complete and/or partial versions of other libraries, and/or unique libraries based on those aforementioned other libraries, such as the C standard library, the STL, and Boost. It can be helpful to have a term to refer to each of these groups in the standard library, but the main issue is that no one actually explains the terms and their origins.

[Justin Time - Reinstate Monica]

It's obvious from the name that the C compatibility headers contain the interfaces for the "C standard library" component of the C++ standard library, and the part from Boost doesn't seem to have its own subgroup name (as far as I'm aware, at least), but most tutorials and other sites don't explain that "the STL" is specifically the portion of the C++ standard library that's based on the original STL, which leads to a lot of confusion when they use the term STL (since, without explanation, it just looks like an abbreviation for STandard Library ).

[Johannes Schaub - litb]

Do you have this account stackoverflow.com/users/560648/lightness-races-in-orbit and also stackoverflow.com/users/18771/tomalak ? Is one of them inactive?

[Lightness Races in Orbit]

@JohannesSchaub-litb: No, the other Tomalak is a German gentleman, with no relationship to myself. It's just a co-incidence. Why do you ask?

[Johannes Schaub - litb]

@LightnessRacesinOrbit haha, I thought I only know a single Tomalak. But now I realize they are actually two person. Thanks :)

[Lightness Races in Orbit]

@JohannesSchaub-litb: Yes unfortunately the phrase "there is only one Tomalak" is not actually true

[cp.engr]

Am I misreading here? This answer seems to say that the "STL" literally only means the original implementation by SGI of C++ templates, iterators, algorithms, etc. However, here is a paper by Stroustroup, et al, which uses the term - though unfortunately doesn't define it. I guess they mean some subset of any iteration of the C++ standard library... open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3351.pdf

[Lightness Races in Orbit]

@cp.engr You must be misreading because most of this answer tells how there are different opinions/approaches/points of view on the topic. You've found one of them. Not sure what else to say about it really

[cp.engr]

@LightnessRacesinOrbit, the answer says "some people habitually employ phrases like 'the STL is included in the C++ Standard Library', which is plain incorrect." And yet they language creators appear to be using it in this way. I agree that there is inconsistency, and that it is not clearly defined (AFAIK), but it seems to me the only way to rectify the situation would be to escalate the issue, and get the language/library maintainers to make a statement on it.

[Lightness Races in Orbit]

@cp.engr Yes, the language creators are often as guilty as anyone else. That's one reason that escalating it will be of little purpose: this argument has been beaten to death and it's not a good use of time to continue fighting it.

[Marc.2377]

github.com/microsoft/STL: MSVC's implementation of the C++ Standard Library. Sigh.

[Lightness Races in Orbit]

@Marc.2377 Sadface :(

[Nicol Bolas]

While it is true that parts of the STL were incorporated into the C++98 standard library, it is not true that STL, IOStream, and C-stuff are the only things in the standard library. Even the C++98 standard library.