Response to Dr. Stroustrup's Memory Safety Comments

The NSA recently published a Cybersecurity Information Sheet about the importance of memory safety, where they recommended moving from memory-unsafe programming languages (like C and C++) to memory-safe ones (like Rust). Dr. Bjarne Stroustrup, the original creator of C++, has made some waves with his response.

To be honest, I was disappointed. As a current die-hard Rustacean and former die-hard C++ programmer, I have thought (and blogged) quite a bit about the topic of Rust vs C++. Unfortunately, I feel that in spite of the exhortation in his title to "think seriously about safety," Dr. Stroustrup was not in fact thinking seriously himself. Instead of engaging conceptually with the article, he seems to have reflexively thrown together some talking points -- some of them very stale -- not realizing that they mostly are not even relevant to the NSA's Cybersecurity Information Sheet, let alone a thoughtful rebuttal of it.

Fortunately, he does eventually discuss his own ideas of how to make C++ memory safe -- in the future. If these ideas are implemented well, it will make C++ a safe programming language as the NSA's Cybersecurity Information Sheet has defined it. But given that they are currently just proposals in an early stage, it's unfair of him to expect the NSA to mention them when advising people on what programming language to use. C++ has been an unsafe language for a long time. Maybe someday that will change, but we'll believe it when we actually see it.

But before I discuss that, I'd like to rebut and discuss my disappointment at the talking points he uses earlier in his response, because I think they unfairly frame the debate, shield C++ from legitimate and important criticism, and slander memory-safe programming languages and downplay memory safety as a concept, even though it's very important.

Multiple Types of Safety?

One of the most interesting and conceptually relevant points that Dr. Stroustrup harps on is that memory safety is not the only type of safety:

Also, as described, “safe” is limited to memory safety, leaving out on the order of a dozen other ways that a language could (and will) be used to violate some form of safety and security.

This might technically be true -- it's not entirly clear what other forms of "safety" he's talking about -- but it's misleading. Memory unsafety is not just one of a dozen equally important forms of "unsafety." Rather, memory unsafety is by far the biggest source of security vulnerabilities and instability in memory unsafe programming languages -- estimates as high as 70 percent in some contexts.

A 70% decrease in security vulnerabilities is worth committing significant resources towards. Memory safety on its own is worth writing a Cybersecurity Information Sheet about, and it is the area where C++ has the most serious deficits. Given that, this feels like a car manufacturer whose cars do not provide air bags responding to a government advisory not to buy the C++ cars by saying "What about other types of safety? By talking just about air bags, the government is clearly not thinking seriously about safety." Sure, there's other types of safety features besides air bags (or memory safety), but air bags are still important!

So, Dr. Stroustrup, what about memory safety in C++? Shouldn't C++ have memory safety? Are you saying it's not important, especially when all of these other programming languages have it?

Of course, he doesn't go into detail about other types of safety, which is telling. Of course, it's because C++ doesn't really have the advantage in any of them. For example, Rust also has a lot of mechanisms for thread safety and type safety, intimately connected with its memory safety mechanisms, and baked into the design of Rust in a way that would be next to impossible to retrofit into another programming language.

And, when you read later on about the "safety profiles" in the C++ Core Guidelines that he makes such a big deal about, most of the focus there is also about memory safety.

Petty Irrelevancies

Let's look at some of the other points he makes.

That specifically and explicitly excludes C and C++ as unsafe.

C++ does not enforce memory safety as a feature of the programming language. This may change in the future (as Dr. Stroustrup discusses), but is the current state of things. Dr. Stroustrup tries to downplay this, but is not convincing.

As is far too common, it lumps C and C++ into the single category C/C++, ignoring 30+ years of progress.

Writing "C/C++" to mean "C and C++" is considered a faux pas among C++ programmers, and among C programmers as well, because it is seen as asserting that these two programming languages are near-identical when there are in fact major differences between them. By pointing out that the NSA does this, Dr. Stroustrup is trying to make them look like they don't know what they're talking about, just because they used a "/" character instead of the word "and."

He's reading too much into the orthography and the NSA's failure to use insider shibboleths of the programming languages they're trying to criticize. Outside of the "C" and "C++" communities, "C/C++" is a fairly common way to refer to the two related programming languages.

And that's the most relevant thing here: C and C++ are indeed related programming languages, and they have a lot in common: They are both compiled programming languages with a focus on performance, and they are (very relevantly) both not particularly focused on guaranteeing memory safety. C and C++ have a substantial common subset, with many memory unsafe features that are popular with programmers, perhaps even more popular because they work similarly in both programming languages. For the purposes of this document, it's often the features that C and C++ have in common that are the problematic ones, so it makes sense for the NSA to lump them together.

While there might be 30+ years of divergence between C and C++, none of C++'s so-called "progress" involved removing memory-unsafe C features from C++, many of which are still in common use, and many of which still make memory safety in C++ near intractible. Sure, new features in C++ have been added that (in some but by no means all cases) do not make it as easy to corrupt memory, but the bad old features are not in any real way being phased out: They are not guarded by any special opt-in syntax, nor in many cases do they result in warnings. Given that, the combined set of features is as strong as its weakest link.

Unfortunately, much C++ use is also stuck in the distant past, ignoring improvements, including ways of dramatically improving safety.

This is a common C++ talking point, but it doesn't help Dr. Stroustrup's position as much as he thinks it does.

He's trying to talk up how much C++ has improved, especially in the last 11 years -- and it has indeed improved. New ways of writing C++, emphasizing relatively new features, can indeed result in more reliable C++ code with less memory corruption.

But unfortunately, this talking point just serves to remind us that these old memory-unsafe features are still in common use. When someone says their project is written in Rust, we can guess that it likely uses only the safe features (including using standard library functions that use unsafe internally -- that truly doesn't count as unsafe), or maybe uses the unsafe features when absolutely necessary. But when someone says their project is written in C++, by Dr. Stroustrup's own admission, there's a high likelihood that it uses old features "stuck in the distant past, ignoring ... ways of dramatically improving safety." This is also a reason to avoid C++.

However, I would also contest his claim about these new features. Memory safety isn't just an absence of memory corruption, but a reliable method for ensuring the absence of memory corruption. "Using new features" isn't good enough. Even if using the new features in preference to the old ones were a guarantee of memory safety -- which it isn't, they're less memory corrupting but not truly memory safe -- the presence of the old ones would still cause problems. You would need some mechanism to ensure that the new features were only used safely, and that the old features were not used, and no such mechanism exists, at least not in the programming language itself. Someone who remembers the old features can always still slip up and use one by accident.

Static Analysis: Not Good Enough

Dr. Stroustrup points out that he's been working very hard on improving memory safety in C++, for a very long time:

After all, I have worked for decades to make it possible to write better, safer, and more efficient C++. In particular, the work on the C++ Core Guidelines specifically aims at delivering statically guaranteed type-safe and resource-safe C++ for people who need that without disrupting code bases that can manage without such strong guarantees or introducing additional tool chains.

Unfortunately, it's not done. The key word here is, of course, "aims." The next sentences admit that this feature is not in fact available:

For example, the Microsoft Visual Studio analyzer and its memory-safety profile deliver much of the CG support today and any good static analyzer (e.g., Clang tidy, that has some CG support) could be made to completely deliver those guarantees....

For memory safety, "much of" is not really good enough, and "could be made" is practically worthless. Fundamentally, the point is that memory safety in C++ is a project being actively worked on, and close to existing. Meanwhile, Rust (and Swift, C#, Java, and others) already implements memory safety.

It's worse than that, though. What Dr. Stroustrup is trying to downplay is that this involves using static analyzers, considered separate from the programming language, something the NSA's original article also discusses. Theoretically, if a static analyzer could be used to guarantee memory safety, that could be just as reliable as a programming language that does it. An engineering team could have a policy that all code must pass this static analysis before being put into production.

But unfortunately, human nature is more fickle than that. If it's not built into the programming language, it's going to get skipped. If a vendor says their software is written in C++, or if an engineer takes a job in C++, how will they know that these static analyzers will in fact be used? A programming language that takes memory safety seriously doesn't provide it as an optional add-on that most people will simply ignore.

But All The C++ Code!

The end of the last quote provides a common talking point in Rust vs C++ arguments:

[Static analyzers] could be made to completely deliver those guarantees at a fraction of the cost of a change to a variety of novel “safe” languages.

Besides the laughably condescending matter of calling Java (which first appeared in 1995), C# (first appeared in 2000), and Ruby (first appeared in 1995) "novel," this is a jab at a common trope that (some immature) Rust programmers go around demanding that people rewrite their projects in Rust (please don't do this!), and an attack on the idea that all code can be written in safe programming languages, given the large body of existing work in unsafe programming languages.

This is a bit of a straw man in this context. The NSA article that Stroustrup is responding to addresses that switching existing codebases might be expensive, even prohibitively so, saying:

It is not trivial to shift a mature software development infrastructure from one computer language to another. Skilled programmers need to be trained in a new language and there is an efficiency hit when using a new language. Programmers must endure a learning curve and work their way through any “newbie” mistakes. While another approach is to hire programmers skilled in a memory safe language, they too will have their own learning curve for understanding the existing code base and the domain in which the software will function.

It then follows this up immediately with an explanation of how tools like static analyzers can be used as a back-up plan for improving memory safety in memory unsafe programming languages -- exactly what Dr. Stroustrup discusses. He's criticizing this NSA document, implying it is not thinking "seriously," while fundamentally making a point that they already made for him.

Of course, this is a terrible endorsement of C++. It's far from ideal to have to use add-on tools to work around a language's flaws. Coming from Dr. Stroustrup, it reads more like a brag that his programming language has locked everyone in than a defense of why C++ is good. Or else, it's an admission that other programming languages should be used for new projects, and that C++'s fate is now to gradually fade like the elves from Middle Earth.

But he's also overstating his case. As I mention before, safe programming languages have existed for a long time. Many programming projects that in the early 90's would have been done in C or C++ have in fact been done in safe programming languages instead, and according to the NSA's recommendation, that was a good idea. As computers have gotten faster and programming language technology has improved, there has been fewer and fewer reasons to settle for languages like C or C++ that don't have memory safety as a feature.

When I was a professional C++ programmer as early as 2013, some people -- even some programmers -- already thought that C++ was a legacy programming language like COBOL or Fortran. And outside of narrow niches like systems programming (e.g. web browsers, operating systems, and lower-level libraries), video games, or high performance programming, it kind of has become one. The former application niches of C++ have been taken over by Java and C#, or more recently by Go. If you have an application program written in C++, chances are that it's a relatively old codebase, or written at a shop that has reasons to write a lot of C++ (such as a high-frequency trading firm).

Now, even C++'s systems niche is under threat, with Rust, a powerful memory-safe programming language that avoids many of C++'s problems. Now, even the niches where C++ isn't at all "legacy" have a viable, memory-safe alternative without a lot of the technical debt that C++ has. Rust is even allowed in the Linux kernel, a project that has only previously accepted C, and whose chief maintainer has always explicitly hated C++.

A Memory-Safe C++

Fortunately, after all of these ill-thought out, tired talking points, Dr. Stroustrup subtly changes his perspective. After his distractions, after bashing memory safe programming languages as "novel," bragging about how C++ is too entrenched to be removable, pretending memory safety is just one of many equally important safety issues, and promising optional add-on tools that will eventually be standardized, he finally begins to tackle the question of how C++ could be made memory safe, in an opt-in fashion:

There is not just one definition of “safety”, and we can achieve a variety of kinds of safety through a combination of programming styles, support libraries, and enforcement through static analysis. P2410r0 gives a brief summary of the approach. I envision compiler options and code annotations for requesting rules to be enforced. The most obvious would be to request guaranteed full type-and-resource safety. P2687R0 is a start on how the standard can support this, R1 will be more specific. Naturally, comments and suggestions are most welcome.


For example, in application domains where performance is the main concern, the P2687R0 approach lets you apply the safety guarantees only where required and use your favorite tuning techniques where needed. Partial adoption of some of the rules (e.g., rules for range checking and initialization) is likely to be important. Gradual adoption of safety rules and adoption of differing safety rules will be important. If for no other reason than the billions of lines of C++ code will not magically disappear, and even “safe” code (in any language) will have to call traditional C or C++ code or be called by traditional code that does not offer specific safety guarantees.

This is a lot closer to what the NSA document actually specifies for memory safe programming languages than he gives the document credit for. For example, the document already provides for opting out of memory safety via annotation, paired with an observation that that will focus scrutiny on the code that opts out.

Dr. Stroustrup did not need to criticize the document for not thinking "seriously" to reach this conclusion, but simply acknowledge that it's true that C++ is not a memory safe programming language yet, but that based on his work, it might soon become one. Maybe the next version of the NSA document will endorse using C++, but only if it's C++ZZ -- where ZZ is some future version of the C++ standard.

I'm glad comments and suggestions are welcome, however, because I have a huge one.

Opt-in for memory safety is unacceptable, and is almost as bad as having a separate static analysis tool to enforce safety. Opt-out is fine -- Rust has a way to opt out of memory safety with the unsafe keyword, and this concept is discussed and defended in the NSA's original document. But the default should be to enforce memory safety unless otherwise specified.

For C++, this means that if these safety features are added in C++ZZ, --std=c++ZZ should cause unsafe constructs to be rejected -- and the C++ standard should require that these constructs be rejected for an implementation to be a conforming implementation of C++ZZ. Perhaps (but only perhaps) other command line arguments could be added to override this constraint on a file-by-file basis. Ideally, a new compiler command (e.g. g++ZZ) should be created for each implementation that defaults to this stricter behavior.

Parts of the codebase that use legacy features should have to have at least a file-level annotation that that file is a legacy file -- and then this annotation could gradually be moved to the function level. As a side benefit, this could also be used to phase out and deprecate weird points of C++ syntax, similar to the Rust edition system: Anyone using, for example, 0 literals to mean nullptr would have to declare some sort of a legacy annotation on their file or in their build system.

Only with this sort of opt-out memory-safety system would I consider C++ a memory safe programming language. I'd be very happy to see a memory-safe C++. I earnestly hope Dr. Stroustrup is successful in his endeavors. I'm not holding my breath, though, and in the meantime, I will continue to use other programming languages, that are already memory-safe, for my new projects, as will the majority of programmers.

In the meantime, it is unfair for Dr. Stroustrup to call safe programming languages novelties or to pretend that C++ isn't already far behind the times on this. This was already an important criticism of C++ decades ago, when Java first came out in the 90's and was referred to as a "managed programming language." This was discussed in detail in my classes when I was a college student in the late aughts. To read Dr. Stroustrup's writing, C++ is being criticized by "novel" upstarts when it is well on its way to getting the feature, but in actuality, the time to act was 1996.