More recent revision: https://isocpp.org/files/papers/P2900R10.pdf. It seems like they've added a few things since this draft.

It's somewhat funny to hear "minimum viable product" in the context of a language standard/specification. I'd never thought of adding a feature to a language in that way before.

The idea of design by contract (DbC) in C++ appears to have been around for a while too. The authors link to a "forthcoming companion paper" (currently a 404 error), but you can find proposals from as early as 2004 with the idea: https://www.open-std.org/JTC1/SC22/WG21/docs/papers/2004/n16...

(and potentially earlier, I just haven't seen one yet)

Contract programming for C++ was implemented back in the 90's.

https://www.digitalmars.com/ctg/contract.html

It also appears in the D programming language:

https://dlang.org/spec/contracts.html

I did my dissertation work on software contracts. They are generally useless: programmers won't write them, or will write them incorrectly. Moreover, the runtime enforcement dominates all but the most-expensive functions, and contracts become an antipattern for helper functions due to their enforcement cost.

Without intense compiler support and contract erasure such as David Van Horne's work, this is a dead idea that needs to stay that way.

Anyone interested in other languages that implement this should take a look at the SPARK subset of Ada. Pre and postconditions work in the same way and enforce the same behaviour described here (or at least that's what I understand it to do from a quick skim through)

For decades we have been using assert(input) and assert(output), so it's not exactly new. Forcing sanitary checks by the language runtime is an option, hence requiring contracts to spell out the range of valid input/output which is checked in debug mode. Otherwise, if it's optional, it will only be used by a subset of engineers. If its mandatory, it can cripple performance (unless we have unsafe scopes). Opt-out is always better than opt-in.

Assuming that this or something substantially identical does land for C++ 26 I think we should start betting on when SG23 (the committee's Study Group for "Safety and Security") is replaced by some hypothetical SG24 "Safety and Security No But Really".

We already know that because C++ Undefined Behaviour can cause what are called "Time Travel" defects and this paper doesn't do anything to prevent it, adding contracts in C++ can make your software less safe and induce more surprise behaviour. We also already know that C++ programmers tend to write erroneous UB tests when trying to grapple with edge cases that might induce Undefined Behaviour, setting off the very calamity they fear. So this new feature, rather than being (as its proponents claim) a way to improve the safety of C++ software, or neutrally becoming a dead letter as the UB "passes" contracts that in fact are not met, might instead become another footgun for the language.

A functioning SG23 should have caused this proposal to stall out until it can explain how it will prevent this problem, rather than merely re-stating the problem (in 3.6.4) and saying well that's unfortunate but maybe somebody else will fix it. That stance might be extremely unpopular with some C++ programmers, who believe they don't make mistakes, but as it stands this work will instead cause all the people who do make mistakes (which is in practice everybody) to regret using contracts. If SG23 isn't interested in preventing C++ from becoming even more unsafe, what's the point in the group existing?

Are there HTML version of these docs? Would make it easier to read without having to download a PDF.

I worked in the telecom business 15 years ago on 4G (LTE) and there it was considered a big savior compared to how it was done before.

Basically before they had a lot of error handling code and it was a significant part of the code base (don’t remember but let’s say 50%) and this error handling code had the worst quality because it is very hard to inject faults everywhere. So basically the error handling code had a lot of bugs in it which made the system fail to recover properly.

But DbC was a godsend in the way that now you didn’t try to handle errors inside the program any longer. Now the only thing that mattered was that a service should be able to handle clients and other services failing. And failure in a few well defined interfaces is much easier to handle. So the quality became much better.

What about the crashes then? Well, by actually crashing and getting really good failure point detection it was much easier to find bugs and remove them. So the failures grew less and less. Also, at that time I believe there were 70 ms between voice packages so as long as the service could recover within that timeframe, no cell phone users would suffer.

Plus of course much less error prone error handling code to write.

And as someone else said, DbC should never be turned of in production. Of course, in embedded systems, speed is not so important as long as it is fast enough to not miss any deadlines. And you need to code it so it doesn’t miss deadlines during integration and verification with DbC so there is no reason to turn them off in production.

It sounds like you're specifically talking about runtime-enforced contracts rather than ones that the compiler uses to prove compliance? Your first sentence makes it sound like you think all contracts are useless, but your last sentence makes it seem like you think they're useful if they're compile time.

Yet companies like Eiffel Software, and Ada vendors with Ada/SPARK, are still in business with people paying for using their tools, in a day and age where many devs refuse to pay for their tooling.

Interesting. I am a fan of DbC (following Meyer's Applying "Design by Contract" - pdf at https://se.inf.ethz.ch/~meyer/publications/computer/contract...) though due to runtime costs (as you point out) i limit it to pre-conditions only everywhere (almost all "policy" part of the code and less on the "mechanism" side) and tighter pre/post-conditions on module boundaries only.

> programmers won't write them, or will write them incorrectly.

I think this is a matter of education and discipline and not an argument for not using DbC.

If it is okay with you could you share your dissertation and maybe highlight the key points which led you to your conclusion of "They are generally useless"? I think it would be useful to know.

I use them for all my code. They are far from useless. You never want to turn them off in production either since that's a source of important information. You are right programmers don't write them, but that says nothing about contracts and a lot about how programmers are trained.

I find it a very dubious assertion particularly with respect to performance. Well-written idiomatic code in most languages should validate inputs anyway; it's just done in an ad-hoc way - stuff like ArgumentException in .NET or ValueError etc in Python, for example. Contracts are just a way to formalize it, not only for the benefit of static analyzers, but also for better integration with existing tooling (generated docs etc).

And no, you don't have to write a code contract for every single function. Code contracts are most useful in the same exact place you'd do other kinds of contracts, like say documenting the public API of a library. From there it's diminishing gains to extend them deeper into the implementation, but in any case, one can always find a balance between performance and enforced contract checks. Not all code needs to be blazing fast, and erring on the side of correctness is preferable.

A reference for your dissertation?

The usefulness (or lack thereof) of contracts was famously discussed in 1997 in this paper: https://www.irisa.fr/pampa/EPEE/Ariane5.html (with additional comments here: https://www.irisa.fr/pampa/EPEE/Ariane5-comments.html ).

I have tried several times (a couple of decades ago) to introduce DBC in Python, using one of the many available libraries (for an overview: see https://lab.abilian.com/Tech/Python/DbC%20in%20Python/ ) but, like you, wasn't convinced.

I believe your argument on the performance penalty is right, and as a corollary, this implies that contracts are mostly useful if associated with a formal proof system. Contracts in production are probably a bad idea, in most cases.

Contracts are widely used in the form of asserts throughout many codebases and are generally seen as improving code quality and enabling more efficient designs.

Even something like accessing an element in an array is subject to the contract that the index is less than the size.

SAL is wildly successful in the Windows world and is basically a contract language embedded in C macros.

Is anyone using Eiffel language in the real world? The community seems so tiny, barely existing.