Contracts for C++ (DbC) [pdf]
isocpp.org
isocpp.org
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)
Many features in Java, C#, C++ have evolved as MVP across several language revisions before.
Lambdas, type inference, modules, unsafe code, constexpr,...
https://www.digitalmars.com/ctg/contract.html
It also appears in the D programming language:
-> Did the proposal authors contact you or refer to your work (or any other previous work)? It not, that would be a shame.
"It also appears in the D programming language"
-> Do people use it?
No
> or refer to your work (or any other previous work)?
No
I presume they simply didn't know about it. I emailed the authors.
> Do people use it?
Yes
The tiny subset of D userbase, which is tiny in its own.
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n19...
"Programming languages such as Eiffel and D have a Contracts facility; this paper proposes a Contracts facility for C++."
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.
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.
Agree! There is a sliding scale between testing contracts and proving them too. My labor of love for the past several years has been a tool for checking Python contracts using an SMT solver (using symbolic inputs over concrete paths): https://github.com/pschanely/CrossHair
That said, these days I think property-based testing gets you the ~same benefits as contracts, and is easier for most people to apply.
The value is just too high to turn them off in production. Do you want bug free code? This is the only way I know how to do it.
> 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.
Even something like accessing an element in an array is subject to the contract that the index is less than the size.
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.
Bertrand Meyer in his usual painstakingly detailed manner explains how to integrate DbC with Exception/Error handling in his paper Applying Design by Contract linked to here - https://news.ycombinator.com/item?id=42133876
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.
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?