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[variant2] Review of Variant2 started today : April 1 - April 10
by Michael Caisse 16 Apr '19

16 Apr '19

The Boost formal review of Peter Dimov's Variant2 library will take place April 1 - 10, 2019. Please consider participating in this review. The success of Boost is partially a result of the quality review process which is conducted by the community. You are part of the Boost community. I will be grateful to receive a review based on whatever level of effort or time you can devote. Variant2 is a never-valueless C++11/14/17 implementation of std::variant. >From the README: The class boost::variant2::variant<T...> is an almost conforming implementation of std::variant with the following differences: * A converting constructor from, e.g. variant<int, float> to variant<float, double, int> is provided as an extension; * The reverse operation, going from variant<float, double, int> to variant<int, float> is provided as the member function subset<U...>. (This operation can throw if the current state of the variant cannot be represented.) * variant<T...> is not trivial when all contained types are trivial. To avoid going into a valueless-by-exception state, this implementation falls back to using double storage unless * one of the alternatives is the type monostate, * one of the alternatives has a nonthrowing default constructor, or * all the contained types are nothrow move constructible. If the first two bullets don't hold, but the third does, the variant uses single storage, but emplace constructs a temporary and moves it into place if the construction of the object can throw. In case this is undesirable, one can force emplace into always constructing in- place by adding monostate as one of the alternatives. You can find the source code here: <https://github.com/pdimov/variant2> The master branch will be stable for the review: <https://github.com/pdimov/variant2/tree/master> Find the documentation here (also stable during the review): <https://pdimov.github.io/variant2/doc/html/variant2.html> Note: The repository contains an expected implementation also; however, that is not being considered in this review. Please provide in your review information you think is valuable to understand your choice to ACCEPT or REJECT including Variant2 as a Boost library. Please be explicit about your decision (ACCEPT or REJECT). Some other questions you might want to consider answering: - What is your evaluation of the design? - What is your evaluation of the implementation? - What is your evaluation of the documentation? - What is your evaluation of the potential usefulness of the library? - Did you try to use the library? With which compiler(s)? Did you have any problems? - How much effort did you put into your evaluation? A glance? A quick reading? In-depth study? - Are you knowledgeable about the problem domain? More information about the Boost Formal Review Process can be found here: <http://www.boost.org/community/reviews.html> Thank you for your effort in the Boost community. Happy coding - michael -- Michael Caisse Ciere Consulting ciere.com

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Mother of all Variants
by Nikita Kniazev 16 Apr '19

16 Apr '19

I do not see any progress in the never-ending never-empty vs everything discussion. It looks like an argument about tastes, and I see no room for compromise here. Number of examples for any point of view is also close to zero. Instead of saying that you would like more one or another, just implement it, and let the user decide which one to use. Or, at least, make a poll that will show needs in promoted by you implementation instead of forecasting from your big, but personal experience. A policy-based design is good in theory, but what in practice? In practice, you want one person to make and maintain bunch of different variants, which you, most likely, will never use, but users, most likely, will have nightmares in using it, especially in API, due to signature verbosity. More code will hurt preprocess and compile times for sure. What surprises me a lot that there was not much attention to the fact that because of number 2 in the name users will come and will ask about it. Do I need an upgrade? Is `variant` deprecated? Is `variant2` an improved version of `variant`?

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GSOD
by Siddhartha Sen 16 Apr '19

16 Apr '19

Any information on how and where to apply for GSOD?

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[variant2] Andrzej's review
by Andrzej Krzemienski 16 Apr '19

16 Apr '19

Hi Everyone, I recommend that variant2 should be REJECTED in its current shape. The main (and only) reason for rejection is the never-empty basic exception-safety guarantee offered and endorsed by the library. I have serious concerns that by adding it into Boost we would be leveraging the concept that is bug prone and encouraging bad habits. I have expressed my concerns in detail in another mail. I find this to be in contrast with what I perceive as goals of Boost. It is my understanding that the never-empty basic guarantee is the core of the library therefore the conditional acceptance would not work. My recommendation would be different if the library dropped the goal of providing the never-empty guarantee, or if it provided a strong guarantee in the cases where assignment emplacement required to change the alternative. I honestly appreciate the work Peter has done to provide this library, but I strongly disagree with the primary design decision. If the library is still accepted I recommend the following things: 1. Call it something slightly different so that at least namespace name clearly indicates that it is not an "improved std::variant" or "improved boost::variant" but just another offer in the market with different design choices in the design space: maybe ne_variant (for "Never Empty") or si_variant (for "Strong Invariant"), or even pdimov_variant (for "Perfectly Defined Invariant Model Of Variant"). 2. Drop the implementation of std::expected. It does not fit into the same library even if 90% of the implementaiton is shared. Propose a second library even if what it does is to #include the variant. 3. Move functions unsafe_get() from namespace detail and make them part of the official interface. You need to use them yourself in the implementation of visit() because otherwise you could not implement it efficiently. They are part of the minimum efficient base interface. Even if required very occasionally. 4. Provide a 2 minute introduction to the library for the newbies. We do not need a full tutorial. A link to someone else's description will do, but a short introduction I think is a must. I offer to help write one. 5. Provide a design rationale explaining why the never-guarantee is offered at all and why in this way. Why you are disregarding the design choices of boost::variant's never-empty guarantee. Also provide a detailed description on which algorithm is chosen under what circumstances and what it does to provide the guarantee. Now, to review questions: - What is your evaluation of the design? As noted, I strongly disagree with the main design choice of the library: the never-empty guarantee. I see no positive value, I see negative value. This aside, I do not see much design choices here: it is a quite small vocabulary type. - What is your evaluation of the implementation? Of all reviews I have participated in I have never found it so easy to understand the implementation. I find it very clean and short. I appreciate that it is a single header library. I can just copy the content of the file and test it in WandBox. Also the implementation is a very good practical tutorial on mp11. Peter already wrote good tutorials on mp11, but here for the first time I have seen it used for something that I consider a real life application. The implementation of variant looks so simple with mp11. - What is your evaluation of the documentation? The reference part meets the high standards of Boost. I am missing even a short introduction that would give us the motivating example. Some have said that it is enough to say "find the tutorial on std::variant". I disagree: there is no good introduction to variant, and I think Boost libraries have a good history for providing a good problem domain description apart form the implementation. I also think that given the controversies around the never-empty guarantee the design choice requires a detailed information in the docs. - What is your evaluation of the potential usefulness of the library? Given that I find std::variant design superior, I suspect this library would be useful in environments that cannot afford to use C++17 yet. It has advantages over boost::variant. It has cleaner implementation. boost::variant is giving me lots of warnings in clang-tidy. I expect these to be gone in variant2. Also the trade-offs made in double buffering may be superior to those in boost::variant: no memory allocation, no additional potential reason for throwing. - Did you try to use the library? With which compiler(s)? Did you have any problems? I tried to break the library with nasty cases in various clang and GCC versions. I reported some minor bugs over GitHub. Peter is quickly addressing them. The library does not compile in GCC 8.2 and higher in C++17 mode due to some bug in GCC. This is already tracked as issue. - How much effort did you put into your evaluation? A glance? A quick reading? In-depth study? I have spent a number of evenings reading the docs, the sources, playing with the library, debugging, discussing the design in the mailing list. I also followed the discussion in the C++ Standards Committee, where similar issues with never-empty guarantee were discussed. - Are you knowledgeable about the problem domain? I have a lot of experience with different implementations of `optional`: I am maintaining the boost version, I have provided the wording and the design for the std version and provided the reference implementation thereof (at https://github.com/akrzemi1/Optional/) which turned out surprisingly popular. Optional needs to solve similar problems in the implementation: manually managing a raw buffer or a union, and has similar problems with exception safety guarantees. However it does not need to address the never-empty guarantee issue. I also use a lot of boost::variant at work. Finally, I want to thank Peter for writing this library and submitting it for review. The review itself was very instructive. Regards, &rzej;

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[doc][test] change log broken from release notes
by Raffi Enficiaud 15 Apr '19

15 Apr '19

Hi, The link to the change logs is broken from the 1.70 release note (points to https://www.boost.org/users/history/libs/test/doc/html/boost_test/change_lo… instead of https://www.boost.org/doc/libs/1_70_0/libs/test/doc/html/boost_test/change_… ) I believe I used the same way I did for the past releases. How can I correct this? Thanks, Raffi

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Re: [boost] [boostorg/python] Building against multiple python implementations broken on 1.70 (#262)
by Stefan Seefeld 15 Apr '19

15 Apr '19

On 2019-04-14 7:50 p.m., Peter Dimov wrote: > > I'll need to look into this. Apparently, |FindBoost.cmake| solves this > by forcing the user to specify the version in |find_package(Boost > COMPONENTS python27)|, but the CMake configuration files installed by > 1.70 only take |find_package(Boost COMPONENTS python)|, so multiple > versions installed at the same time are a problem. > > In the meantime, you can add |--no-cmake-config| to your |b2 install| > command line to skip the installation of CMake configuration files, > which ought to give you the 1.69 behavior back. > This is *exactly* the kind of situation I was afraid of when I argued strongly against adding CMake support to my own projects: Before I had to deal with B2 issues that I had difficulties resolving myself. Now I have to deal with B2 *and* CMake issues, being even more dependent on others. That's not my definition of progress. Stefan -- ...ich hab' noch einen Koffer in Berlin...

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[variant2] never-empty guarantee considered harmful
by Andrzej Krzemienski 15 Apr '19

15 Apr '19

Hi Everyone, I waned to re-iterate my concerns regarding the never-empty guarantee provided by variant2 (as well as by boost::variant). Sorry for starting a yet another thread, but I think it necessary to cleanse, group and structure the information scattered among a number of threads. I think that the never-empty variant deceives people into thinking that it offers something useful and important, but in fact it does not, and more, it can encourage people to trust that they have some guarantee which they haven't, and owing to this to write buggy code. The solution that never-empty variant offers is similar to another common idea of replacing undefined behavior with *any *defined behavior whatsoever, and that is supposed to be "safe" because programmers no longer see symptoms of their bugs. Let me digress on the subject of UB for a second. UB is always caused by a bug in the program. Thus if the program had no bugs, it would have no UB. But many people are very afraid of UB per se, and less afraid of bugs. They will divert much attention to prevent UB at surprisingly high cost, but focus less on preventing their bugs. UB is a useful symptom of a bug and therefore can help detect and remove bugs, but people are sometimes so afraid of UB that they loose this connection to the extent that they are willing to remove the link between the two and get rid of potential UB even at the cost of concealing the bugs. And they consider it "safe". For instance, someone writes a sequence container class and faces the problem. I will need to provide access to elements by index, but people will give me bad indexes (because they can), this would be a UB, and I do not want a UB, therefore I apply a "safety" feature. Whenever someone gives me bad index, I will return a reference to the first element. Sure, it is surprising, and likely not what the caller wanted, but it is better than UB. And in case I have no elements in my container I will default-construct one in the container. Sure it is not nice to to increase the size of the container only because someone gave me the wrong index, but... *anything is better than UB.* I wonder if everyone reading this mail will agree with me that reasoning "anything is better than UB" is wrong. The author of this "safe" container calls his container "safe" because whatever bugs his callers have, they will never trigger UB *in his library*. Maybe in another library, at another level, but not in his component. In the end we will get programs that will cause self-driving cars to crash and kill people, space rockets to explode, but the "safe" program itself will never crash. Instead the author of the "safe" container should be focusing on something else. Why is my caller giving me the wrong index given that he can check my size and easily determine that this index is wrong? The answer is, the programmer probably intended to do something else and this needs to be brought to his attention. Tools like compilers and static analyzers cannot detect bugs in general because they cannot guess our intentions, but they can detect things like UB, so if we keep a correspondence between bugs and UB, we can detect bugs by detecting UB. This has been described in more detail in this paper: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1043r0.html (section 6). Now, the situation is similar with the never-empty guarantee of the variant that offers only a basic exception safety guarantee on assignment. Because the guarantee is basic (that is, it is not the strong guarantee), the only thing we can do with it in order not to risk program bugs is, following the Dave's article (https://www.boost.org/community/exception_safety.html) to either destroy or reset the variant that threw from assignment. And this is what correct programs do, and the technique is quite simple: unless objects provide strong exception safety guarantee (or some other guarantee that allows you to reason about the values of objects that threw) they should die in the stack unwinding process. The damage that the never-empty guarantee (implemented by setting unexpected values) does is to encourage people to think that with variant they can do something more than just destroy or reset. And while technically, variant itself is "clean" in this case (it really doesn't invoke UB itself) it causes the code that is using it to be incorrect. Two examples. I may have the following instance: ``` variant2<X, Y, shared_ptr<Z>> v {X{"params"}}; struct Visitor { int operator()(shared_ptr<Z> const& z) { return z->getCount(); } int operator()(X const&) { return 1; } int operator()(Y const&) { return 1; } }; ``` Where X and Y have nasty properties (are declared to potentially throw from move constructor and do not have default constructors). My v could technically store a null pointer at some point. In the visitor I do not check for this eventuality. Someone could say, "you should always check for null pointer, because anything is better than UB", But I know I never will be storing a null pointer there. I use shared_ptr for its ability to share ownership of an object, not for its ability to be null. I guarantee that I will never set the shared_ptr<Z> alternative to nullptr value. Therefore in the visitor I can assume that the pointer is never null. But if I am encouraged by the variant's never-empty guarantee to depart from Dave's guidance not to trust basic-guarantee objects that threw, I may end up in the situation where I start with alternative X, I want to assign alternative Y, but it throws, and because the only type with the default constructor is shared_ptr<Z>, I will get a nullptr value which I never intended. This is not bad yet. But now, if I apply my visitor on my variant, my precondition that the pointer is not null will be violated: ``` variant2<X, Y, shared_ptr<Z>> v {X{"params"}}; BOOST_SCOPE_GUARD(&v) { visit(Visitor{}, v); }; v.emplace<Y>("params"); // throws ``` Now, variant2 fulfills its guarantee: it stores one of the alternatives (of arbitrary choice and arbitrary value), but it injected me the worst possible value of the type with the weak invariant. Thus, variant2's invariant may be strong, but it exploits the weak invariants of the types of the alternatives. And unfortunately, a lot of types, even in the STD, have weak invariants and use the default constructor to set the degenerate state. Thus, my program has UB, and hopefully a crash, because I was deceived by the never-empty guarantee: I thought it would be "safe" to observe such variant that threw. Second example comes from Peter: ``` template<class T> class monitor { T const& v_; T old_; public: explicit monitor(T const& v) : v_(v), old_(v) {} ~monitor() { if( v_ != old_ ) std::clog << "value changed"; } }; ``` Suppose I use it like this: ``` { variant2<X, Y, shared_ptr<Z>> v {shared_ptr<Z>{}}; // let me check if this code sets the v to a non-degenerate value: monitor m {v}; v.emplace<X>("params"); // succeeds v.emplace<Y>("param"); // fails: null pointer set. } // no value change reported ``` although the only state changes I initiated in this block were to set types X and Y, I got the wrong result from the monitor, because the "safe" guarantee of the variant chose some state that corrupted the monitoring logic. variant2 delivers its guarantee as promised: some value is there, but my monitoring is compromised because of its arbitrary choice of value. Note that std::variant does not have this problem: a distinct state is set upon such exception that is always different than any value of any of the alternatives. Of course, this second example has problems of its own. It assumes you can invoke operator== on an arbitrary T of which we know nothing. It assumes that T's invariant allows the values to be compared, but this is not correct an assumption. Type T can have a weak invariant: one that does not even allow comparing two values. You cannot just invoke arbitrary operations on an unknown T in the destructor, if you do not know they exception safety guarantees. T's comparison could potentially throw. Even destructors of generic components such as vector<T> or optional<T> in their non-trivial implementations only invoke operations on concrete types: pointers and bools, but for the T they only call destructors. Peter argues that a call to the operation on a variant could be invoked indirectly form the destructor: destructor calls function f(), function f calls function g(), function g() calls function f() and function h() makes a visitation on the variant. But can you see what this means? I am calling some function f() from the destructor, which is supposed to clean up resources, and I am not sure what f() does, if it does some hidden things beyond my control. This is a buggy code already. Now, to address another argument in favor of never-empty, arbitrary-value-choosing guarantee. Emil says that with never empty-guarantee we can more easily reason about the variant: it is *always* one of the alternatives: rather that "either one of the alternatives or some degenerate state. He argues that once we allow the faintest possibility for the degenerate state, we have to check for it *everywhere*. I think that the simple binary model, "either the weak state does not exist or is potentially everywhere", while it often works for many types is, not adequate to describe the reality of the variant. People need models (which are simplifications of reality) to help them manage complex reality. But models have the downside of not allowing people to see the reality beyond what the models allows them to see. We ave to distinguish "ordinary" weak state and "unusual circumstance" weak state. Types like unique_ptr or fstream have ordinary weak state: it is created by a constructor. Worse, by the default constructor, so if you just declare the variable name and forget about parameters you immediately get the weak state. It is easy to obtain this state. And therefore in many places we need to be prepare to handle them. But other types (which we have fewer in STD: only std::variant, I think) can have only "unusual circumstance" weak state: you cannot initialize the object to contain this state (unless you move/copy construct from another object that already has one). You can only get it in situations that for other reasons require special caution from programmers. I am aware of two: * when an object is manually moved from (by explicitly casting an lvalue to an rvalue reference, which std::move() does) * when a mutable operation on an object with basic exception safety guarantee threw an exception. In the first situation, in a generic context where we work with an unknown type T, after the move an object is in a valid but unspecified state. Type T is allowed to have a very weak invariant: so weak that even operator== may not be allowed to be invoked. In this state the only correct thing we can do is to either let the object die or reset its value (the only common way for resetting the value for different types is assignment of a non-degenerate value). Working under the assumption that programmers are aware of these constraints and are willing to adhere to them, we get confidence that these moved-from values will not be observed. (Of course this does not apply to types which choose to guarantee more after move.) We can design a resource-handling type, such as a socket, whose default constructor acquires the first available port, and leaves the object in a "strong" state, ready to work. there is no way to initialize objects of this type to a degenerate value. but if these sockets are move-constructible, the moved from object will have to store the degenerate value. But it would be pointless to check it anywhere in the program. We could only see it if programmers did not adhere to the rule, "only assign to or destroy the moved from objects, unless you know your object offers more guarantees". If programmers do not adhere to this rule, rather than covering up for their bugs, socket should make the UB as explicit as possible in order to assist static analyzers and tools in detecting such programmer's misuse of the moved-from state. Similarly with the after-throw state from the basic guarantee operation. The language cannot enforce this, but programmers are required to adhere to the rule: the only correct operations to be invoked in that case is to destroy the object or to reset it. again, if programmers do other things this is a bug and the type should try to reflect that in a controlled UB, rather than pretending that everything is fine and causing bugs later in unpredictable places, as was shown in the examples above. Insisting that unusual-circumstance weak state should be checked *everywhere* is depriving oneself of the ability to see the engineering trade-offs clearly. We do not have to distinguish "ordinary" and "unusual-circumstance" weak states in most of other types (nearly all types) because they do not pose so many implementation difficulties as variant does: implementing never empty guarantee costs too much in terms of run-time or spatial efficiency. For the sake of the simplicity of the model we cannot sacrifice too much efficiency. Given that efficient implementation of variant does not fit into the simple model, "either I have a strong invariant or not", one option is to provide a misleading and/or never-empty guarantee. But another option is to acknowledge that the simple model is no longer sufficient, and adapt a new one: more complex but one that better reflects the reality: we have two invariants: one that applies in normal code (where every function can be freely called and a weaker one that applies in circumstances that already require special caution from programmers. This is the C++ way: we deal with UB which some other languages do not have at all:this is more complex a task but allows for faster binaries and better bug detection. In a similar vein paper "Fixing Relations" ( http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1248r0.html) has been recently adopted in the C++ Standards Committee. The proposal compromised a nice and simple mathematical model, but this part of the model stood in a way to use some STL algorithms efficiently. C++ is not an easy language, and by trying to make it appear simpler than it is, we might only confuse the new programmers. One more thing, it is not the surprising value that variant2 may set upon the exception that bothers me the most. It is rather that indirectly it encourages programmers to think that it is fine or "safe" to read the value of the object that has been moved from or that threw from the basic-guarantee operation. We do not want to suggest that. Finally, all the above reasoning would not apply if variant offered a strong guarantee on assignment/emplacement for cases where the alternative needs to be changed. In that case: 1. It is perfectly fine to observe the state of the object that threw. 2. No surprising, potentially weak-state, value is ever fabricated in this case. Regards, &rzej;

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update of release notes for boost-1.70.0
by Oliver Kowalke 15 Apr '19

15 Apr '19

Hi, I've added 3 pull requests for boost/website... could you apply the patches and re-create the release notes for boost-1.70.0, please?! ty, Oliver

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Re: [boost] [boostorg/python] Building against multiple python implementations broken on 1.70 (#262)
by Stefan Seefeld 15 Apr '19

15 Apr '19

On 2019-04-14 7:50 p.m., Peter Dimov wrote: > > I'll need to look into this. Apparently, |FindBoost.cmake| solves this > by forcing the user to specify the version in |find_package(Boost > COMPONENTS python27)|, but the CMake configuration files installed by > 1.70 only take |find_package(Boost COMPONENTS python)|, so multiple > versions installed at the same time are a problem. > > In the meantime, you can add |--no-cmake-config| to your |b2 install| > command line to skip the installation of CMake configuration files, > which ought to give you the 1.69 behavior back. > This is *exactly* the kind of situation I was afraid of when I argued strongly against adding CMake support to my own projects: Before I had to deal with B2 issues that I had difficulties resolving myself. Now I have to deal with B2 *and* CMake issues, being even more dependent on others. That's not my definition of progress. Stefan -- ...ich hab' noch einen Koffer in Berlin...

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[variant2] Review
by Damian Jarek 14 Apr '19

14 Apr '19

> - What is your evaluation of the design? Variant2 is clearly a superior replacement for the original variant type in boost - Boost.Variant and would be very useful for libraries in Boost and outside of it that are stuck on C++11/14. I strongly believe that triviality propagation should be supported, especially if Variant2 is to be used as the implementation detail of a type like `expected`. As for the hot topic of valuelessness - based on my experience of how variant types are typically used, I think that an implementation should loudly object at compile-time to any operation that could put it in a valueless state - e.g. if emplacing can throw and the type does not have a noexcept move constructor. However, I see that it could be quite inconvenient for many users and I consider the fallback mechanism currently in variant2 to be a good enough compromise. > - What is your evaluation of the implementation? I reviewed the implementation and it looks great. As a test, I replaced the internal variant type in Boost.Beast(beast::detail::variant) with variant2 to compare it against the private lean variant and against std::variant. In terms of compilation time, I've observed no significant difference between std::variant and variant2 when compiling the Boost.Beast test suite. Both were about 10% slower to compile than beast::detail::variant, which can be attributed to the additional header material which must be parsed by the compiler. In terms of runtime performance, I have not observed a significant difference between the 3 variants in terms of requests/second in a test project I use to optimize Boost.Beast performance. Note that Boost.Beast HTTP serialization makes heavy use of a variant to enable sending an HTTP message in as few write calls as possible. As far as I know, all types stored in the variant were "well behaved", so it did not test the double-buffering cases. > - What is your evaluation of the documentation? The documentation is adequate, however, it could use a few examples for users unfamiliar with the concept of sum types. > - What is your evaluation of the potential usefulness of the library? It is a useful replacement for Boost.Variant in C++11/14 libraries both in Boost and outside of Boost. > - How much effort did you put into your evaluation? A glance? A quick > reading? In-depth study? About 6 hours, most of which where spent on the compilation and runtime benchmarks based on Boost.Beast. > - Are you knowledgeable about the problem domain? I have implemented and used a few variant implementations in embedded code. I have also reimplemented the private and lean beast::detail::variant type on top of Boost.mp11. Conclusion: I recommend that Variant2 is CONDITIONALLY ACCEPTED if: - documentation is extended with usage examples - triviality propagation is implemented

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