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Fwd: Boost.Decimal Review -- Attila Feher
by John Maddock 20 Oct '25

20 Oct '25

Forwarded here for archiving, with permission. -------- Forwarded Message -------- Subject: Boost.Decimal Review -- Attila Feher Date: Mon, 20 Oct 2025 14:34:21 -0000 From: Attila Feher (BLOOMBERG/ 919 3RD A) <afeher(a)bloomberg.net> Reply-To: Attila Feher <afeher(a)bloomberg.net> To: jz.maddock(a)googlemail.com CC: matt(a)mattborland.com, mungo.gill(a)me.com Attached is the review. CONDITIONALLY ACCEPTED See conditions in the text file. I hope it helps. Attila

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End of Boost.Decimal Review
by John Maddock 20 Oct '25

20 Oct '25

Just to remind everyone that the Boost.Decimal review ends today, so if there are any last minute reviews, now's the time to get them in! Best, John.

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Decimal Library re-review begins today
by John Maddock 20 Oct '25

20 Oct '25

The second review of the proposed Decimal Number library by Matt Borland and Chris Kormanyos begins today and runs until 15th Oct. You will find documentation here: https://develop.decimal.cpp.al/decimal/overview.html And the code repository is here: https://github.com/cppalliance/decimal/ Boost.Decimal is an implementation of IEEE 754 <https://standards.ieee.org/ieee/754/6210/> <https://standards.ieee.org/ieee/754/6210/%3E> and ISO/IEC DTR 24733 <https://www.open-std.org/JTC1/SC22/WG21/docs/papers/2009/n2849.pdf> <https://www.open-std.org/JTC1/SC22/WG21/docs/papers/2009/n2849.pdf%3E> Decimal Floating Point numbers. The library is header-only, has no dependencies, and requires C++14. This re-review is happening because the original review had an indeterminate result (see my summary at https://lists.boost.org/archives/list/boost@lists.boost.org/message/2Q7UBOT…) and resulted in my filing a number of issues against the library based on the reviewers comments. Now that Matt and Chris have been busy fixing these and addressing the reviewers concerns, the library is now back for a second look, see https://github.com/cppalliance/decimal/issues?q=is%3Aissue%20state%3Aclosed… for a complete list of issues addressed from the last review. I hope both the original reviewers, and new ones will come back for a second look. Please feel free to amend your original review, or if you're starting from scratch then please provide feedback on the following general topics: - 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? - Do you already use it in industry? - 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? Ensure to explicitly include with your review: ACCEPT, REJECT, or CONDITIONAL ACCEPT (with acceptance conditions). Best, John Maddock (review manager).

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New depinst feature
by Архипов Дмитрий 16 Oct '25

16 Oct '25

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Decimal Review extended until Mon 20th.
by John Maddock 15 Oct '25

15 Oct '25

Folks, I've been made aware that there are a couple of knowledgeable people who would like to participate in the Decimal lib review but are currently away, in order to give them a little more time (along with anyone else who needs it), the review period is extended until Monday 20th. I'd like to thank everyone who has participated so far: we've had some great discussion. If you were thinking of submitting a review and haven't got around to it, now's your chance! Best, John Maddock.

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[boost][decimal] LoS's review
by LoS 13 Oct '25

13 Oct '25

Dear Boosters, I'd first like to thank Matt for his effort in designing and implementing the library. Below, there's my review of the "Boost.Decimal" library. ## Documentation An issue is the inconsistent use of `using namespace` declarations in the code snippets. In some examples, the global declaration allows to write types simply as `decimal32_t`; in others, the examples omit the declaration, forcing to repeatedly write the full namespace, such as `boost::decimal::decimal32_t`. I think it'd be better to keep a consistent style throughout the documentation. The current documentation often lacks necessary detail for functions. While the synopsis and a generic description of the main classes (decimal32_t, decimal64_t, decimal128_t) are present, there's lack of information, like preconditions or edge-case behaviors, for many functions. Some examples aren't embedded directly in the documentation, but are merely mentioned as external files. While this prevents large blocks of code from cluttering the documentation, it imposes a burden on the user, who must manually navigate the repository to find these files. A much better solution would be to provide direct inks to these files. In this way, users could quickly jump to the code and return to the documentation, without the need to open a new browser window and manually look for the specific files. Finally, the "API Reference" section organizes some information using tables. While the tabular format is appropriate for structures where multiple fields must be summarized, its use to some types, like enums, seems arbitrary. Also, the visual display of these tables can be confusing upon first glance, since it may lead the user to incorrectly assume a relationship between adjacent, but functionally separate, tables. ## Design I think the use of explicit constructors should be more clearly defined. Some constructors are marked explicit, while others are not, and the reasoning for this difference isn't immediately clear. The "Design notes" section explains the decision to make constructors for floating-point numbers explicit. However, it omits any rationale for why constructors for integer types have been made non-explicit. I think that a complete explanation of the rationale behind constructor explicitness would be beneficial. About the documentation, the constructors, for both floating-point numbers and integers, are marked as explicit. This mismatch should be fixed, too. Best regards, LoS

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[boost][decimal] Mungo's review
by mungo.gill＠me.com 13 Oct '25

13 Oct '25

Boost Decimal Review - Mungo Gill Firstly thank you to Matt for all the effort put into this sizable and clearly well thought out library. Here is my review of the Decimal library. - Background to this review I have used various floating point libraries in the past, mostly in financial applications, and am reasonably familiar with IEEE 754 for binary floating point, although I am less familiar with decimal floating point. I spent about a day writing small test programs, attempting to use it in my own code and stepping through function calls with the debugger. - Building and using I found the library relatively easy to download and use, the only exceptions being Windows ARM and assumptions made regarding fmtlib and format library availability (below). - - Compilation failures on Windows ARM When I tried to compile this on a Windows ARM (64-bit) box (using cmake and MSVC 2022), it failed due to the following error: …boost\decimal\detail\u256.hpp(691): error C3861: '_addcarry_u64': identifier not found (note: arm and arm64 windows probably need to be treated as a special case in the macro checks found in config.hpp lines 88-12. Note that on MSVC ARM the macro __ARM_NEON__ is not defined but arm_neon.h is provided as a header) - - fmtlib library support I tried building a program dependent on decimal on Windows (x64 built with cmake, MSVC 2022, C++20) and encountered an interesting issue: If the fmtlib library happens to be installed on the system then decimal attempts to #include the fmtlib library headers, encountering the following static_assert in fmt/base.h: static_assert(!FMT_UNICODE || use_utf8, "Unicode support requires compiling with /utf-8"); This occurred even though I had made no attempt (by this stage of testing) to write any code dependent upon either fmtlib or std::format. The checks in decimal/fmt_format.hpp should perhaps be tightened so that windows programs compiled without /utf-8 and which do not themselves use the fmtlib library do not give a compilation error. - - format library support The checks for whether or not the <format> header is available checks compiler versions. However, it is feasible that people are using a different standard library from the one you are assuming based on the compiler version (for example clang compilation may or may not have the -stdlib=libc++ flag, or someone might be using a modern clang with an older GNU libstdc++ lacking <format>). It would perhaps be more reliable to check for the __cpp_lib_format feature test macro that C++20 provides. - Points already mentioned by others in the mailing list As these have already been mentioned on the mailing list, I kept these points into their own section. - - Implicit conversions It is slightly confusing having implicit integer construction, but explicit floating point construction. Consider making all construction explicit. Although this leads to slightly more verbose user code, there is less opportunity for unintentioned conversions. Also confusing is the existence of a constructor taking a bool - for example the following code counter-intuitively prints 1 rather than the expected 3. I cannot personally imagine a good use case for wanting to implicitly convert a bool to a decimal. struct MyFloatType { double v_ = 3.14; operator int() const { return static_cast<int>(v_); } }; int main(int argc, char** argv) { MyFloatType f; boost::decimal::decimal64_t g(f); std::cout << g << std::endl; return 0; } - - Rounding I’m also not sure about the rounding convention being a global setting, and I’m not convinced thread-local overrides solve the problem. Perhaps some key functions could take an optional argument to override the global rounding setting ? - Other thoughts on the interface/design I thought this was a nice clean design, which covered most of what I would want covered by a decimal library. - - format library support The std::format support is somewhat lacking in flexibility, for example things like padding character specification (see https://en.cppreference.com/w/cpp/utility/format/spec.html) is not supported. (although I do appreciate that most standard library implementations don’t currently do the unicode width calculations correctly anyway, and supporting them would perhaps be excessive). Note: I did not do the same tests with the fmtlib library. - - locale support The level of locale support is undocumented. I would have included this in my documentation feedback but when I did a test I found that the implementation is also incomplete - i.e. it only appears to impact the decimal point specifier. Here is a simple reproducer (both output lines should be the same): std::locale a("en_US.utf8"); std::cout.imbue(a); std::cout << 1122.89 << std::endl; constexpr boost::decimal::decimal_fast64_t val4{ 112289, -2 }; std::cout << val4 << std::endl; - - cohort-aware functions Although I think the standard arithmetic operations and i/o should behave as they currently designed, there should be facilities to better exploit the fact that decimals can have different cohorts (for example facilities to add/subtract/multiply/divide where the resulting cohort is predictable and deterministic, albeit the preconditions would need to be quite tight to meet such requirements). A use of this would be to output a member of a cohort in a unique manner such that, when parsed, results in a decimal of the same cohort. - Other thoughts on the implementation Generally a good implementation, with clean, easy-to-follow code. I was able to step into operations using the debugger and did not find myself completely confused (although more code comments would always help :-) ) - - internal naming (this is an unduly picky point I know) The 32-bit, 64-bit and 128-bit implementations are called mul_impl, d64_mul_impl and d128_mul_impl. Perhaps for consistency the 32-bit should be d32_mul_impl. More notably, all 3 have comments talking about 128-bit arithmetic which looks to have been copied and pasted but which is not applicable to all 3. Similar points hold true for the division implementations. - - testing Although I understand the argument put forward in the mailing list that testing of special functions exercises the numerical precision, “interface testing” is quite light. For example a test that attempted to use a std::set<decimal32_t> would have quickly found the hashing issue mentioned in the mailing list. - Other thoughts on documentation Again, generally good, easy to read and understand documentation. Coming to this library for the first time I was quickly able to get up to speed. However, I did find myself having to look at the source code or the comments therein to find out how particular methods worked. - - Type conversions I may have missed it in the documentation, but the conversion rules between decimal types should be more clearly specified if they are not already (eg a decimal64_t can be converted to decimal128_fast_t but not to decimal32_fast_t). Also the documentation should specify the type promotion rules. For example, I failed to find in the documentation in the following code? constexpr boost::decimal::decimal_fast64_t val1{ 314, -2 }; constexpr boost::decimal::decimal64_t val2{ 2, 0 }; auto val3 = val1 * val2; - Conclusion My view would be that we should accept this library. Overall the documentation could be improved, and some useful features are missing, but in its current form (assuming the bugs and build issues mentioned above and in the mailing list are addressed) it is in a usable state and would be a useful addition to Boost.

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[boost][decimal] Andrzej's review
by Andrzej Krzemienski 13 Oct '25

13 Oct '25

Hi Everyone, This is a review of the Boost.Decimal library. My knowledge about decimal floats is very superficial. In my job I need to deal with prices in all the World's currencies (string<->numeric conversion and simple additions and scaling) and for that I use a home-brew fixed-point implementation. I participated in the previous review, spent a couple of days studying the documentation, having a superficial look at tests and small pieces of implementation, running simple examples and filing bug reports. I would like to thank Matt and Chris for the huge effort to write and maintain this library, and for undergoing the painstaking process of Boost Review. The community needs this library, and I am happy that it is proposed for Boost. The nature of this library is very unusual: it implements types that belong in processors. We know it is needed for applications where accuracy and correctness is absolutely critical. Against other Boost libraries, it requires extreme measures for verifying correctness across the whole spectrum of values and cases. In a similar vein, in my opinion, it requires extremely detailed and comprehensive documentation. The bar should be higher than for an average Boost library. In this context, I have some concerns. While playing with the library I found three corner cases where it produces incorrect results: * https://github.com/cppalliance/decimal/issues/1091 * https://github.com/cppalliance/decimal/issues/1094 * https://github.com/cppalliance/decimal/issues/1119 Two are already fixed, and Matt has been very responsive, but I feel like it is too easy for a novice to find bugs in the library after a couple of minute's tests. It looks to me like the unit test "sieve" is still too coarse. Some test files (like test_decimal32.cpp or test_decimal64_basis.cpp) are commented out. Fuzz tests for decimal comparisons in random_decimal32_comp.cpp seem to be only testing decimals created from integer values (no fractions). I know that other Boost candidate libraries pass with fewer tests, but I feel this one needs to have a lot of them. I am not even sure how one tests user-defined literals across the spectrum. Maybe the unit tests should be written outside of C++. One can see that a lot of work went into preparing the documentation. It is helpful and already provides a lot of useful information, also about the background, and ideas. It has the "Design Rationale" section which reminds me of the high quality documentation that the original Boost libraries used to have. But I still find it insufficient. The API reference for the key types, like `decimal32_t`, only lists the class members, but there is no detailed description for the behavior of these members. What are the preconditions of the Decimal's constructors? Is any combination of values considered invalid? Or is it nicely degrading to subnormals and infinities? I think the contract of the library (how the cohorts are treated in comparisons, if they are implementation detail, or are observable?) need to be spelled out more clearly. Again, Matt has been very responsive, and the documentation has improved a lot during the review period, and I know it will improve further. Regarding the design, it looks like it has been done by IEEE, so not much to review here. I only had a short glimpse at the implementation, so I cannot say much. One thing that I found surprising is that operator<=> is implemented in terms of operators <, > and ==. Operator <=> was designed for the class authors to go the opposite way: define operator <=> and you get all the others for free. So I wonder what was the reason for the current implementation. I do not feel comfortable giving an accept or reject recommendation. I clearly have a higher acceptance bar for this library than for any other I have seen reviewed in Boost, and this may appear unfair. Do I want to see this library in Boost? -- yes. It is useful, and IEEE seems to agree. Are the authors the right people to implement it? -- definitely! But I would feel more comfortable accepting it, if it went through another review iteration. Could this be considered a conditional acceptance? In the case of the test sieve issue I brought up, I do not think it is a question of writing more tests. It is more a question of designing an approach to testing that would give a guarantee of correctness. This is requesting a lot of work from the authors who already put a lot of effort to get the library where it is. It is huge (both the library and the effort)! Again, this cannot be overstated. Matt and Chris: you did great work! Thank you! I know that sooner or later we will be able to use Boost.Decimal from the official distribution. Also, thanks to John Maddock for managing the review. Regards, &rzej;

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Master regression tests failing
by Tom Kent 12 Oct '25

12 Oct '25

There seems to be a problem on master with all tests failing. It results in this in the log: error: Unable to find file or target named error: '/var/boost/run/boost_root/libs/system/build//boost_system' error: referred to from project at error: '../libs/graph_parallel/test' error: could not resolve project reference '/var/boost/run/boost_root/libs/system/build' Tom

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[charconv] Possible bug
by Gennaro Prota 10 Oct '25

10 Oct '25

Hi, the following program prints "1e-01". Furthermore, it prints "Error: 132" if I change `size` to 20. It prints the expected "0.1" if I change the two using declarations to use the standard facilities. Tested against the master branch, with MSVC 2022. Is this a bug in CharConv? #include "boost/charconv.hpp" #include <charconv> #include <iostream> int main() { // using std::to_chars; // using std::chars_format::general; using boost::charconv::to_chars; using boost::charconv::chars_format::general; int const size = 100; char buffer[ size ]; auto const r = to_chars( buffer, buffer + size, 0.1, general ); if ( r.ec != std::errc() ) { std::cerr << "Error: " << static_cast< int >( r.ec ) << std::endl; return 1; } *r.ptr = '\0'; std::cout << buffer << std::endl; } -- Gennaro Prota <https://prota.dev>

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