I've been waiting in anticipation of pqs for some time. I've had to work with dimensioned values several times and could really have used the compile-time checking pqs offers. It is also, IMHO, one of the best practical examples of template metaprogramming. - Please always explicitly state in your review, whether you think the library should be accepted into Boost. I'm disappointed that for now, I would have to reject the library from Boost. I definitely want to see this library make it in but there is one big drawback that's a showstopper for me. Everything is based on powers of 10. More on this below. There are also some improvements I'd like to see that I don't consider showstoppers. I'll comment on those below as well. - What is your evaluation of the design? The everyday user interface is fine. The myriad of concepts and classes to model them can be overwhelming, however. This might be addressed with better documentation. I don't understand the t1_quantity name. It doesn't convey any useful information to me. It needs a better name. Unit naming: pqs::length::m - Can we also have pqs::length::meter & metre? (Can always typedef this, I suppose) pqs::velocity::m_div_s - m_per_s? I know this is discussed in the documentation but again, typedefs might be nice to have Output: 1 kg.m+2.s-2 - What does this mean? It wasn't clear to me when I first saw it. Why not use ^ or ** for power? Why even have the concept of a prefix_offset? It seems unnecessarily confusing to me. Why does mass default to kg rather than g? If these special cases are really needed, then the documentation needs to explain why. Otherwise, I would prefer that everything work similarly to reduce complexity. The big problem with the design is the assumption that every quantity prefix is based on a power of 10. That's fine for SI units but my hope is that pqs will go beyond just SI. It already does with the incoherent_units so there is some precedent here. Even before pqs was put up for formal review, I had planned to make use of it in some computer simulator work that I'm doing. The ability to create units like Megabytes and Mibibytes would help tremendously. Unfortunately, I could not figure out how to create a unit that uses prefixes that are powers of 2. It might be ok to "fake" it if the calculations always stayed within one system (Mibibytes, Kibibytes, etc.) but as soon as one wants to convert Mibibytes to Megabytes it falls apart. If this can already be done, I'd be overjoyed to be corrected. But I could not puzzle it out from the documentation or a cursory look through the implementation. This is a big gap for me and prevents the use of pqs in most of the day-to-day work that I do. There's also the question of what base_dimension these would fit under. "Amount of substance?" It would be nice to be able to extend the types of base_dimension but I recognize that this could be very difficult given the design of pqs::meta::dimension and classes that depend on it. - What is your evaluation of the implementation? I did not thoroughly review the implementation, just glanced at it to answer some questions. Obviously it does not seem to fulfill my requirements as I stated above. I suspect it will be a bit of work to introduce the new flexibility. - What is your evaluation of the documentation? This needs a lot of work. It seems to assume a deep understanding of SI standards and terminology (beyond the unit names and what they mean) and introduces lots of terms itself (abstract_quantity, named_quantity, concrete_quantity, etc.). The presentation is overwhelming. Some specific suggestions: In the introduction section, we have the following statements about rationale: Increases programmer satisfaction What does this mean, exactly? It's very vague and subjective. enhances productivity How, apart from other rationales given? My suggestion would be to organize the rationale like this: * Increases programmer satisfaction and enhances productivity. * Strong type checking enforces dimensional integrity, catching calculation errors at compile time. * Automated unit conversion reduces drudgery. * Self documenting, helps code clarity. This is done better in the Rationale section itself. Make the presentation in these two sections similar in structure. The ERD was confusing to me. Can this be drawn in UML or a close-to-some-standard way? Reading through the documentation, one is confronted with the ginormous definitions page almost right away. I'd prefer to see some simple examples of use first. The "Getting started" page is a good start, but would be improved by removing references to the more esoteric terms. Those can come later. The definitions page itself needs many more code examples (for example, for named_quantity). It took me a long time to begin to understand what these terms really mean and I'm still not all the way there yet. I found the colored links to the terms all over the documentation to be distracting. It really interrupted the flow of the text. It would be better to make these links that look like ordinary text but are highlighted/underlined when the cursor moves over them. The links need to be there but they should be less intrusive. Typos (fixes in brackets): [t1_quantity features] The t1_quantity has many similar features to an [a] built-in type... Where used the type returned by typeof will not[neither] be a reference nor have cv-qualifiers. [abstract_quantity_id] The abstract_quantity_id has two roles, the first of which is to distinguish between different but dimensionally_equivalent[ ] abstract_quantities. [Unary operations example] [c]ondition_true [Addition and subtraction] If one type has a coherent_unit and the other has an incoherent_unit the operand with the incoherent_unit will first be converted to a t1_quantity in the nearest coherent_unit( simply found by setting the incoherent_unit[']s[ ]unit_multiplier to 1) [unit_output_symbol] A text symbol representing the unit[ ]of a concrete_quantity in stream output. [Lots of these missing spaces, underscores and first letters of example code through the documentation. Too many to enumerate.] [Stream output overview] The output of units for stream output functions for t1_quantities resolves to 3[three] output formats - What is your evaluation of the potential usefulness of the library? Very. IMHO strong typing is essential when designing large systems. - Did you try to use the library? With what compiler? Did you have any problems? I was going to do some experiments to create my own quantities as described above, but I could not figure out how to do it, so I did not do any testing. - How much effort did you put into your evaluation? A glance? A quick reading? In-depth study? In-depth reading of the documentation with a clear idea of how I wanted to extend the framework. Unfortunately, I discovered that I could not do what I'd hoped to accomplish. - Are you knowledgeable about the problem domain? In the same way most scientists and engineers are. I'm certainly no authority on the formal SI standards. I do have a large amount of experience in computer simulation and have run into the need for this kind of package for "computer units" over and over. It would be nice to have a package that went beyond the SI and its rigid assumptions.