mp-units/docs/blog/posts/2.3.0-released.md

---
date: 2024-09-27
authors:
 - mpusz
categories:
 - Releases
comments: true
---

# mp-units 2.3.0 released!

**A new product version can be obtained from
[GitHub](https://github.com/mpusz/mp-units/releases/tag/v2.3.0) and
[Conan](https://conan.io/center/recipes/mp-units?version=2.3.0).**

This release fine-tunes many key features of the library. This post describes the most interesting
improvements, while a much longer list of the changes introduced by the new version can be found in
our [Release Notes](../../release_notes.md#2.3.0).

<!-- more -->

## CMake and Conan options changed

During the review on the ConanCenter, we got feedback that we should improve the handling of
options for which value is automatically determined based on the current configuration.
Instead of explicitly setting the `auto` value, we defer the choice between `True`/`False` until
the configuration stage and set it there once all the settings are known. `auto` value for such
option was removed (:boom: **breaking change** :boom:).

If you didn't set any value at the command line for such options, everything stays the same for
you. However, some changes are needed if you explicitly used `auto` like below:

```bash
conan install . -o 'mp-units:std_format=auto' -s compiler.cppstd=23 -b missing
```

Now you have to either skip such an option to keep automatic deduction:

```bash
conan install . -s compiler.cppstd=23 -b missing
```

or set it explicitly to `True` or `False` to force a specific configuration:

```bash
conan install . -o 'mp-units:std_format=True' -s compiler.cppstd=23 -b missing
```


## MSVC compiler support

The MSVC compiler has the most bugs in the C++20 support from all our compilers. However,
with this release, we could apply many workarounds to the library's code to make it work. 🎉

Please note those workarounds were only applied to the library's code and not to our unit tests
and examples. Trying to build the entire project on MSVC will inevitably fail to compile unless
the MSVC bugs are resolved. MSVC developers still have some work to do.


## Representation type template parameter added to value conversion functions

Previously, changing a representation type was only possible with a `value_cast<NewRep>(q)`
non-member function while a change of unit was supported by all `value_cast<NewU>(q)`,
`q.in(NewU)`, and `q.force_in(NewU)`. The rationale for it was that passing an explicit type to
a member function template requires a `template` disambiguator when we are dealing with a dependent
name (e.g., `quantity` type is determined based on a template parameter).

During a discussion in LEWGI at the St. Louis WG21 Meeting, we decided to provide such additional
overloads despite possible issues when a dependent name is used. In such case, a user needs
to provide a `template` disambiguator or switch back to using `value_cast`:

*[LEWGI]: Library Evolution Working Group Incubator

```cpp
// non-dependent name
auto f(quantity<m, int> q)             { return q.in<double>(km); }
auto g(quantity<m, int> q)             { return value_cast<double, km>(q); }

// dependent name
auto h(QuantityOf<isq::length> auto q) { return q.template in<double>(km); }
auto i(QuantityOf<isq::length> auto q) { return value_cast<double, km>(q); }
```

The table below provides all the value conversion functions in **mp-units** that may be run on
`x` being the instance of either `quantity` or `quantity_point`:

| Forcing | Representation | Unit | Member function    | Non-member function                            |
|:-------:|:--------------:|:----:|--------------------|------------------------------------------------|
|   No    |      Same      | `u`  | `x.in(u)`          |                                                |
|   No    |      `T`       | Same | `x.in<T>()`        |                                                |
|   No    |      `T`       | `u`  | `x.in<T>(u)`       |                                                |
|   Yes   |      Same      | `u`  | `x.force_in(u)`    | `value_cast<u>(x)`                             |
|   Yes   |      `T`       | Same | `x.force_in<T>()`  | `value_cast<T>(x)`                             |
|   Yes   |      `T`       | `u`  | `x.force_in<T>(u)` | `value_cast<u, T>(x)` or `value_cast<T, u>(x)` |


## Quantity reference specifiers

The features described in this chapter directly solve an issue raised on
[std-proposals reflector](https://lists.isocpp.org/std-proposals/2024/06/10118.php). As it was
reported, the code below may look correct, but it provides an invalid result:

```cpp
quantity Volume = 1.0 * m3;
quantity Temperature = 28.0 * deg_C;
quantity n_ = 0.04401 * kg / mol;
quantity R_boltzman = 8.314 * N * m / (K * mol);
quantity mass = 40.0 * kg;
quantity Pressure = R_boltzman * Temperature.in(K) * mass / n_ / Volume;
std::cout << Pressure << "\n";
```

The problem is related to the accidental usage of a `quantity` rather than `quantity_point` for
`Temperature`. This means that after conversion to kelvins, we will get `28 K` instead of
the expected `301.15 K`, corrupting all further calculations.

A correct code should use a `quantity_point`:

```cpp
quantity_point Temperature(28.0 * deg_C);
```

This might be an obvious thing for domain experts, but new users of the library may not be aware
of the affine space abstractions and how they influence temperature handling.

After a lengthy discussion on handling such scenarios, we decided to:

- make the above code deprecated (:boom: **breaking change** :boom:),
- provide an alternative way to create a `quantity` with the `delta` quantity construction helper.

Here are the main points of this new design:

1. All references/units that specify point origin in their definition (i.e., `si::kelvin`,
   `si::degree_Celsius`, and `usc::degree_Fahrenheit`) are excluded from the multiply syntax
 (:boom: **breaking change** :boom:).
2. A new `delta` quantity construction helper is introduced:

    - `delta<m>(42)` results with a `quantity<si::metre, int>`,
    - `delta<deg_C>(5)` results with a `quantity<si::deg_C, int>`.

3. A new `absolute` quantity point construction helper is introduced:

    - `absolute<m>(42)` results with a `quantity_point<si::metre, zeroth_point_origin<kind_of<isq::length>>{}, int>`,
    - `absolute<deg_C>(5)` results with a `quantity<si::metre, si::ice_point, int>`.

!!! info

    Please note that `si::kelvin` is also excluded from the multiply syntax to prevent the
    following surprising issues:

    === "Now"

        ```cpp
        quantity q = delta<K>(300);
        quantity_point qp = absolute<K>(300);
        static_assert(q.in(deg_C) != qp.in(deg_C).quantity_from_zero());
        ```

    === "Before"

        ```cpp
        quantity q(300 * K);
        quantity_point qp(300 * K);
        static_assert(q.in(deg_C) != qp.in(deg_C).quantity_from_zero());
        ```

    We believe that the code enforced with new utilities makes it much easier to understand what
    happens here.

With such changes to the interface design, the offending code will not compile as initially written.
Users will be forced to think more about what they write. To enable the compilation, the users have
to create explicitly:

- a `quantity_point` (the intended abstraction in this example) with any of the below syntaxes:

    ```cpp
    quantity_point Temperature = absolute<deg_C>(28.0);
    auto Temperature = absolute<deg_C>(28.0);
    quantity_point Temperature(delta<deg_C>(28.0));
    ```

- a `quantity` (an incorrect abstraction in this example) with:

    ```cpp
    quantity Temperature = delta<deg_C>(28.0);
    auto Temperature = delta<deg_C>(28.0);
    ```

Thanks to the new design, we can immediately see what happens here and why the result might be
incorrect in the second case.


## `quantity_point_like_traits` are based on numerical value instead of a quantity

In this release, we decided to fine-tune the traits that customize the conversion between custom
quantity point types and the ones provided with **mp-units** (:boom: **breaking change** :boom:).

Previously, such type traits were based on the `quantity` type. This was inconsistent with
`quantity_like_traits`, that is working on raw values. Also, there are cases where a custom
quantity point abstraction is not modelled with a quantity type. In such cases, the previous
approach required additional types to be introduced for no good reason.

=== "Now"

    ```cpp
    template<>
    struct mp_units::quantity_point_like_traits<Timestamp> {
      static constexpr auto reference = si::second;
      static constexpr auto point_origin = default_point_origin(reference);
      using rep = decltype(Timestamp::seconds);

      static constexpr convert_implicitly<rep> to_numerical_value(Timestamp ts)
      {
        return ts.seconds;
      }

      static constexpr convert_explicitly<Timestamp> from_numerical_value(rep v)
      {
        return Timestamp(v);
      }
    };
    ```

=== "Before"

    ```cpp
    template<>
    struct mp_units::quantity_point_like_traits<Timestamp> {
      static constexpr auto reference = si::second;
      static constexpr auto point_origin = default_point_origin(reference);
      using rep = decltype(Timestamp::seconds);

      static constexpr convert_implicitly<quantity<reference, rep>> to_quantity(Timestamp ts)
      {
        return ts.seconds * si::second;
      }

      static constexpr convert_explicitly<Timestamp> from_quantity(quantity<reference, rep> q)
      {
        return Timestamp(q.numerical_value_ref_in(si::second));
      }
    };
    ```

!!! note

    The old behavior is deprecated and will be removed in future releases.


## `mag<pi>`

With this release, we introduced a new strongly-typed constant to create a magnitude involving
scaling by `pi`. The solution used before was not consistent with magnitudes of integral values
and also was leaking a floating-point value of `std::numbers::pi_v<long double>` to the resulting
magnitude type. With the new approach, this is no longer the case, and the user-facing interface
is more consistent:

=== "Now"

    ```cpp
    inline constexpr struct degree final : named_unit<{u8"°", "deg"}, mag<pi> / mag<180> * si::radian> {} degree;
    ```

=== "Before"

    ```cpp
    inline constexpr struct degree final : named_unit<{u8"°", "deg"}, mag_pi / mag<180> * si::radian> {} degree;
    ```

!!! note

    The old `mag_pi` helper is marked as deprecated and will be removed in future releases.


## Common units

Adding or subtracting two quantities of different units will force the library to find a common
unit for those. This is to prevent data truncation. For the cases when one of the units is an
integral multiple of the another, the resulting quantity will use a "smaller" one in its result.
For example:

```cpp
static_assert((1 * kg + 1 * g).unit == g);
static_assert((1 * km + 1 * mm).unit == mm);
static_assert((1 * yd + 1 * mi).unit == yd);
```

However, in many cases an arithmetic on quantities of different units will result in a yet another
unit. This happens when none of the source units is an integral multiple of another. In such cases,
the library returns a special type that denotes that we are dealing with a common unit of such
an equation.

Previously we returned a scaled unit calculated against our arbitrarily appointed reference unit.
This resulted often in a long and messy type exposing the prime-factorized magnitude of the unit
(implementation detail). In this release, we introduced a new `common_unit` wrapper for such cases:

=== "Now"

    ```cpp
    quantity q = 1 * km + 1 * mi;  // quantity<common_unit<international::mile, si::kilo_<si::metre>>{}, int>
    ```

=== "Before"

    ```cpp
    quantity q = 1 * km + 1 * mi;  // quantity<scaled_unit<magnitude<power_v<2, 3>{}, power_v<5, -3>{}>{}, si::metre>{}, int>
    ```

!!! note

    A user should never explicitly instantiate a `common_unit` class template. The library's
    framework will do it based on the provided quantity equation.

Such units need special printing rules for their symbols. As they represent a minimum set of common
units resulting from the addition or subtraction of multiple quantities, from this release, we
print all of them as a scaled version of the source unit. Previously we were printing them relative
to some arbitrary reference unit (implementation detail) that often was not spelled by the user at
all in the source code. For example the following:

```cpp
std::cout << 1 * km + 1 * mi << "\n";
std::cout << 1 * nmi + 1 * mi << "\n";
std::cout << 1 * km / h + 1 * m / s << "\n";
```

will print:

=== "Now"

    ```text
    40771 ([1/25146] mi = [1/15625] km)
    108167 ([1/50292] mi = [1/57875] nmi)
    23 ([1/5] km/h = [1/18] m/s)
    ```

=== "Before"

    ```text
    40771 [8/125] m
    108167 [4/125] m
    23 [1/18] m/s
    ```

Thanks to the above, it might be easier for the user to reason about the magnitude of the resulting
unit and its impact on the value stored in the quantity.

!!! info

    In order to provide `common_unit` strong type unit wrapper we had to rename all the
    `common_XXX()` functions to `get_common_XXX()` (:boom: **breaking change** :boom:).


## Superpowers of the unit `one`

In this release, we also added a long-awaited change. From now on a quantity of a unit `one` can be:

- **implicitly constructed from** the raw value,
- **explicitly converted to** a raw value,
- **compared to** a raw value.

=== "Now"

    ```cpp
    quantity<one> inc(quantity<one> q) { return q + 1; }
    void legacy(double) { /* ... */ }

    if (auto q = inc(42); q != 0)
      legacy(static_cast<int>(q));
    ```

=== "Before"

    ```cpp
    quantity<one> inc(quantity<one> q) { return q + 1 * one; }
    void legacy(double) { /* ... */ }

    if (auto q = inc(42 * one); q != 0 * one)
      legacy(q.numerical_value_in(one));
    ```

This property also expands to usual arithmetic operators.

With the above change, we can now achieve the same results in a terser way:

=== "Now"

    ```cpp
    static_assert(10 * km / (5 * km) == 2);
    const quantity gain = 1. / index;
    ```

=== "Before"

    ```cpp
    static_assert(10 * km / (5 * km) == 2 * one);
    const quantity gain = 1. / index * one;
    ```

!!! note

    Those rules do not apply to all the dimensionless quantities. It would be unsafe and misleading
    to allow such operations on units with a magnitude different than `1` (e.g., `percent` or
    `radian`).


## `import std;` support

This release brings experimental support for `import std;`. The only compiler that supports
it for now is clang-18+. Until all the compilers start to support it and CMake removes
the experimental tag from this feature, we will also keep it experimental.

As all of the C++ compilers are buggy for now, it is not allowed to bring the same definitions
through the `import std;` and regular header files. This applies not only to the current project
but also to all its dependencies. This is why, in order to use it with **mp-units**, we need to
disable all the dependencies as well (enforced with `conanfile.py`). It means that we have to use
`std::format` (instead of [fmtlib](https://github.com/fmtlib/fmt)) and remove functions contract
checking.

With the above assumptions, we can refactor our smoot example to:

```cpp hl_lines="2"
import mp_units;
import std;

using namespace mp_units;

inline constexpr struct smoot final : named_unit<"smoot", mag<67> * usc::inch> {} smoot;

int main()
{
  constexpr quantity dist = 364.4 * smoot;
  std::println("Harvard Bridge length = {::N[.1f]} ({::N[.1f]}, {::N[.2f]}) ± 1 εar",
               dist, dist.in(usc::foot), dist.in(si::metre));
}
```


## `unit_can_be_prefixed` removed

Previously, the `unit_can_be_prefixed` type trait was used to limit the possibility to prefix
some units that are officially known as non-prefixable (e.g., hour, minute).

It turned out that it is not easy to determine whether some units can be prefixed. For example,
for degree Celsius, the ISO 80000-5 standard explicitly states:

> Prefixes are not allowed in combination with the unit °C.

On the other hand [this NIST page](https://www.nist.gov/pml/owm/writing-si-metric-system-units)
says:

> Prefix symbols may be used with the unit symbol ºC and prefix names may be used with the unit
> name “degree Celsius.” For example, 12 mºC (12 millidegrees Celsius) is acceptable.

It seems that it is also a [common engineering practice](https://github.com/search?q=repo%3Atorvalds%2Flinux+millidegree&type=code).

To prevent such issues, we decided to simplify the library's design and remove the
`unit_can_be_prefixed` type trait (:boom: **breaking change** :boom:).

From now on, every named unit in the library can be prefixed with the SI or IEC prefix.


## `iec80000` system renamed to `iec`

As we mentioned IEC already, in this release, we decided to rename the name of the system and its
corresponding namespace from `iec80000` to `iec` (:boom: **breaking change** :boom:). This involves
renaming of a defining header file and of the namespace it provides.

With this change it should be easier to type the namespace name. This name is also more correct
for some quantities and units that are introduced by IEC but not necessarily in the ISO/IEC 80000
series of documents (e.g., `iec::var`).

!!! note

    The old `iec80000` namespace in _iec8000.h_ is marked as deprecated and will be removed in
    future releases.


## Error messages-related improvements

The readability of compile-time error messages is always a challenge for generic C++ libraries.
However, for quantities and units library, generating readable errors is the most important
requirement. If you do not make errors, you do not need such a library in your project :wink:.

This is why we put lots of effort into improving here. Besides submitting compiler bugs to improve
on their part, we also try to do our best here.

Some compilers do not present the type resulting from calling a function within a template
argument. This ends up with statements like `get_quantity_spec(si::second{})` in the error message.
Some less experienced users of the library may not know what this mean, and then why the
conversion error happens.

To improve this, we injected additional helper concepts into the definitions. It results with
a bit longer but a more readable error in the end.

For example:

=== "Now"

    ```text hl_lines="19"
    error: no matching member function for call to 'in'
       15 | const quantity time_to_goal = (distance * speed).in(s);
          |                               ~~~~~~~~~~~~~~~~~~~^~
    note: candidate template ignored: constraints not satisfied [with ToU = struct second]
      221 |   [[nodiscard]] constexpr QuantityOf<quantity_spec> auto in(ToU) const
          |                                                          ^
    note: because 'detail::UnitCompatibleWith<si::second, unit, quantity_spec>' evaluated to false
      219 |   template<detail::UnitCompatibleWith<unit, quantity_spec> ToU>
          |            ^
    note: because '!AssociatedUnit<si::second>' evaluated to false
      164 |   (!AssociatedUnit<U> || UnitOf<U, QS>) && detail::UnitConvertibleTo<FromU, U{}>;
          |    ^
    note: and 'UnitOf<si::second, kind_of_<derived_quantity_spec<power<isq::length, 2>, per<isq::time> > >{}>' evaluated to false
      164 |   (!AssociatedUnit<U> || UnitOf<U, QS>) && detail::UnitConvertibleTo<FromU, U{}>;
          |                          ^
    note: because 'detail::QuantitySpecConvertibleTo<get_quantity_spec(si::second{}), kind_of_<derived_quantity_spec<power<isq::length, 2>, per<isq::time> > >{}>' evaluated to false
      141 |                  detail::QuantitySpecConvertibleTo<get_quantity_spec(U{}), QS> &&
          |                  ^
    note: because 'implicitly_convertible(kind_of_<struct time>{}, kind_of_<derived_quantity_spec<power<isq::length, 2>, per<isq::time> > >{})' evaluated to false
      151 |   implicitly_convertible(From, To);
          |   ^
    1 error generated.
    Compiler returned: 1
    ```

=== "Before"

    ```text hl_lines="16"
    error: no matching member function for call to 'in'
       15 | const quantity time_to_goal = (distance * speed).in(s);
          |                               ~~~~~~~~~~~~~~~~~~~^~
    note: candidate template ignored: constraints not satisfied [with U = struct second]
      185 |   [[nodiscard]] constexpr QuantityOf<quantity_spec> auto in(U) const
          |                                                          ^
    note: because 'detail::UnitCompatibleWith<si::second, unit, quantity_spec>' evaluated to false
      183 |   template<detail::UnitCompatibleWith<unit, quantity_spec> U>
          |            ^
    note: because '!AssociatedUnit<si::second>' evaluated to false
      207 |   (!AssociatedUnit<U> || UnitOf<U, QS>)&&(detail::have_same_canonical_reference_unit(U{}, U2));
          |    ^
    note: and 'UnitOf<si::second, kind_of_<derived_quantity_spec<power<isq::length, 2>, per<isq::time> > >{}>' evaluated to false
      207 |   (!AssociatedUnit<U> || UnitOf<U, QS>)&&(detail::have_same_canonical_reference_unit(U{}, U2));
          |                          ^
    note: because 'implicitly_convertible(get_quantity_spec(si::second{}), kind_of_<derived_quantity_spec<power<length, 2>, per<time> > >{})' evaluated to false
      187 |   implicitly_convertible(get_quantity_spec(U{}), QS) &&
          |   ^
    1 error generated.
    Compiler returned: 1
    ```

!!! note

    The above error messages were stripped a bit of the additional information (file name,
    namespace name, nested curlies) to provide better readability.