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# `mp-units` - A Units Library for C++

## Summary

`Units` is a compile-time enabled Modern C++ library that provides compile-time dimensional
analysis and unit/quantity manipulation. The basic idea and design heavily bases on
`std::chrono::duration` and extends it to work properly with many dimensions.

Here is a small example of possible operations:

```cpp
// simple numeric operations
static_assert(10_km / 2 == 5_km);

// unit conversions
static_assert(1_h == 3600_s);
static_assert(1_km + 1_m == 1001_m);

// dimension conversions
static_assert(1_km / 1_s == 1000_mps);
static_assert(2_kmph * 2_h == 4_km);
static_assert(2_km / 2_kmph == 1_h);

static_assert(1000 / 1_s == 1_kHz);

static_assert(10_km / 5_km == 2);
```

## Repository structure

That repository contains the following independent `cmake`-based projects:
- `./src` - header-only project for `units`
- `.` - project used for development needs that wraps `./src` project together with
  usage examples and unit tests
- `./test_package` - library installation and Conan package verification

Please note that the projects depend on `cmake` git submodule in the `./cmake/common`
subdirectory.

## Building, testing, and installation

For a detailed information on project compilation, testing and reuse please refer to
[doc/INSTALL.md](doc/INSTALL.md).

NOTE: This library as of now compiles correctly only with gcc-9.1 and newer.  

## Library design

`mp-units` library design rationale and documentation can be found in
[doc/DESIGN.md](doc/DESIGN.md)

## Release notes

- 0.4.0 ???
  - Support for derived dimensions in `exp` added
  - Added `pow()` and `sqrt()` operations on quantities
  - `units` removed from a `std::experimental` namespace
  - Downcasting facility refactored so the user does not have to write the boilerplate code anymore
  - From now on base dimensions should inherit from `base_dimension` class template
  - Added unit symbols definitions to `base_dimension` and `derived_unit`

- 0.3.1 Sep 18, 2019
  - cmcstl2 dependency changed to range-v3 0.9.1

- 0.3.0 Sep 16, 2019 (CppCon 2019 design)
  - The design as described on CppCon 2019 talk (<https://youtu.be/0YW6yxkdhlU>)
  - Applied the feedback from the Cologne evening session
    - `upcasting_traits` renamed to `downcasting_traits`
    - `Dimension` template parameter removed from quantity
  - `units` moved to a `std::experimental` namespace
  - Leading underscore prefix removed from UDLs
  - Added a few more derived dimensions
  - `meter` renamed to `metre`
  - Missing `operator*` added
  - Predefined dimensions moved to a dedicated directory
  - `dimension_` prefix removed from names of derived dimensions
  - cmcstl2 library updated to 2019.09.19
  - `base_dimension` is a value provided as `const&` to the `exp` type
  - integrated with Compiler Explorer
  - gsl-lite dependency removed
  - Fractional dimension exponents support added
  - `QuantityOf` concept introduced
  - `quantity_cast<U, Rep>()` support added

- 0.2.0 July 18, 2019 (C++Now 2019 design)
  - The design as described on C++Now 2019 talk (<https://youtu.be/wKchCktZPHU>)
  - Added C++20 features supported by gcc-9.1 (`std::remove_cvref_t`, down with typename, `std::type_identity`)
  - Compile-time performance optimizations (`type_list`, `common_ratio`, `ratio`, `conditional_t`)

- 0.1.0 May 18, 2019
  - Initial library release
  - Begin semantic versioning
  - The last version to work with gcc-8