mp-units/README.md

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# `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

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

## Release notes

- ???
  - 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

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