# Introduction

**mp-units** is a Modern C++ library that provides compile-time dimensional analysis and unit/quantity
manipulation. The initial versions of the library were inspired by the
[`std::chrono::duration`](https://en.cppreference.com/w/cpp/chrono/duration)
but with each release, the interfaces diverged from the original to provide a better user
experience.


## Open Source

**mp-units** is Free and Open Source, with a permissive
[MIT license](https://github.com/mpusz/mp-units/blob/master/LICENSE.md). Check out the source
code and issue tracking (for questions and support, reporting bugs, suggesting feature requests
and improvements) at <https://github.com/mpusz/mp-units>.


## With the User's Experience in Mind

Most of the critical design decisions in the library are dictated by the requirement of
providing the best user experience possible. Other C++ physical units libraries are
"famous" for their enormous and hard-to-understand error messages (one line of the error log often
do not fit on one slide). The ultimate goal of **mp-units** is to improve this and make compile-time
errors and debugging as easy and user-friendly as possible.

To achieve this goal, several techniques are applied:

- usage of C++20 concepts,
- [using strong types for framework entities (instead of type aliases)](../../users_guide/framework_basics/interface_introduction/#strong-types-instead-of-aliases),
- [usage of expression templates to improve the readability of generated types](../../users_guide/framework_basics/interface_introduction/#expression-templates),
- limiting the number of template arguments to the bare minimum.

!!! note

    In many generic C++ libraries compile-time errors do not happen often. It is hard to
    break `std::string` or `std::vector` in a way it won't compile with a huge error
    log. Physical Units libraries are different. **Generation of compile-time errors
    is the main reason to use such a library.**


## Key Features

| Feature                      | Description                                                                                                                                                                                                                                                                                                                                         |
|------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Safety**                   | - [The affine space strong types] (`quantity` and `quantity_point`)<br>- [Compile-time checked conversions of quantities and units]<br>- [Unique support for many quantities of the same kind]<br>- [Type-safe equations on scalar, vector, and tensor quantities and their units]<br>- [Value-preserving conversions]                              |
| **Performance**              | - All the compile-time logic implemented as immediate (`consteval`) functions<br>- As fast or even faster than working with fundamental types<br>- No space size overhead needed to implement high-level abstractions                                                                                                                               |
| **Great User Experience**    | - [Optimized for readable compilation errors and great debugging experience]<br>- [Efficient and composable way to specify a unit of choice]<br>- [Value-based dimension, unit, and quantity equations]                                                                                                                                             |
| **Feature Rich**             | - [Systems of Quantities]<br>- [Systems of Units]<br>- [Scalar, vector, and tensor quantities]<br>- [The affine space]<br>- [Natural units systems support]<br>- [Strong angular system]<br>- [Supports any unit's magnitude (huge, small, floating-point)]<br>- [Faster-than-lightspeed constants]<br>- [Highly adjustable text-output formatting] |
| **Easy to Extend**           | - [Each entity can be defined with a single line of code]<br>- [User can easily extend the systems with custom dimensions, quantities, and units]                                                                                                                                                                                                   |
| **Low Standardization Cost** | - Small number of predefined entities needed thanks to composability<br>- No external dependencies (assuming full C++20 support)<br>- No macros in the user interface (besides portability and standard-compliance issues)<br>- Possibility to be standardized as a [freestanding] part of the C++ Standard Library                                 |


[The affine space strong types]: ../../users_guide/framework_basics/the_affine_space
[Compile-time checked conversions of quantities and units]: ../../users_guide/framework_basics/systems_of_quantities/#converting-between-quantities
[Unique support for many quantities of the same kind]: ../../users_guide/framework_basics/systems_of_quantities/#quantities-of-the-same-kind
[Type-safe equations on scalar, vector, and tensor quantities and their units]: ../../users_guide/framework_basics/quantity_arithmetics
[Value-preserving conversions]: ../../users_guide/framework_basics/value_conversions/#value-preserving-conversions

[Optimized for readable compilation errors and great debugging experience]: ../../users_guide/framework_basics/simple_and_typed_quantities/#easy-to-understand-compilation-error-messages
[Efficient and composable way to specify a unit of choice]: ../../users_guide/framework_basics/systems_of_units/#units-compose
[Value-based dimension, unit, and quantity equations]: ../../users_guide/framework_basics/interface_introduction/#value-based-equations

[Systems of Quantities]: ../../users_guide/framework_basics/systems_of_quantities
[Systems of Units]: ../../users_guide/framework_basics/systems_of_units
[Scalar, vector, and tensor quantities]: ../../users_guide/framework_basics/character_of_a_quantity
[The affine space]: ../../users_guide/framework_basics/the_affine_space
[Natural units systems support]: ../../users_guide/defining_systems/natural_units
[Strong angular system]: ../../users_guide/defining_systems/strong_angular_system
[Supports any unit's magnitude (huge, small, floating-point)]: ../../users_guide/framework_basics/systems_of_units/#scaled-units
[Faster-than-lightspeed constants]: ../../users_guide/framework_basics/faster_than_lightspeed_constants
[Highly adjustable text-output formatting]: ../../users_guide/framework_basics/text_output

[Each entity can be defined with a single line of code]: ../../users_guide/framework_basics/interface_introduction/#new-style-of-definitions
[User can easily extend the systems with custom dimensions, quantities, and units]: ../../users_guide/framework_basics/interface_introduction/#new-style-of-definitions

[freestanding]: https://en.cppreference.com/w/cpp/freestanding