# Look and Feel¶

Here is a small example of operations possible on scalar quantities:

import mp_units;

using namespace mp_units;
using namespace mp_units::si::unit_symbols;

// simple numeric operations
static_assert(10 * km / 2 == 5 * km);

// conversions to common units
static_assert(1 * h == 3600 * s);
static_assert(1 * km + 1 * m == 1001 * m);

// derived quantities
static_assert(1 * km / (1 * s) == 1000 * m / s);
static_assert(2 * km / h * (2 * h) == 4 * km);
static_assert(2 * km / (2 * km / h) == 1 * h);

static_assert(2 * m * (3 * m) == 6 * m2);

static_assert(10 * km / (5 * km) == 2);

static_assert(1000 / (1 * s) == 1 * kHz);

#include <mp-units/systems/si.h>

using namespace mp_units;
using namespace mp_units::si::unit_symbols;

// simple numeric operations
static_assert(10 * km / 2 == 5 * km);

// conversions to common units
static_assert(1 * h == 3600 * s);
static_assert(1 * km + 1 * m == 1001 * m);

// derived quantities
static_assert(1 * km / (1 * s) == 1000 * m / s);
static_assert(2 * km / h * (2 * h) == 4 * km);
static_assert(2 * km / (2 * km / h) == 1 * h);

static_assert(2 * m * (3 * m) == 6 * m2);

static_assert(10 * km / (5 * km) == 2);

static_assert(1000 / (1 * s) == 1 * kHz);


This library requires some C++20 features (concepts and constraints, classes as NTTP, ...). Thanks to them, a user gets a powerful but still easy-to-use interface where all unit conversions and dimensional analysis can be performed without sacrificing accuracy. Please see the below example for a quick preview of basic library features:

#include <format>
#include <iomanip>
#include <iostream>
#include <print>
import mp_units;

using namespace mp_units;

constexpr QuantityOf<isq::speed> auto avg_speed(QuantityOf<isq::length> auto d,
QuantityOf<isq::time> auto t)
{
return d / t;
}

int main()
{
using namespace mp_units::si::unit_symbols;
using namespace mp_units::international::unit_symbols;

constexpr quantity v1 = 110 * km / h;
constexpr quantity v2 = 70 * mph;
constexpr quantity v3 = avg_speed(220. * isq::distance[km], 2 * h);
constexpr quantity v4 = avg_speed(isq::distance(140. * mi), 2 * h);
constexpr quantity v5 = v3.in(m / s);
constexpr quantity v6 = value_cast<m / s>(v4);
constexpr quantity v7 = value_cast<int>(v6);

std::cout << v1 << '\n';                                        // 110 km/h
std::cout << std::setw(10) << std::setfill('*') << v2 << '\n';  // ***70 mi/h
std::cout << std::format("{:*^10}\n", v3);                      // *110 km/h*
std::println("{:%N in %U of %D}", v4);                          // 70 in mi/h of LT⁻¹
std::println("{::N[.2f]}", v5);                                 // 30.56 m/s
std::println("{::N[.2f]U[dn]}", v6);                            // 31.29 m⋅s⁻¹
std::println("{:%N}", v7);                                      // 31
}

#include <mp-units/format.h>
#include <mp-units/ostream.h>
#include <mp-units/systems/international.h>
#include <mp-units/systems/isq.h>
#include <mp-units/systems/si.h>
#include <format>
#include <iomanip>
#include <iostream>
#include <print>

using namespace mp_units;

constexpr QuantityOf<isq::speed> auto avg_speed(QuantityOf<isq::length> auto d,
QuantityOf<isq::time> auto t)
{
return d / t;
}

int main()
{
using namespace mp_units::si::unit_symbols;
using namespace mp_units::international::unit_symbols;

constexpr quantity v1 = 110 * km / h;
constexpr quantity v2 = 70 * mph;
constexpr quantity v3 = avg_speed(220. * isq::distance[km], 2 * h);
constexpr quantity v4 = avg_speed(isq::distance(140. * mi), 2 * h);
constexpr quantity v5 = v3.in(m / s);
constexpr quantity v6 = value_cast<m / s>(v4);
constexpr quantity v7 = value_cast<int>(v6);

std::cout << v1 << '\n';                                        // 110 km/h
std::cout << std::setw(10) << std::setfill('*') << v2 << '\n';  // ***70 mi/h
std::cout << std::format("{:*^10}\n", v3);                      // *110 km/h*
std::println("{:%N in %U of %D}", v4);                          // 70 in mi/h of LT⁻¹
std::println("{::N[.2f]}", v5);                                 // 30.56 m/s
std::println("{::N[.2f]U[dn]}", v6);                            // 31.29 m⋅s⁻¹
std::println("{:%N}", v7);                                      // 31
}


Note

More code examples can be found in the Examples chapter.