refactor: op[U] for quantity and quantity_point replaced with .in(U)

Resolves #469

README.md

@@ -109,7 +109,7 @@ int main()
   constexpr auto v2 = 70 * mph;
   constexpr auto v3 = avg_speed(220. * isq::distance[km], 2 * h);
   constexpr auto v4 = avg_speed(isq::distance(140. * mi), 2 * h);
-  constexpr auto v5 = v3[m / s];
+  constexpr auto v5 = v3.in(m / s);
   constexpr auto v6 = value_cast<m / s>(v4);
   constexpr auto v7 = value_cast<int>(v6);
 
@@ -123,4 +123,4 @@ int main()
 }
 ```
 
-_Try it on the [Compiler Explorer](https://godbolt.org/z/zPnerKqzh)._
+_Try it on the [Compiler Explorer](https://godbolt.org/z/E4rvPGa3n)._

docs/getting_started/code_example.md

@@ -62,7 +62,7 @@ int main()
   constexpr auto v2 = 70 * mph;
   constexpr auto v3 = avg_speed(220. * isq::distance[km], 2 * h);
   constexpr auto v4 = avg_speed(isq::distance(140. * mi), 2 * h);
-  constexpr auto v5 = v3[m / s];
+  constexpr auto v5 = v3.in(m / s);
   constexpr auto v6 = value_cast<m / s>(v4);
   constexpr auto v7 = value_cast<int>(v6);
 
@@ -76,8 +76,8 @@ int main()
 }
 ```
 
-!!! example "[Try it on Compiler Explorer](https://godbolt.org/z/zPnerKqzh)"
+!!! example "[Try it on Compiler Explorer](https://godbolt.org/z/E4rvPGa3n)"
 
 !!! note
 
-    You can find more code examples in the Examples chapter.
+    You can find more code examples in the [Examples](../users_guide/examples/tags_index.md) chapter.

docs/users_guide/framework_basics/faster_than_lightspeed_constants.md

@@ -61,7 +61,7 @@ constexpr auto speed_of_light_in_vacuum = 1 * si::si2019::speed_of_light_in_vacu
 
 QuantityOf<isq::permittivity_of_vacuum> auto q = 1 / (permeability_of_vacuum * pow<2>(speed_of_light_in_vacuum));
 
-std::cout << "permittivity of vacuum = " << q << " = " << q[F / m] << "\n";
+std::cout << "permittivity of vacuum = " << q << " = " << q.in(F / m) << "\n";
 ```
 
 The above first prints the following:
@@ -99,18 +99,18 @@ std::cout << "in `GeV` and `c`:\n"
           << "m = " << m1 << "\n"
           << "E = " << E << "\n";
 
-const auto p2 = p1[GeV / (m / s)];
+const auto p2 = p1.in(GeV / (m / s));
-const auto m2 = m1[GeV / pow<2>(m / s)];
+const auto m2 = m1.in(GeV / pow<2>(m / s));
-const auto E2 = total_energy(p2, m2, c)[GeV];
+const auto E2 = total_energy(p2, m2, c).in(GeV);
 
 std::cout << "\nin `GeV`:\n"
           << "p = " << p2 << "\n"
           << "m = " << m2 << "\n"
           << "E = " << E2 << "\n";
 
-const auto p3 = p1[kg * m / s];
+const auto p3 = p1.in(kg * m / s);
-const auto m3 = m1[kg];
+const auto m3 = m1.in(kg);
-const auto E3 = total_energy(p3, m3, c)[J];
+const auto E3 = total_energy(p3, m3, c).in(J);
 
 std::cout << "\nin SI base units:\n"
           << "p = " << p3 << "\n"

docs/users_guide/framework_basics/interface_introduction.md

@@ -264,7 +264,7 @@ using namespace mp_units::si::unit_symbols;
 auto speed = 60. * isq::speed[km / h];
 auto duration = 8 * s;
 auto acceleration = speed / duration;
-std::cout << "acceleration: " << acceleration << " (" << acceleration[m / s2] << ")\n";
+std::cout << "acceleration: " << acceleration << " (" << acceleration.in(m / s2) << ")\n";
 ```
 
 The `acceleration`, being the result of the above code, has the following type

docs/users_guide/framework_basics/simple_and_typed_quantities.md

@@ -72,7 +72,7 @@ int main()
 
   std::cout << "A car driving " << distance << " in " << duration
             << " has an average speed of " << speed
-            << " (" << speed[km / h] << ")\n";
+            << " (" << speed.in(km / h) << ")\n";
 }
 ```
 
@@ -82,7 +82,7 @@ The code above prints:
 A car driving 110 km in 2 h has an average speed of 15.2778 m/s (55 km/h)
 ```
 
-!!! example "[Try it on Compiler Explorer](https://godbolt.org/z/4zecYqn5z)"
+!!! example "[Try it on Compiler Explorer](https://godbolt.org/z/zWe8ecf93)"
 
 
 ### Easy to understand compilation error messages
@@ -146,7 +146,7 @@ int main()
 
   std::cout << "A car driving " << distance << " in " << duration
             << " has an average speed of " << speed
-            << " (" << speed[km / h] << ")\n";
+            << " (" << speed.in(km / h) << ")\n";
 }
 ```
 
@@ -154,7 +154,7 @@ int main()
 A car driving 110 km in 2 h has an average speed of 15.2778 m/s (55 km/h)
 ```
 
-!!! example "[Try it on Compiler Explorer](https://godbolt.org/z/jhfWjGadz)"
+!!! example "[Try it on Compiler Explorer](https://godbolt.org/z/q3PzMzqsh)"
 
 In case we will accidentally make the same calculation error as before, this time, we will
 get a bit longer error message also containing information about the quantity type:

docs/users_guide/framework_basics/systems_of_units.md

@@ -76,9 +76,9 @@ the following:
 ```cpp
 auto q1 = 42 * W;
 std::cout << q1 << "\n";
-std::cout << q1[J / s] << "\n";
+std::cout << q1.in(J / s) << "\n";
-std::cout << q1[N * m / s] << "\n";
+std::cout << q1.in(N * m / s) << "\n";
-std::cout << q1[kg * m2 / s3] << "\n";
+std::cout << q1.in(kg * m2 / s3) << "\n";
 ```
 
 prints:

docs/users_guide/framework_basics/text_output.md

@@ -44,7 +44,7 @@ associated with this quantity.
     before passing it to the text output:
 
     ```cpp
-    std::cout << v1[km / h] << '\n';             // 110 km/h
+    std::cout << v1.in(km / h) << '\n';          // 110 km/h
     std::cout << value_cast<m / s>(v1) << '\n';  // 30.5556 m/s
     ```
 

docs/users_guide/framework_basics/value_conversions.md

@@ -4,7 +4,7 @@
 
 ```cpp
 auto q1 = 5 * km;
-std::cout << q1[m] << '\n';
+std::cout << q1.in(m) << '\n';
 quantity<si::metre, int> q2 = q1;
 ```
 
@@ -23,7 +23,7 @@ In case a user would like to perform an opposite transformation:
 
 ```cpp
 auto q1 = 5 * m;
-std::cout << q1[km] << '\n';
+std::cout << q1.in(km) << '\n';
 quantity<si::kilo<si::metre>, int> q2 = q1;
 ```
 
@@ -34,7 +34,7 @@ representation type:
 
 ```cpp
 auto q1 = 5. * m;
-std::cout << q1[km] << '\n';
+std::cout << q1.in(km) << '\n';
 quantity<si::kilo<si::metre>> q2 = q1;
 ```
 

example/capacitor_time_curve.cpp

@@ -47,15 +47,15 @@ int main()
     std::cout << "at " << tt << " voltage is ";
 
     if (Vt >= 1 * V)
-      std::cout << Vt[V];
+      std::cout << Vt.in(V);
     else if (Vt >= 1 * mV)
-      std::cout << Vt[mV];
+      std::cout << Vt.in(mV);
     else if (Vt >= 1 * uV)
-      std::cout << Vt[uV];
+      std::cout << Vt.in(uV);
     else if (Vt >= 1 * nV)
-      std::cout << Vt[nV];
+      std::cout << Vt.in(nV);
     else
-      std::cout << Vt[pV];
+      std::cout << Vt.in(pV);
     std::cout << "\n";
   }
 }

example/clcpp_response.cpp

@@ -81,24 +81,24 @@ void quantities_with_typed_units()
 
   std::cout << meter << '\n';
 
-  std::cout << " = " << meter[astronomical_unit] << '\n';
+  std::cout << " = " << meter.in(astronomical_unit) << '\n';
-  std::cout << " = " << meter[iau::angstrom] << '\n';
+  std::cout << " = " << meter.in(iau::angstrom) << '\n';
-  std::cout << " = " << meter[imperial::chain] << '\n';
+  std::cout << " = " << meter.in(imperial::chain) << '\n';
-  std::cout << " = " << meter[imperial::fathom] << '\n';
+  std::cout << " = " << meter.in(imperial::fathom) << '\n';
-  std::cout << " = " << meter[usc::fathom] << '\n';
+  std::cout << " = " << meter.in(usc::fathom) << '\n';
-  std::cout << " = " << meter[international::foot] << '\n';
+  std::cout << " = " << meter.in(international::foot) << '\n';
-  std::cout << " = " << meter[usc::survey1893::us_survey_foot] << '\n';
+  std::cout << " = " << meter.in(usc::survey1893::us_survey_foot) << '\n';
-  std::cout << " = " << meter[international::inch] << '\n';
+  std::cout << " = " << meter.in(international::inch) << '\n';
-  std::cout << " = " << meter[iau::light_year] << '\n';
+  std::cout << " = " << meter.in(iau::light_year) << '\n';
-  std::cout << " = " << meter[international::mile] << '\n';
+  std::cout << " = " << meter.in(international::mile) << '\n';
-  std::cout << " = " << meter[international::nautical_mile] << '\n';
+  std::cout << " = " << meter.in(international::nautical_mile) << '\n';
-  std::cout << " = " << meter[iau::parsec] << '\n';
+  std::cout << " = " << meter.in(iau::parsec) << '\n';
-  std::cout << " = " << meter[typographic::pica_dtp] << '\n';
+  std::cout << " = " << meter.in(typographic::pica_dtp) << '\n';
-  std::cout << " = " << meter[typographic::pica_us] << '\n';
+  std::cout << " = " << meter.in(typographic::pica_us) << '\n';
-  std::cout << " = " << meter[typographic::point_dtp] << '\n';
+  std::cout << " = " << meter.in(typographic::point_dtp) << '\n';
-  std::cout << " = " << meter[typographic::point_us] << '\n';
+  std::cout << " = " << meter.in(typographic::point_us) << '\n';
-  std::cout << " = " << meter[imperial::rod] << '\n';
+  std::cout << " = " << meter.in(imperial::rod) << '\n';
-  std::cout << " = " << meter[international::yard] << '\n';
+  std::cout << " = " << meter.in(international::yard) << '\n';
 }
 
 void calcs_comparison()
@@ -119,8 +119,8 @@ void calcs_comparison()
                "or small values in other units to the base unit.\n"
                "This is both inefficient and inaccurate\n\n";
 
-  const auto L1B = L1A[m];
+  const auto L1B = L1A.in(m);
-  const auto L2B = L2A[m];
+  const auto L2B = L2A.in(m);
   const auto LrB = L1B + L2B;
   std::cout << MP_UNITS_STD_FMT::format("{:%.30eQ %q}\n + {:%.30eQ %q}\n   = {:%.30eQ %q}\n\n", L1B, L2B, LrB);
 

example/conversion_factor.cpp

@@ -46,7 +46,7 @@ int main()
   std::cout << "conversion factor in mp-units...\n\n";
 
   constexpr auto lengthA = 2.0 * m;
-  constexpr auto lengthB = lengthA[mm];
+  constexpr auto lengthB = lengthA.in(mm);
 
   std::cout << MP_UNITS_STD_FMT::format("lengthA( {} ) and lengthB( {} )\n", lengthA, lengthB)
             << "represent the same length in different units.\n\n";

example/hello_units.cpp

@@ -48,7 +48,7 @@ int main()
   constexpr auto v2 = 70 * mph;
   constexpr auto v3 = avg_speed(220. * km, 2 * h);
   constexpr auto v4 = avg_speed(isq::distance(140. * mi), 2 * isq::duration[h]);
-  constexpr auto v5 = v3[m / s];
+  constexpr auto v5 = v3.in(m / s);
   constexpr auto v6 = value_cast<m / s>(v4);
   constexpr auto v7 = value_cast<int>(v6);
 

example/measurement.cpp

@@ -135,7 +135,7 @@ void example()
   const auto t = measurement{1.2, 0.1} * s;
 
   const QuantityOf<isq::velocity> auto v = a * t;
-  std::cout << a << " * " << t << " = " << v << " = " << v[km / h] << '\n';
+  std::cout << a << " * " << t << " = " << v << " = " << v.in(km / h) << '\n';
 
   const auto length = measurement{123., 1.} * m;
   std::cout << "10 * " << length << " = " << 10 * length << '\n';

example/si_constants.cpp

@@ -20,9 +20,13 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+// !!! Before you commit any changes to this file please make sure to check if it !!!
+// !!! renders correctly in the documentation "Examples" section.                 !!!
+// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
 #include <mp-units/format.h>
-#include <mp-units/systems/si/constants.h>
+#include <mp-units/systems/si/si.h>
-#include <mp-units/systems/si/unit_symbols.h>
 #include <iostream>
 
 template<class T>
@@ -37,18 +41,18 @@ int main()
   std::cout << "The seven defining constants of the SI and the seven corresponding units they define:\n";
   std::cout << MP_UNITS_STD_FMT::format("- hyperfine transition frequency of Cs: {} = {:%.0Q %q}\n",
                                         1. * si2019::hyperfine_structure_transition_frequency_of_cs,
-                                        (1. * si2019::hyperfine_structure_transition_frequency_of_cs)[Hz]);
+                                        (1. * si2019::hyperfine_structure_transition_frequency_of_cs).in(Hz));
   std::cout << MP_UNITS_STD_FMT::format("- speed of light in vacuum:             {} = {:%.0Q %q}\n",
                                         1. * si2019::speed_of_light_in_vacuum,
-                                        (1. * si2019::speed_of_light_in_vacuum)[m / s]);
+                                        (1. * si2019::speed_of_light_in_vacuum).in(m / s));
   std::cout << MP_UNITS_STD_FMT::format("- Planck constant:                      {} = {:%.8eQ %q}\n",
-                                        1. * si2019::planck_constant, (1. * si2019::planck_constant)[J * s]);
+                                        1. * si2019::planck_constant, (1. * si2019::planck_constant).in(J * s));
   std::cout << MP_UNITS_STD_FMT::format("- elementary charge:                    {} = {:%.9eQ %q}\n",
-                                        1. * si2019::elementary_charge, (1. * si2019::elementary_charge)[C]);
+                                        1. * si2019::elementary_charge, (1. * si2019::elementary_charge).in(C));
   std::cout << MP_UNITS_STD_FMT::format("- Boltzmann constant:                   {} = {:%.6eQ %q}\n",
-                                        1. * si2019::boltzmann_constant, (1. * si2019::boltzmann_constant)[J / K]);
+                                        1. * si2019::boltzmann_constant, (1. * si2019::boltzmann_constant).in(J / K));
   std::cout << MP_UNITS_STD_FMT::format("- Avogadro constant:                    {} = {:%.8eQ %q}\n",
-                                        1. * si2019::avogadro_constant, (1. * si2019::avogadro_constant)[1 / mol]);
+                                        1. * si2019::avogadro_constant, (1. * si2019::avogadro_constant).in(1 / mol));
   // TODO uncomment the below when ISQ is done
   // std::cout << MP_UNITS_STD_FMT::format("- luminous efficacy:                    {} = {}\n",
   // si2019::luminous_efficacy(1.),

example/spectroscopy_units.cpp

@@ -60,8 +60,9 @@ template<QuantityOf<isq::energy> T1, QuantityOf<isq::wavenumber> T2, QuantityOf<
          QuantityOf<isq::thermodynamic_temperature> T4, QuantityOf<isq::wavelength> T5>
 void print_line_si(const std::tuple<T1, T2, T3, T4, T5>& t)
 {
-  MP_UNITS_STD_FMT::println("| {:<15} | {:<15} | {:<15} | {:<15} | {:<15} |", std::get<0>(t)[eV],
+  MP_UNITS_STD_FMT::println("| {:<15} | {:<15} | {:<15} | {:<15} | {:<15} |", std::get<0>(t).in(eV),
-                            std::get<1>(t)[1 / cm], std::get<2>(t)[THz], std::get<3>(t)[K], std::get<4>(t)[um]);
+                            std::get<1>(t).in(1 / cm), std::get<2>(t).in(THz), std::get<3>(t).in(K),
+                            std::get<4>(t).in(um));
 }
 
 int main()

example/storage_tank.cpp

@@ -126,10 +126,10 @@ int main()
   const auto fill_ratio = fill_level / height;
 
   std::cout << MP_UNITS_STD_FMT::format("fill height at {} = {} ({} full)\n", fill_time, fill_level,
-                                        fill_ratio[percent]);
+                                        fill_ratio.in(percent));
-  std::cout << MP_UNITS_STD_FMT::format("fill weight at {} = {} ({})\n", fill_time, filled_weight, filled_weight[N]);
+  std::cout << MP_UNITS_STD_FMT::format("fill weight at {} = {} ({})\n", fill_time, filled_weight, filled_weight.in(N));
   std::cout << MP_UNITS_STD_FMT::format("spare capacity at {} = {}\n", fill_time, spare_capacity);
   std::cout << MP_UNITS_STD_FMT::format("input flow rate = {}\n", input_flow_rate);
   std::cout << MP_UNITS_STD_FMT::format("float rise rate = {}\n", float_rise_rate);
-  std::cout << MP_UNITS_STD_FMT::format("tank full E.T.A. at current flow rate = {}\n", fill_time_left[s]);
+  std::cout << MP_UNITS_STD_FMT::format("tank full E.T.A. at current flow rate = {}\n", fill_time_left.in(s));
 }

example/strong_angular_quantities.cpp

@@ -44,5 +44,5 @@ int main()
   const auto torque = isq_angle::torque(lever * force * angular::sin(angle) / (1 * isq_angle::cotes_angle));
 
   std::cout << "Applying a perpendicular force of " << force << " to a " << lever << " long lever results in "
-            << torque[N * m / rad] << " of torque.\n";
+            << torque.in(N * m / rad) << " of torque.\n";
 }

example/total_energy.cpp

@@ -48,31 +48,31 @@ void si_example()
   using namespace mp_units::si::unit_symbols;
   constexpr auto GeV = si::giga<si::electronvolt>;
   constexpr QuantityOf<isq::speed> auto c = 1. * si::si2019::speed_of_light_in_vacuum;
-  auto c2 = pow<2>(c);
+  constexpr auto c2 = pow<2>(c);
 
   const auto p1 = isq::momentum(4. * GeV / c);
   const QuantityOf<isq::mass> auto m1 = 3. * GeV / c2;
   const auto E = total_energy(p1, m1, c);
 
-  std::cout << "\n*** SI units (c = " << c << " = " << c[si::metre / s] << ") ***\n";
+  std::cout << "\n*** SI units (c = " << c << " = " << c.in(si::metre / s) << ") ***\n";
 
   std::cout << "\n[in `GeV` and `c`]\n"
             << "p = " << p1 << "\n"
             << "m = " << m1 << "\n"
             << "E = " << E << "\n";
 
-  const auto p2 = p1[GeV / (m / s)];
+  const auto p2 = p1.in(GeV / (m / s));
-  const auto m2 = m1[GeV / pow<2>(m / s)];
+  const auto m2 = m1.in(GeV / pow<2>(m / s));
-  const auto E2 = total_energy(p2, m2, c)[GeV];
+  const auto E2 = total_energy(p2, m2, c).in(GeV);
 
   std::cout << "\n[in `GeV`]\n"
             << "p = " << p2 << "\n"
             << "m = " << m2 << "\n"
             << "E = " << E2 << "\n";
 
-  const auto p3 = p1[kg * m / s];
+  const auto p3 = p1.in(kg * m / s);
-  const auto m3 = m1[kg];
+  const auto m3 = m1.in(kg);
-  const auto E3 = total_energy(p3, m3, c)[J];
+  const auto E3 = total_energy(p3, m3, c).in(J);
 
   std::cout << "\n[in SI base units]\n"
             << "p = " << p3 << "\n"

src/core/include/mp-units/quantity.h

@@ -164,15 +164,15 @@ public:
 #endif
 
   template<Unit U>
-    requires requires(quantity q) { q[U{}]; }
+    requires requires(quantity q) { q.in(U{}); }
   [[nodiscard]] constexpr rep number_in(U) const noexcept
   {
-    return (*this)[U{}].number();
+    return (*this).in(U{}).number();
   }
 
   template<Unit U>
     requires detail::QuantityConvertibleTo<quantity, quantity<::mp_units::reference<quantity_spec, U{}>{}, Rep>>
-  [[nodiscard]] constexpr quantity<::mp_units::reference<quantity_spec, U{}>{}, Rep> operator[](U) const
+  [[nodiscard]] constexpr quantity<::mp_units::reference<quantity_spec, U{}>{}, Rep> in(U) const
   {
     return quantity<quantity_spec[U{}], Rep>{*this};
   }

src/core/include/mp-units/quantity_point.h

@@ -164,9 +164,9 @@ public:
 
   template<Unit U>
     requires detail::QuantityConvertibleTo<quantity_type, quantity<::mp_units::reference<quantity_spec, U{}>{}, Rep>>
-  [[nodiscard]] constexpr quantity_point<::mp_units::reference<quantity_spec, U{}>{}, PO, Rep> operator[](U) const
+  [[nodiscard]] constexpr quantity_point<::mp_units::reference<quantity_spec, U{}>{}, PO, Rep> in(U) const
   {
-    return make_quantity_point<PO>(quantity_from_origin()[U{}]);
+    return make_quantity_point<PO>(quantity_from_origin().in(U{}));
   }
 
   // member unary operators

src/utility/include/mp-units/math.h

@@ -304,7 +304,7 @@ template<ReferenceOf<angular_measure> auto R, typename Rep>
   requires requires { sin(q.number()); } || requires { std::sin(q.number()); }
 {
   using std::sin;
-  return make_quantity<one>(static_cast<Rep>(sin(q[si::radian].number())));
+  return make_quantity<one>(static_cast<Rep>(sin(q.in(si::radian).number())));
 }
 
 template<ReferenceOf<angular_measure> auto R, typename Rep>
@@ -313,7 +313,7 @@ template<ReferenceOf<angular_measure> auto R, typename Rep>
   requires requires { cos(q.number()); } || requires { std::cos(q.number()); }
 {
   using std::cos;
-  return make_quantity<one>(static_cast<Rep>(cos(q[si::radian].number())));
+  return make_quantity<one>(static_cast<Rep>(cos(q.in(si::radian).number())));
 }
 
 template<ReferenceOf<angular_measure> auto R, typename Rep>
@@ -322,7 +322,7 @@ template<ReferenceOf<angular_measure> auto R, typename Rep>
   requires requires { tan(q.number()); } || requires { std::tan(q.number()); }
 {
   using std::tan;
-  return make_quantity<one>(static_cast<Rep>(tan(q[si::radian].number())));
+  return make_quantity<one>(static_cast<Rep>(tan(q.in(si::radian).number())));
 }
 
 template<ReferenceOf<dimensionless> auto R, typename Rep>
@@ -362,7 +362,7 @@ template<ReferenceOf<angle> auto R, typename Rep>
   requires requires { sin(q.number()); } || requires { std::sin(q.number()); }
 {
   using std::sin;
-  return make_quantity<one>(static_cast<Rep>(sin(q[radian].number())));
+  return make_quantity<one>(static_cast<Rep>(sin(q.in(radian).number())));
 }
 
 template<ReferenceOf<angle> auto R, typename Rep>
@@ -371,7 +371,7 @@ template<ReferenceOf<angle> auto R, typename Rep>
   requires requires { cos(q.number()); } || requires { std::cos(q.number()); }
 {
   using std::cos;
-  return make_quantity<one>(static_cast<Rep>(cos(q[radian].number())));
+  return make_quantity<one>(static_cast<Rep>(cos(q.in(radian).number())));
 }
 
 template<ReferenceOf<angle> auto R, typename Rep>
@@ -380,7 +380,7 @@ template<ReferenceOf<angle> auto R, typename Rep>
   requires requires { tan(q.number()); } || requires { std::tan(q.number()); }
 {
   using std::tan;
-  return make_quantity<one>(static_cast<Rep>(tan(q[radian].number())));
+  return make_quantity<one>(static_cast<Rep>(tan(q.in(radian).number())));
 }
 
 template<ReferenceOf<dimensionless> auto R, typename Rep>

test/unit_test/runtime/linear_algebra_test.cpp

@@ -31,17 +31,14 @@
 #include <cmath>
 #include <matrix>
 
-
 template<typename Rep = double>
 using vector = STD_LA::fixed_size_column_vector<Rep, 3>;
 
 template<typename Rep>
 inline constexpr bool mp_units::is_vector<vector<Rep>> = true;
 
-namespace STD_LA {
-
 template<typename Rep>
-std::ostream& operator<<(std::ostream& os, const ::vector<Rep>& v)
+std::ostream& operator<<(std::ostream& os, const vector<Rep>& v)
 {
   os << "|";
   for (auto i = 0U; i < v.size(); ++i) {
@@ -51,8 +48,6 @@ std::ostream& operator<<(std::ostream& os, const ::vector<Rep>& v)
   return os;
 }
 
-}  // namespace STD_LA
-
 namespace {
 
 using namespace mp_units;
@@ -88,7 +83,7 @@ TEST_CASE("vector quantity", "[la]")
     SECTION("non-truncating")
     {
       const auto v = vector<int>{3, 2, 1} * isq::position_vector[km];
-      CHECK(v[m].number() == vector<int>{3000, 2000, 1000});
+      CHECK(v.in(m).number() == vector<int>{3000, 2000, 1000});
     }
 
     SECTION("truncating")

test/unit_test/static/quantity_point_test.cpp

@@ -576,21 +576,21 @@ static_assert(is_of_type<(ground_level + isq::height(short(42) * m)).point_for(m
 // converting to a different unit
 ///////////////////////////////////
 
-static_assert((mean_sea_level + 2. * km)[km].quantity_from_origin().number() == 2.);
+static_assert((mean_sea_level + 2. * km).in(km).quantity_from_origin().number() == 2.);
-static_assert((mean_sea_level + 2. * km)[m].quantity_from_origin().number() == 2000.);
+static_assert((mean_sea_level + 2. * km).in(m).quantity_from_origin().number() == 2000.);
-static_assert((mean_sea_level + 2000. * m)[km].quantity_from_origin().number() == 2.);
+static_assert((mean_sea_level + 2000. * m).in(km).quantity_from_origin().number() == 2.);
-static_assert((ground_level + 2. * km)[km].quantity_from_origin().number() == 2.);
+static_assert((ground_level + 2. * km).in(km).quantity_from_origin().number() == 2.);
-static_assert((ground_level + 2. * km)[m].quantity_from_origin().number() == 2000.);
+static_assert((ground_level + 2. * km).in(m).quantity_from_origin().number() == 2000.);
-static_assert((ground_level + 2000. * m)[km].quantity_from_origin().number() == 2.);
+static_assert((ground_level + 2000. * m).in(km).quantity_from_origin().number() == 2.);
-static_assert((tower_peak + 2. * km)[km].quantity_from_origin().number() == 2.);
+static_assert((tower_peak + 2. * km).in(km).quantity_from_origin().number() == 2.);
-static_assert((tower_peak + 2. * km)[m].quantity_from_origin().number() == 2000.);
+static_assert((tower_peak + 2. * km).in(m).quantity_from_origin().number() == 2000.);
-static_assert((tower_peak + 2000. * m)[km].quantity_from_origin().number() == 2.);
+static_assert((tower_peak + 2000. * m).in(km).quantity_from_origin().number() == 2.);
 
 #if MP_UNITS_COMP_GCC != 10 || MP_UNITS_COMP_GCC_MINOR > 2
 template<template<auto, auto, typename> typename QP>
 concept invalid_unit_conversion = requires {
-  requires !requires { QP<isq::height[m], mean_sea_level, int>(2000 * m)[km]; };  // truncating conversion
+  requires !requires { QP<isq::height[m], mean_sea_level, int>(2000 * m).in(km); };  // truncating conversion
-  requires !requires { QP<isq::height[m], mean_sea_level, int>(2 * m)[s]; };      // invalid unit
+  requires !requires { QP<isq::height[m], mean_sea_level, int>(2 * m).in(s); };      // invalid unit
 };
 static_assert(invalid_unit_conversion<quantity_point>);
 #endif

test/unit_test/static/quantity_test.cpp

@@ -198,17 +198,17 @@ static_assert(quantity<isq::length[km]>(1500 * m).number() == 1.5);
 // converting to a different unit
 ///////////////////////////////////
 
-static_assert(quantity<isq::length[km]>(2. * km)[km].number() == 2.);
+static_assert(quantity<isq::length[km]>(2. * km).in(km).number() == 2.);
-static_assert(quantity<isq::length[km]>(2. * km)[m].number() == 2000.);
+static_assert(quantity<isq::length[km]>(2. * km).in(m).number() == 2000.);
-static_assert(quantity<isq::length[m]>(2000. * m)[km].number() == 2.);
+static_assert(quantity<isq::length[m]>(2000. * m).in(km).number() == 2.);
-static_assert(quantity<isq::length[km], int>(2 * km)[km].number() == 2);
+static_assert(quantity<isq::length[km], int>(2 * km).in(km).number() == 2);
-static_assert(quantity<isq::length[km], int>(2 * km)[m].number() == 2000);
+static_assert(quantity<isq::length[km], int>(2 * km).in(m).number() == 2000);
 
 #if MP_UNITS_COMP_GCC != 10 || MP_UNITS_COMP_GCC_MINOR > 2
 template<template<auto, typename> typename Q>
 concept invalid_unit_conversion = requires {
-  requires !requires { Q<isq::length[m], int>(2000 * m)[km]; };  // truncating conversion
+  requires !requires { Q<isq::length[m], int>(2000 * m).in(km); };  // truncating conversion
-  requires !requires { Q<isq::length[m], int>(2 * m)[s]; };      // invalid unit
+  requires !requires { Q<isq::length[m], int>(2 * m).in(s); };      // invalid unit
 };
 static_assert(invalid_unit_conversion<quantity>);
 #endif
@@ -648,7 +648,7 @@ static_assert((7 * one % (2 * one)).number() == 1);
 static_assert((10 * m2 * (10 * m2)) / (50 * m2) == 2 * m2);
 
 static_assert((10 * km / (5 * m)).number() == 2);
-static_assert((10 * km / (5 * m))[one].number() == 2000);
+static_assert((10 * km / (5 * m)).in(one).number() == 2000);
 static_assert((10 * s * (2 * kHz)).number() == 20);
 
 // commutativity and associativity
@@ -872,10 +872,10 @@ static_assert(!(123 * km >= 321'000 * m));
 
 static_assert(is_of_type<10 * km / (5 * km), quantity<one, int>>);
 
-static_assert((50. * m / (100. * m))[percent].number() == 50);
+static_assert((50. * m / (100. * m)).in(percent).number() == 50);
 static_assert(50. * m / (100. * m) == 50 * percent);
 
-static_assert(((50. * percent)[one]).number() == 0.5);
+static_assert((50. * percent).in(one).number() == 0.5);
 
 
 //////////////////