docs: new formatting syntax ideas

README.md

@@ -121,8 +121,8 @@ int main()
   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}%?%U}", v5);                            // 30.56 m/s
-  std::println("{:{%N:.2f}%?{%U:dn}}", v6);                       // 31.29 m⋅s⁻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
 }
 ```

docs/blog/posts/2.2.0-released.md

@@ -242,9 +242,9 @@ quantity q = (90. * km / h).in(mph);
 std::cout << "Number: " << q.numerical_value_in(mph) << "\n";
 std::cout << "Unit: " << q.unit << "\n";
 std::cout << "Dimension: " << q.dimension << "\n";
-std::println("{:{%N:.2f}%?%U}", q);
+std::println("{::N[.2f]}", q);
 std::println("{:.4f} in {} of {}", q.numerical_value_in(mph), q.unit, q.dimension);
-std::println("{:{%N:.4f} in %U of %D}", q);
+std::println("{:%N in %U of %D:N[.4f]}", q);
 ```
 
 ```text

docs/getting_started/look_and_feel.md

@@ -100,8 +100,8 @@ performed without sacrificing accuracy. Please see the below example for a quick
       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}%?%U}", v5);                            // 30.56 m/s
-      std::println("{:{%N:.2f}%?{%U:dn}}", v6);                       // 31.29 m⋅s⁻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
     }
     ```
@@ -144,8 +144,8 @@ performed without sacrificing accuracy. Please see the below example for a quick
       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}%?%U}", v5);                            // 30.56 m/s
-      std::println("{:{%N:.2f}%?{%U:dn}}", v6);                       // 31.29 m⋅s⁻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
     }
     ```

docs/index.md

@@ -38,7 +38,7 @@ The library source code is hosted on [GitHub](https://github.com/mpusz/mp-units)
     int main()
     {
       constexpr quantity dist = 364.4 * smoot;
-      std::println("Harvard Bridge length = {:{%N:.5} %U} ({:{%N:.5} %U}, {:{%N:.5} %U}) ± 1 εar",
+      std::println("Harvard Bridge length = {::N[.5]} ({::N[.5]}, {::N[.5]}) ± 1 εar",
                    dist, dist.in(usc::foot), dist.in(si::metre));
     }
     ```
@@ -58,7 +58,7 @@ The library source code is hosted on [GitHub](https://github.com/mpusz/mp-units)
     int main()
     {
       constexpr quantity dist = 364.4 * smoot;
-      std::println("Harvard Bridge length = {:{%N:.5} %U} ({:{%N:.5} %U}, {:{%N:.5} %U}) ± 1 εar",
+      std::println("Harvard Bridge length = {::N[.5]} ({::N[.5]}, {::N[.5]}) ± 1 εar",
                    dist, dist.in(usc::foot), dist.in(si::metre));
     }
     ```

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::println("permittivity of vacuum = {} = {:{%N:.3e} %U}", q, q.in(F / m));
+std::println("permittivity of vacuum = {} = {::N[.3e]}", q, q.in(F / m));
 ```
 
 The above first prints the following:

docs/users_guide/framework_basics/simple_and_typed_quantities.md

@@ -75,7 +75,7 @@ Here is a simple example showing how to deal with such quantities:
       const quantity duration = 2 * h;
       const quantity speed = avg_speed(distance, duration);
 
-      std::println("A car driving {} in {} has an average speed of {:{%N:.4} %U} ({:{%N:.4} %U})",
+      std::println("A car driving {} in {} has an average speed of {::N[.4]} ({::N[.4]})",
                    distance, duration, speed, speed.in(km / h));
     }
     ```
@@ -103,7 +103,7 @@ Here is a simple example showing how to deal with such quantities:
       const quantity duration = 2 * h;
       const quantity speed = avg_speed(distance, duration);
 
-      std::println("A car driving {} in {} has an average speed of {:{%N:.4} %U} ({:{%N:.4} %U})",
+      std::println("A car driving {} in {} has an average speed of {::N[.4]} ({::N[.4]})",
                    distance, duration, speed, speed.in(km / h));
     }
     ```
@@ -194,7 +194,7 @@ The previous example can be re-typed using typed quantities in the following way
       const quantity duration = isq::time(2 * h);
       const quantity speed = avg_speed(distance, duration);
 
-      std::println("A car driving {} in {} has an average speed of {:{%N:.4} %U} ({:{%N:.4} %U})",
+      std::println("A car driving {} in {} has an average speed of {::N[.4]} ({::N[.4]})",
                    distance, duration, speed, speed.in(km / h));
     }
     ```
@@ -223,7 +223,7 @@ The previous example can be re-typed using typed quantities in the following way
       const quantity duration = isq::time(2 * h);
       const quantity speed = avg_speed(distance, duration);
 
-      std::println("A car driving {} in {} has an average speed of {:{%N:.4} %U} ({:{%N:.4} %U})",
+      std::println("A car driving {} in {} has an average speed of {::N[.4]} ({::N[.4]})",
                    distance, duration, speed, speed.in(km / h));
     }
     ```

docs/users_guide/framework_basics/text_output.md

@@ -529,20 +529,19 @@ std::println("{:d}", kg * m2 / s2);  // kg⋅m²/s²
 ### Quantity formatting
 
 ```bnf
-quantity-format-spec        ::= [fill-and-align] [width] [quantity-specs]
+quantity-format-spec        ::= [fill-and-align] [width] [quantity-specs] [defaults-specs]
 quantity-specs              ::= conversion-spec
                                 quantity-specs conversion-spec
                                 quantity-specs literal-char
 literal-char                ::= <any character other than '{', '}', or '%'>
-conversion-spec             ::= placement-spec
-                                subentity-replacement-field
-placement-spec              ::= '%' placement-type
-placement-type              ::= 'N' | 'U' | 'D' | '?' | '%'
-subentity-replacement-field ::= '{' '%' subentity-id [format-specifier] '}'
-subentity-id                ::= literal-char
-                                subentity-id literal-char
-format-specifier            ::= ':' format-spec
-format-spec                 ::= <as specified by the formatter for the argument type; cannot start with '}'>
+conversion-spec             ::= '%' placement-type
+placement-type              ::= subentity-id | '?' | '%'
+defaults-specs              ::= ':' default-spec-list
+default-spec-list           ::= default-spec
+                                default-spec-list default-spec
+default-spec                ::= subentity-id '[' format-spec ']'
+subentity-id                ::= 'N' | 'U' | 'D'
+format-spec                 ::= <as specified by the formatter for the argument type>
 ```
 
 In the above grammar:
@@ -556,12 +555,9 @@ In the above grammar:
     - '?' inserts an optional separator between the number and a unit based on the value of
       `space_before_unit_symbol` for this unit,
     - '%' just inserts '%' character.
-- `subentity-replacement-field` token allows the composition of formatters. The following identifiers
-  are recognized by the quantity formatter:
-    - 'N' passes `format-spec` to the `formatter` specialization for the quantity representation
-      type,
-    - 'U' passes `format-spec` to the `formatter` specialization for the unit type,
-    - 'D' passes `format-spec` to the `formatter` specialization for the dimension type.
+- `defaults-specs` token allows overwriting defaults for the underlying formatters with the custom
+  format string. Each override starts with a subentity identifier ('N', 'U', or 'D') followed by
+  the format string enclosed in square brackets.
 
 #### Default formatting
 
@@ -573,7 +569,6 @@ This is why the following code lines produce the same output:
 std::cout << "Distance: " << 123 * km << "\n";
 std::cout << std::format("Distance: {}\n", 123 * km);
 std::cout << std::format("Distance: {:%N%?%U}\n", 123 * km);
-std::cout << std::format("Distance: {:{%N}%?{%U}}\n", 123 * km);
 ```
 
 !!! note
@@ -610,7 +605,7 @@ Thanks to the grammar provided above, the user can easily decide to either:
 - provide custom formatting for components:
 
     ```cpp
-    std::println("Speed: {:{%N:.2f} {%U:n}}", 100. * km / (3 * h));
+    std::println("Speed: {::N[.2f]U[n]}", 100. * km / (3 * h));
     ```
 
     ```text
@@ -630,25 +625,11 @@ Thanks to the grammar provided above, the user can easily decide to either:
     - dimension: LT⁻¹
     ```
 
-!!! note
-
-    The above grammar allows repeating the same field many times, possibly with a different
-    format spec. For example:
-
-    ```cpp
-    std::println("Speed: {:%N {%N:.4f} {%N:.2f} {%U:n}}", 100. * km / (3 * h));
-    ```
-
-    ```text
-    Speed: 33.333333333333336 33.3333 33.33 km h⁻¹
-    ```
-
-
 #### Formatting of the quantity numerical value
 
 The representation type used as a numerical value of a quantity must provide its own formatter
 specialization. It will be called by the quantity formatter with the format-spec provided
-by the user in the `%N` replacement field.
+by the user in the `N` defaults specification.
 
 In case we use C++ fundamental arithmetic types with our quantities the standard formatter
 specified in [format.string.std](https://wg21.link/format.string.std) will be used. The rest
@@ -657,8 +638,8 @@ of this chapter assumes that it is the case and provides some usage examples.
 `sign` token allows us to specify how the value's sign is being printed:
 
 ```cpp
-std::println("{0:%N %U},{0:{%N:+} %U},{0:{%N:-} %U},{0:{%N: } %U}", 1 * m);   // 1 m,+1 m,1 m, 1 m
-std::println("{0:%N %U},{0:{%N:+} %U},{0:{%N:-} %U},{0:{%N: } %U}", -1 * m);  // -1 m,-1 m,-1 m,-1 m
+std::println("{0},{0::N[+]},{0::N[-]},{0::N[ ]}", 1 * m);   // 1 m,+1 m,1 m, 1 m
+std::println("{0},{0::N[+]},{0::N[-]},{0::N[ ]}", -1 * m);  // -1 m,-1 m,-1 m,-1 m
 ```
 
 where:
@@ -672,54 +653,54 @@ where:
 `precision` token is allowed only for floating-point representation types:
 
 ```cpp
-std::println("{:{%N:.0} %U}", 1.2345 * m);   // 1 m
-std::println("{:{%N:.1} %U}", 1.2345 * m);   // 1 m
-std::println("{:{%N:.2} %U}", 1.2345 * m);   // 1.2 m
-std::println("{:{%N:.3} %U}", 1.2345 * m);   // 1.23 m
-std::println("{:{%N:.0f} %U}", 1.2345 * m);  // 1 m
-std::println("{:{%N:.1f} %U}", 1.2345 * m);  // 1.2 m
-std::println("{:{%N:.2f} %U}", 1.2345 * m);  // 1.23 m
+std::println("{::N[.0]}", 1.2345 * m);   // 1 m
+std::println("{::N[.1]}", 1.2345 * m);   // 1 m
+std::println("{::N[.2]}", 1.2345 * m);   // 1.2 m
+std::println("{::N[.3]}", 1.2345 * m);   // 1.23 m
+std::println("{::N[.0f]}", 1.2345 * m);  // 1 m
+std::println("{::N[.1f]}", 1.2345 * m);  // 1.2 m
+std::println("{::N[.2f]}", 1.2345 * m);  // 1.23 m
 ```
 
 `type` specifies how a value of the representation type is being printed.
 For integral types:
 
 ```cpp
-std::println("{:{%N:b} %U}", 42 * m);    // 101010 m
-std::println("{:{%N:B} %U}", 42 * m);    // 101010 m
-std::println("{:{%N:d} %U}", 42 * m);    // 42 m
-std::println("{:{%N:o} %U}", 42 * m);    // 52 m
-std::println("{:{%N:x} %U}", 42 * m);    // 2a m
-std::println("{:{%N:X} %U}", 42 * m);    // 2A m
+std::println("{::N[b]}", 42 * m);    // 101010 m
+std::println("{::N[B]}", 42 * m);    // 101010 m
+std::println("{::N[d]}", 42 * m);    // 42 m
+std::println("{::N[o]}", 42 * m);    // 52 m
+std::println("{::N[x]}", 42 * m);    // 2a m
+std::println("{::N[X]}", 42 * m);    // 2A m
 ```
 
 The above can be printed in an alternate version thanks to the `#` token:
 
 ```cpp
-std::println("{:{%N:#b} %U}", 42 * m);   // 0b101010 m
-std::println("{:{%N:#B} %U}", 42 * m);   // 0B101010 m
-std::println("{:{%N:#o} %U}", 42 * m);   // 052 m
-std::println("{:{%N:#x} %U}", 42 * m);   // 0x2a m
-std::println("{:{%N:#X} %U}", 42 * m);   // 0X2A m
+std::println("{::N[#b]}", 42 * m);   // 0b101010 m
+std::println("{::N[#B]}", 42 * m);   // 0B101010 m
+std::println("{::N[#o]}", 42 * m);   // 052 m
+std::println("{::N[#x]}", 42 * m);   // 0x2a m
+std::println("{::N[#X]}", 42 * m);   // 0X2A m
 ```
 
 For floating-point values, the `type` token works as follows:
 
 ```cpp
-std::println("{:{%N:a} %U}",   1.2345678 * m);      // 1.3c0ca2a5b1d5dp+0 m
-std::println("{:{%N:.3a} %U}", 1.2345678 * m);      // 1.3c1p+0 m
-std::println("{:{%N:A} %U}",   1.2345678 * m);      // 1.3C0CA2A5B1D5DP+0 m
-std::println("{:{%N:.3A} %U}", 1.2345678 * m);      // 1.3C1P+0 m
-std::println("{:{%N:e} %U}",   1.2345678 * m);      // 1.234568e+00 m
-std::println("{:{%N:.3e} %U}", 1.2345678 * m);      // 1.235e+00 m
-std::println("{:{%N:E} %U}",   1.2345678 * m);      // 1.234568E+00 m
-std::println("{:{%N:.3E} %U}", 1.2345678 * m);      // 1.235E+00 m
-std::println("{:{%N:g} %U}",   1.2345678 * m);      // 1.23457 m
-std::println("{:{%N:g} %U}",   1.2345678e8 * m);    // 1.23457e+08 m
-std::println("{:{%N:.3g} %U}", 1.2345678 * m);      // 1.23 m
-std::println("{:{%N:.3g} %U}", 1.2345678e8 * m);    // 1.23e+08 m
-std::println("{:{%N:G} %U}",   1.2345678 * m);      // 1.23457 m
-std::println("{:{%N:G} %U}",   1.2345678e8 * m);    // 1.23457E+08 m
-std::println("{:{%N:.3G} %U}", 1.2345678 * m);      // 1.23 m
-std::println("{:{%N:.3G} %U}", 1.2345678e8 * m);    // 1.23E+08 m
+std::println("{::N[a]}",   1.2345678 * m);      // 1.3c0ca2a5b1d5dp+0 m
+std::println("{::N[.3a]}", 1.2345678 * m);      // 1.3c1p+0 m
+std::println("{::N[A]}",   1.2345678 * m);      // 1.3C0CA2A5B1D5DP+0 m
+std::println("{::N[.3A]}", 1.2345678 * m);      // 1.3C1P+0 m
+std::println("{::N[e]}",   1.2345678 * m);      // 1.234568e+00 m
+std::println("{::N[.3e]}", 1.2345678 * m);      // 1.235e+00 m
+std::println("{::N[E]}",   1.2345678 * m);      // 1.234568E+00 m
+std::println("{::N[.3E]}", 1.2345678 * m);      // 1.235E+00 m
+std::println("{::N[g]}",   1.2345678 * m);      // 1.23457 m
+std::println("{::N[g]}",   1.2345678e8 * m);    // 1.23457e+08 m
+std::println("{::N[.3g]}", 1.2345678 * m);      // 1.23 m
+std::println("{::N[.3g]}", 1.2345678e8 * m);    // 1.23e+08 m
+std::println("{::N[G]}",   1.2345678 * m);      // 1.23457 m
+std::println("{::N[G]}",   1.2345678e8 * m);    // 1.23457E+08 m
+std::println("{::N[.3G]}", 1.2345678 * m);      // 1.23 m
+std::println("{::N[.3G]}", 1.2345678e8 * m);    // 1.23E+08 m
 ```

docs/users_guide/framework_basics/the_affine_space.md

@@ -491,7 +491,7 @@ room_temp room_ref{};
 room_temp room_low = room_ref - number_of_steps * step_delta;
 room_temp room_high = room_ref + number_of_steps * step_delta;
 
-std::println("Room reference temperature: {} ({}, {:{%N:.2f}%?%U})\n",
+std::println("Room reference temperature: {} ({}, {::N[.2f]})\n",
              room_ref.quantity_from_zero(),
              room_ref.in(usc::degree_Fahrenheit).quantity_from_zero(),
              room_ref.in(si::kelvin).quantity_from_zero());
@@ -501,7 +501,7 @@ std::println("| {:<18} | {:^18} | {:^18} | {:^18} |",
 std::println("|{0:=^20}|{0:=^20}|{0:=^20}|{0:=^20}|", "");
 
 auto print_temp = [&](std::string_view label, auto v) {
-  std::println("| {:<18} | {:^18} | {:^18} | {:^18{%N:.2f}%?%U} |", label,
+  std::println("| {:<14} | {:^18} | {:^18} | {:^18:N[.2f]} |", label,
                v - room_reference_temp, (v - si::ice_point).in(deg_C), (v - si::absolute_zero).in(deg_C));
 };
 

example/clcpp_response.cpp

@@ -118,7 +118,7 @@ void calcs_comparison()
   const auto L1A = 2.f * fm;
   const auto L2A = 3.f * fm;
   const auto LrA = L1A + L2A;
-  std::cout << MP_UNITS_STD_FMT::format("{:{%N:.30} %U}\n + {:{%N:.30} %U}\n   = {:{%N:.30} %U}\n\n", L1A, L2A, LrA);
+  std::cout << MP_UNITS_STD_FMT::format("{::N[.30]}\n + {::N[.30]}\n   = {::N[.30]}\n\n", L1A, L2A, LrA);
 
   std::cout << "The single unit method must convert large\n"
                "or small values in other units to the base unit.\n"
@@ -127,17 +127,17 @@ void calcs_comparison()
   const auto L1B = L1A.in(m);
   const auto L2B = L2A.in(m);
   const auto LrB = L1B + L2B;
-  std::cout << MP_UNITS_STD_FMT::format("{:{%N:.30e} %U}\n + {:{%N:.30e} %U}\n   = {:{%N:.30e} %U}\n\n", L1B, L2B, LrB);
+  std::cout << MP_UNITS_STD_FMT::format("{::N[.30e]}\n + {::N[.30e]}\n   = {::N[.30e]}\n\n", L1B, L2B, LrB);
 
   std::cout << "In multiplication and division:\n\n";
 
   const quantity<isq::area[square(fm)], float> ArA = L1A * L2A;
-  std::cout << MP_UNITS_STD_FMT::format("{:{%N:.30} %U}\n * {:{%N:.30} %U}\n   = {:{%N:.30} %U}\n\n", L1A, L2A, ArA);
+  std::cout << MP_UNITS_STD_FMT::format("{::N[.30]}\n * {::N[.30]}\n   = {::N[.30]}\n\n", L1A, L2A, ArA);
 
   std::cout << "similar problems arise\n\n";
 
   const quantity<isq::area[m2], float> ArB = L1B * L2B;
-  std::cout << MP_UNITS_STD_FMT::format("{:{%N:.30e} %U}\n * {:{%N:.30e} %U}\n   = {:{%N:.30e} %U}\n\n", L1B, L2B, ArB);
+  std::cout << MP_UNITS_STD_FMT::format("{::N[.30e]}\n * {::N[.30e]}\n   = {::N[.30e]}\n\n", L1B, L2B, ArB);
 }
 
 }  // namespace