"Text output" chapter finished

docs/framework/text_output.rst

@@ -20,17 +20,34 @@ stream::
     std::cout << v2 << '\n';  // 70 mi/h
 
 The text output will always print the :term:`value of a quantity` followed
-by the symbol of a :term:`unit` associated with this quantity. We will learn
-more about units in the :ref:`Units` chapter, but for now, it is important
-to remember that it is a good practice to always `quantity_cast()` a quantity
+by a space and then the symbol of a :term:`unit` associated with this quantity.
+
+.. important::
+
+    Remember that it is a good practice to always `quantity_cast()` a quantity
     of an unknown ``auto`` type before passing it to the text output::
 
         std::cout << quantity_cast<si::kilometre_per_hour>(v1) << '\n';  // 110 km/h
         std::cout << quantity_cast<si::metre_per_second>(v1) << '\n';    // 30.5556 m/s
 
 
-Formatting the output
----------------------
+Stream output formatting
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Only a basic formatting can be applied for output streams. It includes control
+over width, fill, and alignment::
+
+    os << "|" << std::setw(10) << 123q_m << "|";                       // |     123 m|
+    os << "|" << std::setw(10) << std::left << 123q_m << "|";          // |123 m     |
+    os << "|" << std::setw(10) << std::setfill('*') << 123q_m << "|";  // |*****123 m|
+
+
+fmt::format
+-----------
+
+The **mp-units** via ``fmt::format`` provides a fine-grained control over what
+and how is being printed on the text output.
+
 
 Grammar
 ^^^^^^^
@@ -61,14 +78,133 @@ In the above grammar:
 - ``A`` token of :token:`units-unit-modifier` forces ASCII-only output (instead of the
   default Unicode symbols defined by the :term:`SI` specification).
 
+
 Default formatting
 ^^^^^^^^^^^^^^^^^^
 
 To format `quantity` values the formatting facility uses :token:`units-format-spec`.
 In case it is left empty the default formatting of ``{:%Q %q}`` is applied. The same
-default formatting is also applied to output streams. This is why the following lines
-produce the same output::
+default formatting is also applied to the output streams. This is why the following
+code lines produce the same output::
 
     std::cout << "Distance: " << 123q_km << "\n";
     fmt::print("Distance: {}\n", 123q_km);
     fmt::print("Distance: {:%Q %q}\n", 123q_km);
+
+
+Quantity value, symbol, or both?
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The user can easily decide to either print a whole quantity (value and symbol) or
+only its parts. Also a different quantity formatting might be applied::
+
+    fmt::print("{:%Q}", 123q_km);    // 123
+    fmt::print("{:%q}", 123q_km);    // km
+    fmt::print("{:%Q%q}", 123q_km);  // 123km
+
+
+Controlling width, fill, and alignment
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To control width, fill, and alignment the C++ standard grammar tokens ``fill-and-align``
+and ``width`` are being used and they treat a quantity value and symbol as a contiguous
+text::
+
+    fmt::print("|{:0}|", 123q_m);     // |123 m|
+    fmt::print("|{:10}|", 123q_m);    // |     123 m|
+    fmt::print("|{:<10}|", 123q_m);   // |123 m     |
+    fmt::print("|{:>10}|", 123q_m);   // |     123 m|
+    fmt::print("|{:^10}|", 123q_m);   // |  123 m   |
+    fmt::print("|{:*<10}|", 123q_m);  // |123 m*****|
+    fmt::print("|{:*>10}|", 123q_m);  // |*****123 m|
+    fmt::print("|{:*^10}|", 123q_m);  // |**123 m***|
+
+
+ASCII-only quantity symbols
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Unit symbols of some quantities are specified to use Unicode signs by the :term:`SI`
+standard (i.e. ``Ω`` symbol for the resistance quantity). **mp-units** library follows
+this by default. From the engineering point of view sometimes Unicode text  might
+not be a solution as terminals of many (especially embedded) devices are ASCII-only.
+In such a case the unit symbol can be forced to be printed using ASCII-only characters::
+
+    fmt::print("{}", 10q_R);                 // 10 Ω
+    fmt::print("{:%Q %Aq}", 10q_R);          // 10 ohm
+    fmt::print("{}", 125q_us);               // 125 µs
+    fmt::print("{:%Q %Aq}", 125q_us);        // 125 us
+    fmt::print("{}", 9.8q_m_per_s2);         // 9.8 m/s²
+    fmt::print("{:%Q %Aq}", 9.8q_m_per_s2);  // 9.8 m/s^2
+
+
+Controlling on how the quantity value is being printed
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+``sign`` token allows us to specify on how the value's sign is being printed::
+
+    fmt::print("{0:%Q %q},{0:%+Q %q},{0:%-Q %q},{0:% Q %q}", 1q_m);   // 1 m,+1 m,1 m, 1 m
+    fmt::print("{0:%Q %q},{0:%+Q %q},{0:%-Q %q},{0:% Q %q}", -1q_m);  // -1 m,-1 m,-1 m,-1 m
+
+where:
+
+- ``+`` indicates that a sign should be used for both non-negative and negative numbers,
+- ``-`` indicates that a sign should be used for negative numbers and negative zero only
+  (this is the default behavior),
+- ``<space>`` indicates that a leading space should be used for non-negative numbers other
+  than negative zero, and a minus sign for negative numbers and negative zero.
+
+``precision`` token is allowed only for floating-point representation types::
+
+    fmt::print("{:%.0Q %q}", 1.2345q_m);  // 1 m
+    fmt::print("{:%.1Q %q}", 1.2345q_m);  // 1.2 m
+    fmt::print("{:%.2Q %q}", 1.2345q_m);  // 1.23 m
+
+
+:token:`units-rep-type` specifies how a value of the representation type is being
+printed. For integral types::
+
+    fmt::print("{:%bQ %q}", 42q_m);    // 101010 m
+    fmt::print("{:%BQ %q}", 42q_m);    // 101010 m
+    fmt::print("{:%dQ %q}", 42q_m);    // 42 m
+    fmt::print("{:%oQ %q}", 42q_m);    // 52 m
+    fmt::print("{:%xQ %q}", 42q_m);    // 2a m
+    fmt::print("{:%XQ %q}", 42q_m);    // 2A m
+
+The above can be printed in an alternate version thanks to the ``#`` token::
+
+    fmt::print("{:%#bQ %q}", 42q_m);   // 0b101010 m
+    fmt::print("{:%#BQ %q}", 42q_m);   // 0B101010 m
+    fmt::print("{:%#oQ %q}", 42q_m);   // 052 m
+    fmt::print("{:%#xQ %q}", 42q_m);   // 0x2a m
+    fmt::print("{:%#XQ %q}", 42q_m);   // 0X2A m
+
+For floating-point values the :token:`units-rep-type` token works as follows::
+
+    fmt::print("{:%aQ %q}",   1.2345678q_m);      // 0x9.e065152d8eae841p-3 m
+    fmt::print("{:%.3aQ %q}", 1.2345678q_m);      // 0x9.e06p-3 m
+    fmt::print("{:%AQ %q}",   1.2345678q_m);      // 0X9.E065152D8EAE841P-3 m
+    fmt::print("{:%.3AQ %q}", 1.2345678q_m);      // 0X9.E06P-3 m
+    fmt::print("{:%eQ %q}",   1.2345678q_m);      // 1.234568e+00 m
+    fmt::print("{:%.3eQ %q}", 1.2345678q_m);      // 1.235e+00 m
+    fmt::print("{:%EQ %q}",   1.2345678q_m);      // 1.234568E+00 m
+    fmt::print("{:%.3EQ %q}", 1.2345678q_m);      // 1.235E+00 m
+    fmt::print("{:%gQ %q}",   1.2345678q_m);      // 1.23457 m
+    fmt::print("{:%gQ %q}",   1.2345678e8q_m);    // 1.23457e+08 m
+    fmt::print("{:%.3gQ %q}", 1.2345678q_m);      // 1.23 m
+    fmt::print("{:%.3gQ %q}", 1.2345678e8q_m);    // 1.23e+08 m
+    fmt::print("{:%GQ %q}",   1.2345678q_m);      // 1.23457 m
+    fmt::print("{:%GQ %q}",   1.2345678e8q_m);    // 1.23457E+08 m
+    fmt::print("{:%.3GQ %q}", 1.2345678q_m);      // 1.23 m
+    fmt::print("{:%.3GQ %q}", 1.2345678e8q_m);    // 1.23E+08 m
+
+
+Special signs
+^^^^^^^^^^^^^
+
+Beside adding any list of regular characters as a separator between the value and the
+symbol, it is possible to type a few special signs there too::
+
+    fmt::print("{:%Q_%q}", 123q_km);    // 123_km
+    fmt::print("{:%Q%t%q}", 123q_km);   // 123\tkm  <tab>
+    fmt::print("{:%Q%n%q}", 123q_km);   // 123\nkm  <new line>
+    fmt::print("{:%Q%% %q}", 123q_km);  // 123% km