Add member operator behavior

Author: mborland

doc/modules/ROOT/pages/decimal_fast128_t.adoc

@@ -108,11 +108,144 @@ explicit constexpr operator std::float64_t() const noexcept;
 explicit constexpr operator std::bfloat16_t() const noexcept;
 
 explicit constexpr operator decimal32_t() const noexcept;
+explicit constexpr operator decimal_fast32_t() const noexcept;
 explicit constexpr operator decimal64_t() const noexcept;
+explicit constexpr operator decimal_fast64_t() const noexcept;
+explicit constexpr operator decimal128_t() const noexcept;
 
 }; // class decimal_fast128_t
 
 } //namespace decimal
 } //namespace boost
 
 ----
+
+
+== Operator Behavior
+
+=== Construction to and from binary floating-point type
+
+[source, c++]
+----
+// 3.2.2.2 Conversion from floating-point type
+template <typename Float>
+explicit BOOST_DECIMAL_CXX20_CONSTEXPR decimal_fast128_t(Float val) noexcept;
+
+// 3.2.6 Conversion to floating-point type
+explicit BOOST_DECIMAL_CXX20_CONSTEXPR operator float() const noexcept;
+explicit BOOST_DECIMAL_CXX20_CONSTEXPR operator double() const noexcept;
+explicit BOOST_DECIMAL_CXX20_CONSTEXPR operator long double() const noexcept;
+
+// If C++23 and <stdfloat> are available
+explicit constexpr operator std::float16_t() const noexcept;
+explicit constexpr operator std::float32_t() const noexcept;
+explicit constexpr operator std::float64_t() const noexcept;
+explicit constexpr operator std::bfloat16_t() const noexcept;
+----
+
+See: xref:binary_floating_conversions.adoc[]
+
+=== Construction From Integral Type
+
+[source,c++]
+----
+// 3.2.2.3 Conversion from integral type
+template <typename Integer>
+explicit constexpr decimal_fast128_t(Integer val) noexcept;
+----
+
+Constructs a decimal value subject to the current rounding mode (if necessary).
+
+=== Construction From String
+
+[source,c++]
+----
+// Extension: Construction from (c)string
+explicit constexpr decimal_fast128_t(const char* str);
+
+#ifndef BOOST_DECIMAL_HAS_STD_STRING_VIEW
+explicit inline decimal_fast128_t(const std::string& str);
+#else
+explicit constexpr decimal_fast128_t(std::string_view str);
+#endif
+----
+
+Constructs a decimal value that matches `str` subject to:
+
+. If `str` is a `nullptr` or of length 0 either:
+.. `throw std::runtime_error`
+.. Constructs a `QNAN` in a no exception environment
+. If `str` is an invalid string either:
+.. `throw std::runtime_error`
+.. Constructs a `QNAN` in a no exception environment
+. On overflow constructs `INF`
+. On underflow constructs `0`
+. Rounds value represented by `str` according to current rounding mode
+
+=== Conversion to Integral Type
+
+[source,c++]
+----
+// 3.2.2.4 Conversion to integral type
+
+explicit constexpr operator int() const noexcept;
+explicit constexpr operator unsigned() const noexcept;
+explicit constexpr operator long() const noexcept;
+explicit constexpr operator unsigned long() const noexcept;
+explicit constexpr operator long long() const noexcept;
+explicit constexpr operator unsigned long long() const noexcept;
+----
+
+Constructs an integer representation of the decimal value subject to:
+
+. If the decimal value is `INF` returns `std::numeric_limits<IntegerType>::max()`
+. If the decimal value is `NAN` returns `std::numeric_limits<IntegerType>::max()`
+. If the decimal value exceeds the range of the `IntegerType` returns `std::numeric_limits<IntegerType>::max()`
+
+=== Increment and Decrement Operators
+
+[source,c++]
+----
+// 3.2.2.5 increment and decrement operators:
+constexpr decimal_fast128_t& operator++();
+constexpr decimal_fast128_t  operator++(int);
+constexpr decimal_fast128_t& operator--();
+constexpr decimal_fast128_t  operator--(int);
+----
+
+Increments/Decrements the decimal value subject to:
+
+. If the decimal value is `NAN` returns `QNAN`
+. If the decimal value is `INF` returns `INF`
+
+=== Compound Operators
+
+[source, c++]
+----
+// 3.2.2.6 compound assignment:
+constexpr decimal_fast128_t& operator+=(RHS rhs);
+constexpr decimal_fast128_t& operator-=(RHS rhs);
+constexpr decimal_fast128_t& operator*=(RHS rhs);
+constexpr decimal_fast128_t& operator/=(RHS rhs);
+----
+
+Matches the behavior of xref:generic_decimal.adoc#operator_behavior[addition, subtraction, multiplication, and division].
+
+=== Conversion to Other Decimal Types
+
+[source,c++]
+----
+explicit constexpr operator decimal32_t() const noexcept;
+explicit constexpr operator decimal_fast32_t() const noexcept;
+explicit constexpr operator decimal64_t() const noexcept;
+explicit constexpr operator decimal_fast64_t() const noexcept;
+explicit constexpr operator decimal128_t() const noexcept;
+----
+
+Conversion to `decimal128_t` is lossless in all cases.
+
+Conversion to any other decimal type is subject to:
+
+. Current rounding mode if the number of digits exceeds the precision of the target type.
+. Overflow constructs `INF`.
+. Underflow constructs `0`.