Represents an IEEE 754-2008 decimal128 floating-point value (BSON TypeId 0x13, available since MongoDB 3.4). Used wherever exact decimal representation matters — currency, ledger entries, scientific data — since binary double cannot represent 0.1 exactly.

Storage is two Poco::UInt64 halves matching the BSON wire format (16 bytes, little-endian, low half first then high half). The class supports BSON serialization, canonical string conversion per IEEE 754-2008 Section 5.12.4, parsing simple decimal literals, and equality comparison. It does not implement decimal arithmetic; users who need to operate on values are expected to round-trip through std::string or use an external decimal-arithmetic library.

Creates a Decimal128 representing positive zero (0E0).

Creates a Decimal128 from the raw 128 bits, expressed as low and high 64-bit halves matching the on-wire layout.

Parses a decimal string into a Decimal128. Accepts the canonical forms produced by toString(): "NaN" (case insensitive), "Infinity" / "-Infinity" / "Inf" / "-Inf", and finite decimal literals with optional sign, optional fractional part, and optional exponent ("E"/"e" prefix). Throws Poco::SyntaxException on malformed input or Poco::RangeException when the coefficient exceeds 34 decimal digits or the exponent is out of the decimal128 range.

Includes -0, -Inf and -NaN.

Returns the canonical decimal string per IEEE 754-2008 Section 5.12.4: special values render as "NaN", "Infinity", "-Infinity"; finite values render in scientific or non-scientific notation depending on the adjusted exponent.