Rounding & scale (BigDecimal)

BigDecimal keeps a fixed scale and rounds when you:

• parse from string with more fractional digits than scale
• rescale to smaller scale
• multiply/divide (keeps scale, rounds by default to HalfEven)
• compute sqrt() or pow() with negative exponent

Practical rules

• Keep the same scale across all values you want to add/subtract.
• Use rescaled() to align scales:

BigDecimal a("1.23", 2);
BigDecimal b("4.5678", 4);

auto b2 = b.rescaled(2);      // 4.57 (HalfEven)
auto sum = a + b2;            // ok

HalfEven vs HalfUp

• HalfUp: ties always go away from zero
• HalfEven: ties go to the nearest even last digit (banker’s rounding)