Will implicit conversions lose information?

Question

According to https://www.learncpp.com/cpp-tutorial/44-implicit-type-conversion-coercion/

"The important thing to remember about promotions is that they are always safe, and no data loss will result."

However https://www.geeksforgeeks.org/type-conversion-in-c/ states that:

"It is possible for implicit conversions to lose information, signs can be lost (when signed is implicitly converted to unsigned), and overflow can occur (when long long is implicitly converted to float)."

I understand that implicit conversions are safe, but I don't see how no data loss will result. Which is accurate?

Answer 1

The first article talks about promotions, which are a specific type of implicit conversion. There are other types of conversions out there that are also implicit conversions but aren't promotions. A promotion is a specific type of implicit conversion and it can't lose information as you are always going to a wider type, i.e. a type where all the values representable by the type being promoted are representable by the promoted to type (int -> long long for example)

Other implicit conversions include: going from signed to unsigned, narrowing conversions, floating point to integer conversions. These conversion may lose information unlike promotions.