Have you ever wondered how libc++ implements functions that are similar to libc? For example, consider std::from_chars(float) and strtof. They both take a string and output a float, but from_chars also takes an end pointer and a format argument. They operate similarly behind the scenes. Their interfaces mean that from_chars can’t easily be implemented in terms of strtof. This talk will explain Project Hand-In-Hand, an LLVM-libc and libc++ collaboration to share the internal code for very similar functions that have incompatible interfaces.