How to use Claude to port C code to C++

The author had shelved a complex C application because its intertwined logic made it difficult to understand. They later used Claude in agent mode to port that application into more structured C++. Although the initial conversion was useful, the agent still struggled with parts of the original logic.

To keep the port faithful, the author had Claude design a test suite comparing outputs from the old and new applications. When results differed and the cause was unclear, Claude generated hex dump comparisons, wrote small Python programs, and analyzed the evidence. It repeatedly built the code, ran the tests, compared outputs, and revised the implementation until the differences were resolved.

The target was byte-for-byte equivalence rather than simply improving behavior. The author says some discrepancies were likely bugs in the original application that Claude had fixed during conversion. To preserve compatibility, those behaviors had to be reintroduced into the new implementation. The account reports successful resolution of the observed differences, but does not quantify the test suite or claim that every possible input was covered.

Who it's for: Developers porting legacy programs where preserving existing output matters more than correcting historical behavior during the migration.

Prompt
Suggested prompt (written by Agents In Use, not the author's)

Source: Hacker News · wvenable