Rewriting a FIX Engine in C++23: What Got Simpler (and What Didn't)

QuickFIX has been around forever. If you've touched FIX protocol in the last 15 years, you've probably used it. It works. It also carries a lot of code that made sense in C++98 but feels heavy now. I wanted to see how far C++23 could take a FIX engine from scratch. Not a full QuickFIX replacement (not yet anyway), but a parser and session layer where I could actually use modern tools. The project ended up at about 5K lines of headers, covers 9 message types, parses an ExecutionReport in ~246 ns. QuickFIX does the same parse in ~730 ns on identical synthetic input. Microbenchmark numbers, so grain of salt. Single core, pinned affinity, RDTSCP timing, warmed cache, 100K iterations. But the code changes that got there were more interesting to me than the final numbers. The allocation thing QuickFIX parses by allocating. Every field value becomes a std::string. Fields live in a std::map<int, std::string>, so each field access is O(log n) with pointer chasing through tree nodes. A typ