What is std::flat_map and how does it differ from std::map?

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**std::flat_map** — sorted map backed by vector. **std::map** — tree-based map. ```cpp #include #include // std::flat_map std::flat_map fm; fm["Alice"] = 95; fm["Bob"] = 87; // std::map std::map m; m["Alice"] = 95; m["Bob"] = 87; // Insertion: flat_map O(n), map O(log n) fm.insert({"Charlie", 90}); // May shift elements m.insert({"Charlie", 90}); // Tree rebalance // Search: both O(log n) if (fm.contains("Alice")) { /* ... */ } if (m.contains("Alice")) { /* ... */ } // Iteration: flat_map faster (cache-friendly) for (const auto& [k, v] : fm) { // Contiguous memory } // Memory: flat_map smaller (no tree nodes) std::cout << sizeof(fm); // ~vector size std::cout << sizeof(m); // ~3 pointers per element // Capacity fm.reserve(100); // Pre-allocate fm.shrink_to_fit(); // Release excess ``` **When to use flat_map:** - Few insertions, many lookups. - Cache performance matters. - Memory is constrained. - Iteration-heavy workload.

C++: What is std::flat_map and how does it differ from std::map?