Implement std::string

Implementing a high-performance string class with small-buffer optimization

This hard C++ coding problem asks you to build a production-quality string container from scratch, including constructors, assignment operators, element access, and mutation methods. It tests deep understanding of memory layout, move semantics, and the Small Buffer Optimization (SBO) — a technique used in real standard-library implementations to avoid heap allocation for short strings.

The problem requires careful bit-flag manipulation to distinguish between short strings (stored inline in a 23-byte stack buffer) and long strings (heap-allocated with dynamic capacity). You must handle SSO-to-heap transitions, self-assignment and self-move edge cases, iterator invalidation during insertion, and self-referential range operations where the source overlaps the destination. Strong solutions maintain the invariant that the buffer is always null-terminated, even as the string grows or shrinks.

• Memory layout and union-based storage
• Move semantics and reference semantics
• Capacity doubling and reallocation strategy
• Iterator validity and range operations
• Bit manipulation for type discrimination