Implement std::variant

Implementing a type-safe variant container in C++

This hard coding problem tests whether you can build a production-quality generic container that mirrors std::variant. It is a common interview question at firms that value deep C++ knowledge, including high-frequency trading shops that rely on zero-overhead abstractions.

The challenge requires you to manage multiple unrelated types in a single memory footprint, track which type is active, safely construct and destroy the contained value, and provide type-safe access. Strong solutions use template metaprogramming to handle the arbitrary list of alternatives, union-like storage to avoid heap allocation, and careful lifetime management to ensure correct copy, move, and swap semantics. You must also handle the interaction between changing the active type and cleaning up the previous one.

• Union-based storage and index tracking
• Template specialization and type-to-index lookup
• Copy and move constructor/assignment semantics
• Explicit lifetime control and destructor dispatch
• Const and mutable reference overloads for type and index access