std::variant与继承的性能对比及URI场景选型问询

Great question—this is exactly the kind of tradeoff that comes up when balancing code maintainability, performance, and idiomatic C++ for real-world types like RFC 3986 URIs. Let’s break this down based on your specific constraints and performance findings:

优先选择：继承 + 虚函数（适配你的场景） #

Given your focus on const-heavy usage, preference for std::string_view, and your benchmark showing virtual functions outperform std::visit, this is the clearer choice here:

• Performance alignment: You’ve already proven virtual functions are faster, and in a const-dominant workflow, branch prediction will be extremely accurate (most calls will hit the same derived type). The runtime overhead of virtual calls will be negligible in practice, especially compared to the URI parsing/access logic itself.
• Clean, maintainable interface: Define an abstract base class URI with all your const-compliant RFC 3986 methods (e.g., scheme(), authority(), etc.). Then implement two derived classes:
  • OwningURI: Holds a std::string for cases where you need to own the URI data.
  • ViewURI: Holds a std::string_view for const, non-owning access.
    This keeps your public API consistent—users can work with const URI& without caring about the underlying storage.
• Const correctness by design: The base class can enforce const-only methods, and std::string_view naturally fits this pattern. You won’t have to juggle variant states to prevent accidental modifications to the owned string.
• Extensibility: If you ever need to add another storage type (e.g., a memory-mapped string), adding a new derived class is far cleaner than modifying a variant’s type list and updating every std::visit call site.

When to even consider std::variant? #

Only if your use case meets all these narrow criteria:

• You’re absolutely certain you’ll never need more than 2 storage types (string + string_view).
• Your URI access paths are extremely simple (no complex nested calls that would amplify std::visit overhead).
• You’re willing to accept slightly worse performance (per your benchmarks) for a value-type-based approach.

In your scenario, none of these apply—your performance tests already rule out variant as the top choice, and avoiding duplicate code is better handled via inheritance than variant wrapping.

Extra Tips for Your URI Implementation #

• Default to string_view: Make ViewURI the go-to type for most const use cases, and provide implicit conversion from OwningURI to ViewURI (since an owning string can safely produce a view).
• Avoid unnecessary polymorphism: For hot paths where you know you’re dealing with a view, expose non-polymorphic overloads or helpers to skip the virtual call entirely.
• Double-check RFC 3986 edge cases: Both approaches need to handle things like relative URIs, percent-encoding, and empty segments—ensure your base class interface covers all required operations so derived classes don’t diverge in behavior.

内容的提问来源于stack exchange，提问作者The Moisrex