C++26: Constexpr Virtual Inheritance
C++26 introduces constexpr virtual inheritance, allowing virtual base classes and virtual inheritance to be used in constant expressions. This expands the capabilities of constexpr code by removing restrictions that previously prevented virtual inheritance from being evaluated at compile time.
Background
- The C++ programming language is updated every three years with a new "standard" (C++11, C++14, C++17, C++20, C++23, C++26). Each standard adds features that compiler vendors (Microsoft, GCC, Clang) eventually implement.
- `constexpr` is a keyword that tells the compiler a function can be evaluated at compile time rather than runtime, enabling faster programs and more compile-time error checking. Over the years, more C++ features have been made `constexpr`-compatible.
- **Virtual inheritance** is a C++ feature used when a class inherits from a base class through multiple paths (the "diamond problem"). It has been prohibited inside `constexpr` functions — until C++26.
- This change matters because it removes a long-standing restriction, allowing more complex object-oriented code to run at compile time. For practical C++ developers, it means fewer workarounds when writing compile-time logic that uses virtual inheritance hierarchies.