Open-hierarchy custom RTTI framework for C++17 and up.

If you have ever attempted to use the C++'s build in RTTI on a resource constrained (embedded) system you will most likely have noticed it is massively inefficient. Hence this implementation of a hand-rolled form of RTTI which is much more efficient and flexible, although it requires a bit more work from you as a class author. The current implementation supports the following features:

• Compiletime (stable) ID generation based on the FNV1a hash of the type signature
• Multiple inheritance, including virtual
• Full dynamic casting support
• Parent constructors are accessible
• No external dependencies, single header
• Static asserts on the parents passed to TypeInfo structure.
• Works on bare-metal systems
• Currently supports GCC/Clang based compilers
• One convenience marco ¯\(ツ)/¯

Note: This project was initially inspired by open-hierarchy examples in the guidelines defined for RTTI by the LLVM project^[1]. However this solution has one major drawback which is that the parent constructors are no longer accessible given that RTTI classes are injected in between the parent and child. An initial implementation of the RTTI library was based on this design and is still available for reference under git tag llvm-style-inheritance.

[1] https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html

---

How to use

• Add -fno-rtti to your compile options in order to disable C++'s build in RTTI. (Optional, as it can work in conjunction with native RTTI)
• RTTI::Enable describes the abstract interface for performing runtime RTTI checks and type casing. It is to be virtually derived from by the highest member(s) in the class hierarchy.
• For each type part of the hierarchy the RTTI_DECLARE_TYPEINFO(Type, Parents...) macro should be added after the opening brace to define a type alias to RTTI::TypeInfo structure and overload the virtual methods of the interface described by RTTI::Enable.
• RTTI::TypeInfo holds the type information of each member and provides statis methods for performing RTTI checks and type casting. It uses the “Curiously Recurring Template Idiom”, taking the class being defined as its first template argument and optionally the parent classes as the arguments there after.

Basic example:

struct Shape : virtual RTTI::Enable {
    RTTI_DECLARE_TYPEINFO(Shape);
};

struct Square : Shape {
    RTTI_DECLARE_TYPEINFO(Square, Shape);
};

struct OtherParent : virtual RTTI::Enable {
    RTTI_DECLARE_TYPEINFO(OtherParent);
}

struct Circle : Shape, OtherParent {
    RTTI_DECLARE_TYPEINFO(Circle, Shape, OtherParent);
};

int main() {
    Circle c;
    Shape* shape = &c;

    if (shape->is<Circle>()) {
        std::cout << "Yes, the shape is a circle!" << std::endl;
    }

    if (shape->cast<Square>() == nullptr) {
        std::cout << "No, it was not a square... :(" << std::endl;        
    }

    if (auto circle = shape->cast<Circle>()) {
        std::cout << "Woot, we have the circle back! \\0/" << std::endl;
    }

    OtherParent *p = &c;
    if (auto s = p->cast<Shape>()) {
        std::cout << "Pointer offsets are take into account for multiple inheritance hierarchies." << std::endl;
    }

    return 0;
}

Note that the RTTI::TypeInfo<T>::Id() method can also be used to identify any other types not part of an RTTI hierarchy, for example a very basic interface and implementation of a variant type:

struct AnyVariant {
    virtual ~AnyVariant() {}
    virtual RTTI::TypeId valueTypeId() const noexcept =0;
};

template<typename T>
struct Variant : AnyVariant {
    T value;

    virtual RTTI::TypeId valueTypeId() const noexcept override {
        return RTTI::TypeInfo<T>::Id();
    }
};

void testValueTypeId() {
    Variant<int> v;
    assert(v.valueTypeId() == TypeInfo<int>::Id());
    assert(v.valueTypeId() != TypeInfo<bool>::Id());
}

Benchmark Results

Running ./build/benchmark/rtti-benchmark
Run on (6 X 4300 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x6)
  L1 Instruction 32 KiB (x6)
  L2 Unified 256 KiB (x6)
  L3 Unified 9216 KiB (x1)
Load Average: 0.48, 0.26, 0.17
------------------------------------------------------------
Benchmark                  Time             CPU   Iterations
------------------------------------------------------------
NativeDynamicCast        133 ns          133 ns      5252735
RttiDynamicCast         6.08 ns         6.08 ns    114135771

Contribute

Have you found a bug/mistake or any other proposal and want to contribute? Feel free to open an issue or pull request! Happy coding!