What is the Resource Consumption Overhead of RTTI in Embedded Systems?

The Resource Consumption of RTTI

The overhead of utilizing RTTI (Run-Time Type Information) is a concern, especially on embedded systems with limited resources. However, the extent of this overhead remains unclear.

Resource Usage

Resource consumption through RTTI is highly implementation-specific. An important optimization lies in using static_cast instead of dynamic_cast whenever possible. Static_cast incurs the cost of a single std::type_info comparison, while dynamic_cast necessitates traversing an inheritance tree.

GCC Implementation Details

GCC employs a preferred C ABI that guarantees consistent and unique typeid() objects for each type across dynamic linking boundaries. This enables the efficient comparison of typeid(a) == typeid(b). Moreover, in GCC's preferred ABI, every class vtable inherently contains a pointer to a per-type RTTI structure. Accordingly, a typeid() operation should only entail the overhead of a vtable lookup, comparable to calling a virtual member function.

Size analysis of compiled binaries reveals that disabling RTTI (via -fno-rtti) may paradoxically increase binary size. This could potentially stem from altered behavior in the GCC STL code without RTTI support.

Conclusion

While RTTI may be considered resource-intensive, the overhead is implementation-dependent and can be mitigated through careful coding practices. For platforms with GCC's preferred ABI, RTTI introduces minimal space requirements and exceptionally efficient typeid() comparisons. However, it is still advisable to consider design alternatives to avoid excessive RTTI usage.

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