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Runtime Polymorphism · 3:40
How Virtual Calls Work
How does one call find the right function while the program runs? We watch an object grow by eight bytes when its first virtual function appears, follow the hidden vptr into its class's table of function addresses, read the three arm64 instructions clang emits for a virtual call, and measure what it really costs, including the inlining it gives up.
You will learn
- Explain what the vptr and the vtable are and where they live
- Trace a virtual call as load vptr, load slot, jump
- Predict how sizeof changes when virtual functions are added
- Describe the real costs of virtual calls, including lost inlining