Cheap Type Erasure with std::any
Using std::any as the storage for a type erased object matching an interface, and capturing the de-erasure with a lambda where the underlying type is known seems to be very inexpensive and well visible to the compiler for optimization.
The general technique is grabbing the type to be any_cast, when the type is known, to initialize a function pointer, using the decay of a non-capturing lambda to erase the type used in any_cast.
[](std::any const& d, int i) {
return std::any_cast<std::remove_reference_t<Duck>>(&d)->quack(
i);
}This can be done directly for the holding type erasing object, at the cost of space for each member function, or the vtbl construction can be delegated to a templated function instantiating a static identical for each duck type.
For comparison:
Using any as the store and unwrapping via lambda
3: ---------------------------------------------------------------------------------------
3: Benchmark Time CPU Iterations
3: ---------------------------------------------------------------------------------------
3: Benchmark_Construct_AnyDuck 2.36 ns 2.36 ns 6341642
3: Benchmark_Construct_AnyDuck_LValue 2.78 ns 2.78 ns 6154584
3: Benchmark_Construct_AnyDuck_Temp 3.64 ns 3.64 ns 3686884
3: Benchmark_Construct_HiddenAnyDuck_LValue 8.36 ns 8.36 ns 1678108
3: Benchmark_Construct_HiddenAnyDuck_Temp 8.09 ns 8.09 ns 1796347
3: Benchmark_Construct_AnyDuckVtbl 3.81 ns 3.81 ns 3659894
3: Benchmark_Construct_AnyDuckVtbl_LValue 3.82 ns 3.82 ns 3712408
3: Benchmark_Construct_AnyDuckVtbl_Temp 4.45 ns 4.45 ns 3142312
3: Benchmark_Construct_HiddenAnyDuckVtbl_LValue 8.10 ns 8.11 ns 1697762
3: Benchmark_Construct_HiddenAnyDuckVtbl_Temp 8.15 ns 8.16 ns 1740605
"Traditional" Type erasure Parent Style (hidden inheritance)
6: ------------------------------------------------------------------------------------
6: Benchmark Time CPU Iterations
6: ------------------------------------------------------------------------------------
6: Benchmark_Construct_TEPSDuck 11.6 ns 11.6 ns 1361988
6: Benchmark_Construct_TEPSDuck_LValue 14.4 ns 14.4 ns 1155166
6: Benchmark_Construct_TEPSDuck_Temp 13.9 ns 13.9 ns 1023115
6: Benchmark_Construct_HiddenTEPSDuck_LValue 24.0 ns 24.0 ns 644624
6: Benchmark_Construct_HiddenTEPSDuck_Temp 22.0 ns 22.0 ns 681301
Actual use of inheritance
9: ---------------------------------------------------------------------------------
9: Benchmark Time CPU Iterations
9: ---------------------------------------------------------------------------------
9: Benchmark_Construct_IDuck 12.6 ns 12.6 ns 1226115
9: Benchmark_Construct_IDuck_LValue 15.4 ns 15.4 ns 1014344
9: Benchmark_Construct_IDuck_Temp 16.7 ns 16.7 ns 829242
9: Benchmark_Construct_HiddenIDuck_LValue 20.0 ns 20.0 ns 702076
9: Benchmark_Construct_HiddenIDuck_Temp 20.8 ns 20.8 ns 708747
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