"Now if we pass our Bar1 to foo it will first wastefully construct an overhead object, with a pointer to overhead, when bar.doSomething() is called inside foo, it will trigger overhead lookup to find some overhead wrapper which then finally calls the piece of code which is exactly what we want."
Sure, as written, there is lots of overhead, because it will copy `t` (by my count) three times, plus do a heap allocation, every time you call `foo`. The heap allocation part might make sense if you're planning to store the type-erased object somewhere and then use it repeatedly. The copying part makes no sense, though it's understandable, because C++ is terrible and makes copying the default.
But the general pattern of implementing type erasure using a wrapper object is sound, and can be made zero-overhead or near-zero-overhead depending on the use case. Here is a simple version where the wrapper object just stores a pointer (making it suitable for cases where the function doesn't store the object beyond its own execution):
In this case, the `Bar` object is passed to `foo` as two registers, a function pointer and the pointer to the object itself. `Bar::doSomething` is inlined into `foo`, and `Example::doSomething` is inlined into `Bar::doSomethingWrapper<Example>`, so it winds up with `foo` calling a function pointer that leads directly to the implementation of Example::doSomething.
If `Example::doSomething` couldn't be inlined, `Bar::doSomethingWrapper<Example>` would likely be compiled as a single instruction jumping to it, which is quite minimal overhead.
Comments
Sure, as written, there is lots of overhead, because it will copy `t` (by my count) three times, plus do a heap allocation, every time you call `foo`. The heap allocation part might make sense if you're planning to store the type-erased object somewhere and then use it repeatedly. The copying part makes no sense, though it's understandable, because C++ is terrible and makes copying the default.
But the general pattern of implementing type erasure using a wrapper object is sound, and can be made zero-overhead or near-zero-overhead depending on the use case. Here is a simple version where the wrapper object just stores a pointer (making it suitable for cases where the function doesn't store the object beyond its own execution):
https://gcc.godbolt.org/z/hEWv1hqEo
In this case, the `Bar` object is passed to `foo` as two registers, a function pointer and the pointer to the object itself. `Bar::doSomething` is inlined into `foo`, and `Example::doSomething` is inlined into `Bar::doSomethingWrapper<Example>`, so it winds up with `foo` calling a function pointer that leads directly to the implementation of Example::doSomething.
If `Example::doSomething` couldn't be inlined, `Bar::doSomethingWrapper<Example>` would likely be compiled as a single instruction jumping to it, which is quite minimal overhead.