why do non-const iovector_view memcpy_to/memcpy_from mutate original IOVector?

[austinhmh]

• what is the design rationale for allowing the non-const view to modified original IOVector? IT seem very counter-intuitive for a view to modify original data. That make me very confused.

void DeepCopyIOVe(IOVector* src, IOVector* dest) {
    dest->memcpy_from(src, src->sum);
    std::string s;
    s.resize(dest->sum());
    dest.memcpy_to(s.data(), dest->sum());
    // dest actually deep copy all bytes from src but iov_len equal 0 which mean dest->sum is also 0 and s is "".
    // if want to use these two variables properly late on, I had to use const IOVector* value to memcpy，
    // like static_cast<const IOVector*>(dest)->memcpy_from(static_cast<const IOVector*>dest(src), src->sum()).
}

the reason why sum equal to 0 is that func implementation internally uses by a view object. It can directly access the underlying iovec* iovec() which should only exclusively managed by IOVector Itself.

• are there similar patterns in other libraries? Or that is unique to photonLibOS?

[lihuiba]

what is the design rationale for allowing the non-const view to modified original IOVector?

The design dates back to many years ago, when the concept of view just appears in C++ (e.g. string_view). And I didn't master the concept very well, I was expecting read-write access to the underlay object (what I see is what I get 😂). Perhaps I should have named it as slice.

are there similar patterns in other libraries? Or that is unique to photonLibOS?

I don't have related information.

[austinhmh]

Well, I think that make sense if obj's name is slice. My concept of 'view' is derived from c++'s string_view, perhaps that's why I find this design counter-intuitive.

are there similar patterns in other libraries? Or that is unique to photonLibOS?

I just want to ask whether have other libraries use view objects to manage underlying data. It seems like this is just a case of a name-functionality mismatch in this repo.

But by the way, this mismatch greatly complicated my understanding and use of iovector_view and IOVectorEntity 😢

[beef9999]

int main(int argc, char** arg) {
    IOVector iov1, iov2;
    iov1.push_back(10);
    iov2.push_back(10);
    iov1.memcpy_to(&iov2);
    std::cout << iov1.sum() << std::endl;   //  Will print 0, which is beyond expectation.
}

[lihuiba]

The const versions realize non consuming semantics. I believe that should be made more clear.