Traits seem to not support references and const-references.

[dkorolev]

Hi,

I just had to change

cereal::traits::is_output_serializable<T, cereal::JSONOutputArchive>::value;

into

cereal::traits::is_output_serializable<typename std::remove_const<typename std::remove_re ference<T>::type>::type, cereal::JSONOutputArchive>::value;

Should it be handled by Cereal internally?

Thanks, Dima

[AzothAmmo]

Can you give an example of the code that required this change to be made?

[dkorolev]

https://github.com/dkorolev/sandbox_cereal/blob/master/standalone/traits.cc

#include <iostream>

#include "../cereal/archives/json.hpp"

struct X {
  std::string s;
  template<class A> void serialize(A& ar) {
    ar(CEREAL_NVP(s));
  }
};

template<typename T> void foo(T&& x) {
  std::cout
    << std::boolalpha
    << cereal::traits::is_output_serializable<T, cereal::JSONOutputArchive>::value
    << std::endl;
}

int main() {
  X x;
  const X& y(x);
  foo(x);  // true
  foo(y);  // false
}

As long as `foo() does any compile-type dispatching into cerealizable/non-cerealizable, the code would result in a nontrivial issue to catch.

[AzothAmmo]

We made a design decision at the beginning of cereal not to support raw pointers or references since we did not want to deal with the overhead (in terms of runtime and code) of having to track ownership ourselves to prevent duplicate loads/saves. There's more on this on the main web page at the bottom.

I think at best we should supply a proper static assertion saying that references are not supported when someone attempts to serialize them - it is not our intention to allow a reference to be serializable.

[dkorolev]

Hi Shane,

Thank you for circling back!

For me, the issue is that cereal::traits::is_output_serializable<T, cereal::JSONOutputArchive>::value is true for non-cereal types.

My usecase is HTTP response generation from C++. The logic is simple: Send Cereaizable objects as JSON-s, strings as strings, fail on the rest. Cereal's traits turn out to not be useful since, for example, std::vector gets JSON-ified as well.

Is there a way to use Cereal's traits to only tell if this type specifically defines Cereal-friendly serialize() or save()?

Thanks, Dima

Thanks, Dima

http://dimakorolev.com/ | http://www.linkedin.com/in/dimakorolev | dmitry.korolev@gmail.com | +1 (312) 593 2783

On Sat, Feb 28, 2015 at 11:53 AM, Shane Grant notifications@github.com wrote:

We made a design decision at the beginning of cereal not to support raw pointers or references since we did not want to deal with the overhead (in terms of runtime and code) of having to track ownership ourselves to prevent duplicate loads/saves. There's more on this on the main web page http://uscilab.github.io/cereal/pointers.html at the bottom.

I think at best we should supply a proper static assertion saying that references are not supported when someone attempts to serialize them - it is not our intention to allow a reference to be serializable.

— Reply to this email directly or view it on GitHub https://github.com/USCiLab/cereal/issues/171#issuecomment-76542718.

[AzothAmmo]

The traits are actually already checking if something specifically provides a cereal friendly serialize/save/etc. I think the issue you might be having is that when you pull in the default support for vectors via <cereal/types/vector.hpp>, this defines support for all POD types used with vectors as well. Am I correct or is there a different issue?

[dkorolev]

Yes, I pull in support for vector-s.

Sorry for introducing the confusion: We are talking about two subject in parallel here, due to my mistake misreading your previous comment.

Subject 1: cereal::traits::is_output_serializable<> does not support const reference types. Subject 2: cereal::traits::is_output_serializable<> returns true for basic types like strings or vectors, while I was hoping it would only return true for user-defined types that define template<typename A> void serialize() or template<typename A> void save() const.

To me:

• Subject 1 shows the behaviour of the libary, which I found confusing, and other users might see it as confusing as well.
• Subject 2, while being separate, probably deserves more attention (perhaps move it to a different issue?). I would be glad to have a simple way to tell whether certain user-defined class defines template<typename A> void serialize() or template<typename A> void save() const: in other words, cereal::traits::is_output_serializable<> that has ::value = false for strings, vectors and other primitive types.

Thanks, Dima