Cereal seems to not properly serialize an std::string

Question

I am trying to serialize a class into a binary, to that effect I first started trying to serialize an std::string member within the class, I wrote this serialization method:

template<typename Archive>
void ShaderProgram::serialize(Archive& archive, ShaderProgram& program)
{
    archive(CEREAL_NVP(program.program_name));
}

Then I am trying to serialize and immediately read the class:

ShaderProgram program;
std::filesystem::create_directories(fs::path(cached_shader_path).parent_path());
std::ofstream os(cached_shader_path, std::ios::binary);
cereal::BinaryOutputArchive archive_out( os );
ShaderProgram::serialize(archive_out, program);

std::ifstream is(cached_shader_path, std::ios::binary);
cereal::BinaryInputArchive archive_in( is );
ShaderProgram::serialize(archive_in, program);

Which results in:

terminate called after throwing an instance of 'cereal::Exception'
  what():  Failed to read 8 bytes from input stream! Read 0

The class I am testing this with is trivial:

struct ShaderProgram
{
    std::string program_name = "name";

    template<typename Archive>
        static void serialize(Archive& archive, ShaderProgram& program);
};
template<typename Archive>
void ShaderProgram::serialize(Archive& archive, ShaderProgram& program)
{
    archive(CEREAL_NVP(program.program_name));
}

I don;t understand why this fails.

Reading and writing to the same file simultaneously can be problematic. Here you haven't even flushed the serialised data and expect to be able to read it. — ALX23z, Oct 17 '21 at 19:12
`ofstream` has function `flush` that does exactly that. It is also called during destructor. — ALX23z, Oct 18 '21 at 13:04

score 1 · Accepted Answer · answered Nov 02 '22 at 13:25

Here is an example. All is fine with cereal. In plain C++ remove Rcpp connections.

// [[Rcpp::depends(Rcereal)]]
#include <string>
#include <fstream>
#include <cereal/archives/binary.hpp>
#include <cereal/types/string.hpp>
#include <cereal/access.hpp>
#include <Rcpp.h>

struct ShaderProgram
{
  ShaderProgram(){};
  ShaderProgram(std::string program_name)
    : program_name{program_name}{};
  ~ShaderProgram() = default;
  
  std::string get_program_name() const {
    return program_name;
  }
  
private:
  std::string program_name{};
  
  friend class cereal::access;

  template<class Archive>
  void serialize(Archive& archive)
  {
    archive(program_name);
  }
  
};


// [[Rcpp::export]]
int main() {
  
  {
    ShaderProgram sp("King Kong 8");
    
    std::ofstream os("Backend/Serialize_StringProgram.bin", std::ios::binary);
    cereal::BinaryOutputArchive oarchive(os);
    oarchive(sp);
  }
  
  {
    ShaderProgram sp{};
    std::ifstream is("Backend/Serialize_StringProgram.bin", std::ios::binary);
    cereal::BinaryInputArchive iarchive(is);
    iarchive(sp);
    
    Rcpp::Rcout << sp.get_program_name() << std::endl;
  }

}

Cereal seems to not properly serialize an std::string

1 Answers1