How to resolve JSON serialization error with cereal c++?

Question

I encountered the following runtime error with cereal when trying serialize the class with JSON:

libc++abi: terminating with uncaught exception of type cereal::RapidJSONException: rapidjson internal assertion failure: IsObject()

Interestingly, absolutely same code works with binary archive.

I'm trying to make a template function for my code. Something like that:

enum SER_Archive_Type {BIN, JSON};

template<typename T>
bool SerializeObject(T &obj, std::string filename, const SER_Archive_Type TYPE) {
    std::fstream fs;

    switch (TYPE) {
    case JSON: {
        fs.open(filename, std::ios::out);
        if (fs.is_open()) {
            cereal::JSONOutputArchive jarchive(fs);
            jarchive(obj);
            fs.close();
            return true;
        } else {
            return false;
        }
        break;
    }
    case BIN: {
        fs.open(filename, std::ios::out | std::ios::binary);
        if (fs.is_open()) {
            cereal::BinaryOutputArchive barchive(fs);
            barchive(obj);
            fs.close();
            return true;
        } else {
            return false;
        }
        break;
    }
    default: 
        break;
    }

    return false;
}

template<typename T>
bool DeserializeObject(T &obj, std::string filename, const SER_Archive_Type TYPE) {
    std::fstream fs;

    switch (TYPE) {
    case JSON: {
        fs.open(filename, std::ios::in);
        if (fs.is_open()) {
            cereal::JSONInputArchive jarchive(fs);
            jarchive(obj);
            fs.close();
            return true;
        } else {
            return false;
        }
        break;
    }
    case BIN: {
        fs.open(filename, std::ios::in | std::ios::binary);
        if (fs.is_open()) {
            cereal::BinaryInputArchive barchive(fs);
            barchive(obj);
            fs.close();
            return true;
        } else {
            return false;
        }
        break;
    }
    default: 
        break;
    }

    return false;
}

Example code looks like that:

int main()
{
    uint32_t a = 123;
    SerializeObject(a, "a.bin", BIN);   // ok

    uint32_t b;
    DeserializeObject(b, "a.bin", BIN); // ok
    cout << b << endl; 

    uint32_t c = 321;
    SerializeObject(c, "c.txt", JSON);   // ok

    uint32_t d;
    DeserializeObject(d, "c.txt", JSON); // error
    cout << b << endl;
}

I found out that the top level node in the generated JSON file is not closed. See the content of c.txt file from the example:

{
    "value0": 321

This must be incorrect behavior or I'm missing something? I don't know how to resolve this correctly. Thank you in advance for your help!

score 0 · Answer 1 · answered May 14 '23 at 03:00

Apparently, I haven't understood how archives work. The solution will be:

case JSON: {
        fs.open(filename, std::ios::out);
        if (fs.is_open()) { 
            { // to finish json inside of brackets and call destructor... 
                cereal::JSONOutputArchive jarchive(fs);
                jarchive(obj);
            }
            fs.close();
            return true;
        } else {
            return false;
        }
        break;
    }

score 0 · Accepted Answer · answered May 14 '23 at 14:39

Lifetime issue can be solved much easier:

if (std::ofstream file{filename}) 
    //file belongs to `if` scope: no need to close.
    cereal::JSONOutputArchive{file}(obj);
    /*Archive is rvalue and is destructed
    immediately if not captured by a reference.*/

Note: since I use ofstream instead of fstream, the ios_base::out flag is redundant. Similarly for input files, you can use ifstream. The ios_base::binary is still needed for binary files:

if (std::ofstream file{filename, std::ios_base::binary})...

How to resolve JSON serialization error with cereal c++?

2 Answers2