jsoncpp write incrementally

Question

I have to log down what my application does into a json file. Is expected that the application goes on for weeks and so I want to write the json file incrementally.

For the moment I'm writing the json manually, but there is some log-reader application that is using Jsoncpp lib and should be nice to write the log down with Jsoncpp lib too.

But in the manual and in some examples I haven't found anything similar.. It is always something like:

Json::Value root;
// fill the json

ofstream mFile;
mFile.open(filename.c_str(), ios::trunc);
mFile << json_string;
mFile.close();

That is not what I want because it unnecessary fills the memory. I want to do it incrementally.. Some advice?

Answer 1

If you can switch to plain JSON to JSON lines, as described in How I can I lazily read multiple JSON objects from a file/stream in Python? (thanks ctn for the link), you can do something like that :

const char* myfile = "foo.json";

// Write, in append mode, opening and closing the file at each write
{   
    Json::FastWriter l_writer;
    for (int i=0; i<100; i++)
    {
        std::ofstream l_ofile(myfile, std::ios_base::out | std::ios_base::app);

        Json::Value l_val;
        l_val["somevalue"] = i;
        l_ofile << l_writer.write(l_val);

        l_ofile.close();
    }       
}

// Read the JSON lines
{
    std::ifstream l_ifile(myfile);
    Json::Reader l_reader;
    Json::Value l_value;
    std::string l_line;
    while (std::getline(l_ifile, l_line))
        if (l_reader.parse(l_line, l_value))
            std::cout << l_value << std::endl;  
}    

In this case, you do not have a single JSON in the file anymore... but it works. Hope this helps.

Answer 2

I am a maintainer of jsoncpp. Unfortunately, it does not write incrementally. It does write into a stream without using extra memory, but that doesn't help you.