What is the best way to read metadata from onnx model from C++?

Question

I have model in onnx format that contains attribute "list_classes". I run it with opencv dnn. I need to read this list using C++.

• I tried opencv dnn library, but it seems like there is no instruments for that.
• I tried onnxruntime, but I can't even create a session. I'm not very familiar with its syntax so maybe I did something wrong.Here is test program:

#include <onnxruntime_cxx_api.h>
#include <iostream>

int main()
{
    auto model_path = L"model.onnx";
    std::unique_ptr<Ort::Env> ort_env;
    Ort::SessionOptions session_options;
    Ort::Session session(*ort_env, model_path, session_options);

    return 0;
}

This code throws

Answer 1

I kind of figured it out

#include <iostream>
#include "onnxruntime_cxx_api.h"

std::vector<std::string> split(std::string str, std::string delimiter) {
    size_t pos = 0;
    std::string token;
    while ((pos = str.find(delimiter)) != std::string::npos) {
        token = str.substr(0, pos);
        output.push_back(token);
        str.erase(0, pos + delimiter.length());
    }
    output.push_back(str);
    return output;
}

int main() {
    std::string model_path = "path/to/model.onnx";
    std::wstring widestr = std::wstring(model_path.begin(), model_path.end());
    const wchar_t* widecstr = widestr.c_str();
    Ort::Env env;
    Ort::SessionOptions ort_session_options;
    Ort::Session session = Ort::Session(env, widecstr, ort_session_options);
    Ort::AllocatorWithDefaultOptions ort_alloc;

    std::cout << "CLASS NAMES: " << std::endl;
    Ort::ModelMetadata model_metadata = session.GetModelMetadata();
    Ort::AllocatedStringPtr search = model_metadata.LookupCustomMetadataMapAllocated("classes", ort_alloc);
    std::vector<std::string> classNames;
    if (search != nullptr) {
        const std::array<const char*, 1> list_classes = { search.get() };
        classNames = split(std::string(list_classes[0]), ",");
        for (int i = 0; i < classNames.size(); i++)
            std::cout << "\t" << i << " | " << classNames[i] << std::endl;
    }

    return 0;
}

The output looks like this:

CLASS NAMES:
    0 | class_1
    1 | class_2
    2 | class_3