如何使用Curlpp编写json数据文件？

命令行选项正在工作，下面是wokring请求

curl -s -H "Content-Type: application/json"   https://speech.googleapis.com/v1/speech:recognize?key=apikey -d @sync-request.json

与iam尝试使用libcurl做的相同，添加json数据文件失败。。。 在curl:：PostFields中直接以字符串形式传递json数据时工作

在下面的示例中，apikey替换为mydev密钥。 对于较大的文件大小，您需要此选项

#include <iostream>
 #include <curlpp/Options.hpp>
 #include <curlpp/Easy.hpp>
 #include <curlpp/cURLpp.hpp>
 #include <sstream>
 #include <future>
 #include <curlpp/Exception.hpp>
 #include <cstdlib>
 #include <cstdio>
 #include <cstring>
 #include <cerrno>

    size_t WriteCallback(char* ptr, size_t size, size_t nmemb, void *f)
    {
        FILE *file = (FILE *)f;
        cout<<"file write"<<endl;
        return fwrite(ptr, size, nmemb, file);
    }

    std::future<std::string> invoke(std::string const& url) {
      return std::async(std::launch::async, 
        [](std::string const& url) mutable {
          std::list<std::string> header;
          header.push_back("Content-Type: application/json");

          FILE* file = fopen("sync-request.json", "wb");
          curlpp::options::WriteFunctionCurlFunction  myFunction(WriteCallback);
          curlpp::OptionTrait<void *, CURLOPT_WRITEDATA> myData(file);

          curlpp::Cleanup clean;
          curlpp::Easy r;
          r.setOpt(new curlpp::options::Url(url));
          r.setOpt(new curlpp::options::HttpHeader(header));
          r.setOpt(myFunction);
          r.setOpt(myData);

          std::ostringstream response;
          r.setOpt(new curlpp::options::WriteStream(&response));

          r.perform();
          std::cout<<std::string(response.str());
          return std::string(response.str());
        }, url);
    }
    int main(int argc, char **argv) {
        invoke("https://speech.googleapis.com/v1/speech:recognize?key=apikey");
        return 0;
    }

---

如果将Json文件内容加载到字符串中，只需将数据设置为POST有效负载

    string payload = json.data();
    [...]
    request.setOpt(new curlpp::options::PostFields(payload));
    request.setOpt(new curlpp::options::PostFieldSize(payload.length()));
    [...]
    request.perform();

    string payload = json.data();
    [...]
    request.setOpt(new curlpp::options::PostFields(payload));
    request.setOpt(new curlpp::options::PostFieldSize(payload.length()));
    [...]
    request.perform();