在 C++ 中以块的形式将蒸汽写入磁盘

Question

I am quite new to the C++ world, and I am trying to write a file stream to a disk.I am able to do the following:

  const Aws::String objectKey2("filename.tar");
  Aws::S3::S3Client s3_client2;
  Aws::S3::Model::GetObjectRequest object_request2;
  object_request2.SetBucket(bucket_name);
  object_request2.SetKey(objectKey2);
  Aws::S3::Model::GetObjectOutcome get_object_outcome2 =
      s3_client2.GetObject(object_request2);
  auto &retrieved_file2 = get_object_outcome2.GetResult().GetBody();


  std::ios_base::sync_with_stdio(false);
  auto myfile = std::fstream("file.tar", std::ios::out | std::ios::binary);
  myfile.write((char*) &retrieved_file2, 1024*100);
  myfile.close();

But this is writing some 1k to disk and the file is unreadable (obviously). The file is a tar from from aws s3, which I try to write to disk in chunks.

Answer 1

Given the following...

auto &retrieved_file2 = get_object_outcome2.GetResult().GetBody();

retrieved_file2 is actually of type Aws::IOStream & with Aws::IOStream being typedef'd via...

typedef std::basic_iostream<char, std::char_traits<char>> Aws::IOStream;

Hence, you can't simply cast a pointer to retrieved_file2 to char * and hope that the result will make sense when written to a stream.

If the result of the query is the contents of a tar file then you can probably just write the associated stream_buf using (untested)...

std::fstream myfile("file.tar", std::ios::out | std::ios::binary);
myfile << retrieved_file2.rdbuf();