A simple cpp example to uncap a capped collection in MongoDB. This version is using the legacy mongo-cxx-driver https://github.com/mongodb/mongo-cxx-driver.

uncapCollectionExample.cpp

#include "mongo/client/dbclient.h"

int main() {

  /* Initialize Driver */
  mongo::Status status = mongo::client::initialize();
  if (!status.isOK()){
    std::cout<< "Failed to initialize driver." << std::endl;
    return  EXIT_FAILURE;
  }

  /* Establish db connection */
  mongo::DBClientConnection c;
  try {
    c.connect("localhost");
  } catch( const mongo::DBException &e ) {
    std::cout << "caught " << e.what() << std::endl;
  }

  /* The database name is 'capped' and the capped collection name is 'test',
     using a temporary collection in the same database with a name 'temp'.
  */
  const std::string CAPPED_COLL = "capped.test";
  const std::string UNCAPPED_COLL = "capped.temp";

  /* Copy collection from capped to uncapped collection */
  std::auto_ptr<mongo::DBClientCursor> cursor = c.query(CAPPED_COLL, mongo::BSONObj());
  while(cursor->more()){
    mongo::BSONObj p = cursor->next();
    c.insert(UNCAPPED_COLL, p);
  }

  /* Drop capped collection, and rename uncapped to capped collection */
  mongo::BSONObj result;
  c.runCommand("admin",
               BSON("renameCollection"<<UNCAPPED_COLL
                    <<"to"<<CAPPED_COLL
                    <<"dropTarget"<<true), 
                result);

  std::cout<< "Completed" <<std::endl;
  return EXIT_SUCCESS;
}