写入和读取映射文件C ++

Question

I have this struct:

struct DataItem
{
  std::string tag;        
  vector<unsigned char> data_block;
  time_t input_time;     
  int version_mark; 
};

What i am trying to do is to create a 10MB file with boost (1.47.0 version & Windows platform) map that file to memory and write some data of this struct type in a method. It is this one: (I did not fill all the atrributes of the struct since I am just trying the functionality)

void putData()
{

    managed_mapped_file mfile(create_only,"MyMappedFile", 10485760);

    typedef allocator<int, managed_mapped_file::segment_manager> MnmapfileAllocator;
    typedef vector<DataItem,MnmapfileAllocator> MyVector;

    const MnmapfileAllocator alloc_inst (mfile.get_segment_manager());

    MyVector *myvector = mfile.construct<MyVector>("MyVector")(alloc_inst);

    time_t t = time(0);
    struct tm* now = localtime(&t);


    for (int j=0; j<10; j++)
    {
        DataItem data; 

        data.version_mark = j;
        data.timestamp = now->tm_sec;

        myvector->push_back(data);

    }

}

And from another function i am trying to read that data from the file like this: (I just pop_back the last data to see at least if there is data on the file)

void getData()
{
    DataItem data;
    managed_mapped_file mfile(open_only,"MyMappedFile");

    typedef allocator<int, managed_mapped_file::segment_manager> MnmapfileAllocator;
    typedef vector<DataItem,MnmapfileAllocator> MyVector2;

    const MnmapfileAllocator alloc_inst (mfile.get_segment_manager());
    MyVector2 *myvector2 = mfile.find<MyVector2>("MyVector2").first;

    myvector2->pop_back();
    std::cout << myvector2->back().version_mark << std::endl;
}

The file is created to my hard drive physically but the problem is myVector2 is empty every time (i can observe it from watch pane), so i can not test even i succeed to write data to the file or not. By the way also the compiler gaves an access violation error in the last line (cout line)

Can anyone tell me please am i misunderstanding the managed_map_file concept, or what is wrong with this code?

Answer 1

You are creating a vector named "MyVector" in the putData function but you are searching a vector named "MyVector2" in the getData function. The names have to be identical. It's like an identifier.