CODEWITHSUNDEEP
c++fstream
Mushy
Mushy

Reputation: 2655

trouble reading binary data

The reader and writer

#include<string>
#include<fstream>
#include<memory>

class BinarySearchFile{

     BinarySearchFile::BinarySearchFile(std::string file_name){

     // concatenate extension to fileName
     file_name += ".dat";

     // form complete table data filename
     data_file_name = file_name;

     // create or reopen table data file for reading and writing
     binary_search_file.open(data_file_name, std::ios::binary);  // create file

     if(!binary_search_file.is_open()){

          binary_search_file.clear();
          binary_search_file.open(data_file_name, std::ios::out | std::ios::binary);
          binary_search_file.close();
          binary_search_file.open(data_file_name), std::ios::out | std::ios::in | std::ios::binary | std::ios::ate;
     }

    std::fstream binary_search_file;

    void BinarySearchFile::writeT(std::string attribute){

        if(binary_search_file){
            binary_search_file.write(reinterpret_cast<char *>(&attribute), attribute.length() * 2);
        }
    }

    std::string BinarySearchFile::readT(long filePointerLocation, long sizeOfData) 
    {
        if(binary_search_file){
           std::string data;
           data.resize(sizeOfData);
           binary_search_file.seekp(filePointerLocation);
           binary_search_file.seekg(filePointerLocation);
           binary_search_file.read(&data[0], sizeOfData);
           return data; 
    }
};

The reader call

while (true){
    std::unique_ptr<BinarySearchFile> data_file(new BinarySearchFile("classroom.dat"));

    std::string attribute_value = data_file->read_data(0, 20);

}

The writer call

    data_file->write_data("packard   ");

The writer writes a total of 50 bytes 

"packard   101       500  "

The reader is to read the first 20 bytes and the result is "X packard X" where X represents some malformed bytes of data. Why is the data read back in x-number of bytes corrupt?

Upvotes: 0

Views: 155

Answers (2)

alexrider
alexrider

Reputation: 4463

binary_search_file.write(reinterpret_cast<char *>(&attribute), attribute.length() * 2);
It is incorrect to cast std::string to char* if you need char* you must use attribute.c_str().
std::string apart from string pointer contains other data members, for example, allocator, your code will write all of that data to file. Also I don't see any reason to multiply string length by 2. +1 makes sense if you want to output terminating zero.

Upvotes: 0

James Kanze
James Kanze

Reputation: 154047

You can't simply write data out by casting it's address to a char* and hoping to get anything useful. You have to define the binary format you want to use, and implement it. In the case of std::string, this may mean outputing the length in some format, then the actual data. Or in the case where fixed length fields are needed, forcing the string (or a copy of the string) to that length using std::string::resize, then outputting that, using std::string::data() to get your char const*.

Reading will, of course, be similar. You'll read the data into a std::vector<char> (or for fixed length fields, a char[]), and parse it.

Upvotes: 2

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