I wanna read data from my binary file after the writing process complete and also read data without the writing process

Question

Here is my part of the code from binary file writing. here I'm passing a 2d vector table that contains text format data or binary data. if the main table contains binary data I wanna read and load that data into one vector table. I already done that if the data is text file I can load that data from that file, but it's a binary data I don't know how can I load. I am also using index table. That means that table contains size of the each field in main table.

For eg: emp.idx

field - Size

name - 20

age - 2

sex- 10

mainTbl - main table containing the binary data.

typedef vector <string> record_t;
typedef vector <record_t> table_t;
table_t mainTbl;

table_t fileStruct::FormatData(table_t &mainTbl)
{

fstream fs("emp.bin",ios::binary | ios::out | ios::in); 

size_t rowLength=mainTbl.size();
size_t colLength=idxTbl.size();
count_t  colSize;
    for (size_t j=0;j<colLength;j++)

    {
        colSize.push_back(idxTbl[j].fsize);
        //cout<<"colum size "<<colSize[j]<<endl;
    }


    for(size_t i=0;i<rowLength;i++)

    {

    for (size_t j=0;j<colLength;j++)

        {

        string data=mainTbl[i].at(j);

        data.resize(colSize.at(j),' ');

        mainTbl[i].at(j)=data;
        int len = data.length();
        fs.write(reinterpret_cast<char*> (&len),len);
        fs.write(const_cast<char*>(data.c_str()),len);

        //cout<<data;
        //fu<<mainTbl[i].at(j);

        }
    fs<<endl;
    //cout<<endl;
    }

    return mainTbl;

 }

Answer 1

You made a mistake in your writing code

fs.write(reinterpret_cast<char*> (&len),len);

should be

fs.write(reinterpret_cast<char*>(&len), sizeof len);

To read you can read into a temporary vector and create the string from that.

vector<char> temp;
fs.read(reinterpret_cast<char*>(&len), sizeof len);
if (len > 0)
{
    temp.resize(len);
    fs.read(&temp[0], len);
}
mainTbl[i].at(j) = string(temp.begin(), temp.end());