0

I have created a class which has two string objects. This class writes and reads the two string objects to/from a file in Binary Mode. All this is defined in the header file "a_details.h" 

    #include <iostream>
    #include <fstream>
    #include <string>
    using namespace std;

class AVENGERS{

public:
string codename;
string realname;
const size_t JUMP = 2*sizeof(string);
AVENGERS(string code=" ",string real=" "){codename=code;realname=real;}

void read(istream & is){
is.read((char*)&codename,sizeof(string));
is.read((char*)&realname,sizeof(string));
}

void write(ostream & os){
os.write((char*)&codename,sizeof(string));
os.write((char*)&realname,sizeof(string));
}

};
istream & operator >> (istream & is,AVENGERS & A)
{

    is>>A.codename>>A.realname;
   return is;

}

ostream & operator << (ostream & os,AVENGERS & A)
{

    os<<A.codename<<"-"<<A.realname;
   return os;

}

below is the file "a_write.cpp "which opens a file in binary mode and writes 3 AVENGERS objects(and hence, 6 strings). 

#include <iostream>
#include <fstream>
#include <string>
#include "a_details.h"
using namespace std;

int main(){
fstream ffile("avengers.bin", ios_base::binary | ios_base::in |ios_base::out);
if(ffile.is_open()){
AVENGERS("Iron Man","Tony Stark").write(ffile);
AVENGERS("Captain America","Steve Rogers").write(ffile);
AVENGERS("Hulk","Bruce Banner").write(ffile);

cout<<"writing done\n";

ffile.close();
}

}

After that, i checked the size of avengers.bin to be 24 bytes to verify if writing operation was done properly. But then , when i create another file "a_read.cpp" to read in the data, it gives me errors. below is the code. 

#include <iostream>
#include <fstream>
#include <string>
#include "a_details.h"
using namespace std;

int main(){

fstream ffile("avengers.bin" , ios_base::binary | ios_base :: in |ios_base::out);

if(ffile.is_open()){
        cout<<"file is going to open"<<endl;
while(!ffile.eof())
    {
        cout<<"entered loop\n";
     AVENGERS TEMPO(" "," ");
      TEMPO.read(ffile);
cout<<"read data\n";
if(ffile.gcount()==0)break;

cout<<"this means file is not at end of file\n";
    }

    cout<<"this means file is at end of file\n";
ffile.close();


}
cout<<"reading done";

return 0;
}

The cout statements in between are for my reference,since i wanted to check where was the problem. But i am unable to understand. The program loops 1 time, then crashes. Can somebody please help me? This is the Error message

Venkatesh Naresh
  • 1
  • 1
  • 1
    Why `is.read((char*)&codename,sizeof(string));` instead of `is.read(codename.c_str(), codename.size())`? That first version is really wrong. – Cory Kramer Mar 26 '18 at 16:10
  • i learnt the first version only. Could you tell why the first version is wrong?I didnt know about the second version – Venkatesh Naresh Mar 26 '18 at 16:14
  • `sizeof(string)` is *not* the number of characters in the string, and that C-style cast `(char*)` results in a `reinterpret_cast` which doesn't make sense. As I mentioned, `.c_str()` is the appropriate way to get at the underlying character buffer, and `size()` is how you query the number of characters. – Cory Kramer Mar 26 '18 at 16:18
  • ok...while working with other data types like double , we consider their size using sizeof(). But while using strings why do we consider length of string and not size of string data type? – Venkatesh Naresh Mar 26 '18 at 16:23
  • Also please take some time to read [Why is iostream::eof inside a loop condition considered wrong?](https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong) – Some programmer dude Mar 26 '18 at 16:25
  • For all container types in the STL (`vector`, `string`, `set`, etc) the `size()` method is how you get the number of elements. For `std::string` the `sizeof` function [will give you a different number](https://stackoverflow.com/questions/3770781/why-is-sizeofstring-32). You have to understand what `sizeof` does, it is the *object* size, not the size of the *array it allocates* elsewhere – Cory Kramer Mar 26 '18 at 16:26

0 Answers0