Reading From A Binary File Without Writing

[Playtner]

Hi, I've been trying to create program that can access a binary file with a vector in it without first saving it to the file. If I just try to read from the file without saving something, the vector will have information in it but when I output it to screen it will be either gibberish or not output at all. If I do save information to the file in the same program as when I read from it, the output will be what I want. Here is my code:

#include <iostream>
#include <vector>
#include <conio>
#include <fstream>
#include <vcl.h>
//-----------------------------------------------------------------------
struct strWord{
           string word;
           string type;
           string tense;
}aWord;
vector<strWord> words;
vector<strWord> wordsB;
//--------------------------------------------------------------------------
int main(){
          aWord.word = "cow"; 
          aWord.type = "noun"; 
          aWord.tense = "singular";

          words.push_back(aWord);

          ofstream out("BinaryDict.txt", ios::binary);
          out.write((char*)&words, sizeof(words));
          out.close();
          
          ifstream in("BinaryDict.txt", ios::in | ios::binary);
          while(!in.eof()){
                    in.read((char*)&wordsB, sizeof(wordsB));
          }
          in.close();

          cout << wordsB[0].word << '\n';
          cout << wordsB[0].type << '\n';
          cout << wordsB[0].tense << '\n';

          getch();
}

[ArkM]

It's totally wrong approach: class std::vector object is not the same as its contents, sizeof(words) does not bear a relation to the contained data size (print sizeof(words) value, it's constant). Never serialize/deserialize STL containers (include std::string) in a such manner.

Probably the simplest way to serialize your struct strWord object is to output it as three text lines to a text (not binary) fstream. To serialize a vector of n strWord repeat this operation n times. Now to deserialize this vector (to load it in the memory) make reverse operations.

PS. Think: a std::string object contains a string size (unsigned int) and a pointer to its contents. You have wrote only this "header" and this pointer value is absolutely senseless when you will load it into the memory (especially for the next run).

[Playtner]

Ok. I see what you meant about sizeof(words) being constant. Thanks a lot.