Why does my program not read binary files properly?

Question

When i read the binary file it doesn't read anything... This is the reading:

if (file.is_open())
{
    Challenge* newChallenge;
    while (!file.eof())
    {
        file.read((char*)&newChallenge, sizeof(Challenge));
        if (!challenges.contains(newChallenge))
        {
            challenges.push_back(newChallenge);
        }
    }
    delete newChallenge;
    std::cout << "Successfully loaded " << fileName << std::endl;
    file.close();
}

This is the writing:

else if(action == "write"){
    std::ofstream file("Challenges.bin", std::ios::binary);
    if(file.is_open()){
        for (size_t i = 0; i < challenges.length(); i++)
        {
            file.write((char*)&challenges[i], sizeof(Challenge));   
        }
        std::cout << "Successfully written to file!" << std::endl;
    }else {
        std::cout << "Failed to open file!" << std::endl;
    }
    file.close();
}

And this is the challenge class:

#ifndef CHALLENGE_H
#define CHALLENGE_H
#include "String.h"

class Challenge
{
private:
    double rating = 0;
    int status = 1, finishes = 0;
    String init;

public:
    Challenge(String _init = "") : init(_init) {}

    void addToStatus() { status++; }
    void addToRating(double rate)
    {
        finishes++;
        rating = ((rating * (finishes - 1)) + rate) / finishes;
    }

    String getChallenge() { return init; }
    int getStatus() { return status; }
    double checkRating() { return rating; }
};

#endif

Note: The String class is a class that I've made myself using char*, its not from std.. I am not allowed to use the std one.

Are you sure the file is opening correctly? Have you tried debugging? Please provide a [mre] — Alan Birtles, Apr 18 '20 at 08:26
Warning: [`while (!file.eof())` is usually a bug](https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-i-e-while-stream-eof-cons) — user4581301, Apr 18 '20 at 08:30
The file is indeed opening correctly, but the information is not going through... Is it normal that sizeof(Challenge) = 4 ?? This is a bit off to me — Stukata, Apr 18 '20 at 08:35
The things that im doing are from https://www.youtube.com/watch?v=BpxULagsPLs&list=LLOzgYbvfj3KwKUNnCdTVxCw&index=2&t=0s — Stukata, Apr 18 '20 at 08:38
Not sure I can trust tutorials from someone who spells advanced wrong. — user4581301, Apr 18 '20 at 18:01

score 1 · Accepted Answer · answered Apr 18 '20 at 08:25

1

Challenge* newChallenge;
while (!file.eof())
{
    file.read((char*)&newChallenge, sizeof(Challenge));
...

This code is declaring a pointer to a class instance not initialized to point at anything and then apparently wanted to reads over it some bytes from a disk file.

This is very very wrong on at least three levels:

you did not allocate any memory
objects are not byte chunks and to create them you need to invoke constructors
the reading is targeting the pointer, not the memory pointed to (there's an & in excess in the fread call)

answered Apr 18 '20 at 08:25

6502

104,192
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251

okey, soo you're saying that i remove the &, make the Challenge* newChallenge = new Challenge; ? – Stukata Apr 18 '20 at 08:34
@Stukata That would address point 1, but point 2 would still stop your program from working. *objects are not byte chunks*, in general you cannot make an object by reading bytes. – john Apr 18 '20 at 08:39

Why does my program not read binary files properly?

1 Answers1