程序第一次完美运行但是从二进制文件中读取类时第二次想知道seg错误？

Question

The issue I am having is that the program runs great the first time but seg faults the second time. The one difference is the first time the binary file is created and the second time the data is appended to the binary file.

#include <iostream>
#include <vector>
#include <fstream>

//count of the number of car classes written to bin file
int num_of_cars_written;
// This will be the class written to binary
class Car{
    public:
    std::string model;
    int num_of_wheels;
    std::vector<std::string> list_of_features;
    Car(std::string model, int num_of_wheels){
        this->model = model;
        this->num_of_wheels = num_of_wheels;
    }
    Car(){
        this->model = "";
        this->num_of_wheels = 0;
    }
};

void write_to_binary_file(Car& car){
    std::fstream ofile("cars.bin", std::ios::binary | std::ios::app | std::ios::out);
    ofile.write(reinterpret_cast<char*>(&car), sizeof(Car));
    ofile.close();
    ++num_of_cars_written;
}

std::vector<Car> read_bin_of_cars(std::string bin_file_path){
    std::vector<Car> car_list;
    std::fstream file(bin_file_path, std::ios::binary | std::ios::in);
    int size_of_file = sizeof(file);
    int number_of_cars_read_so_far = 0;
    Car* car;
    while(!file.eof()){
        file.read (reinterpret_cast<char*>(car), sizeof(Car));
        car_list.push_back(*car);
        number_of_cars_read_so_far += size_of_file / num_of_cars_written;
        if(number_of_cars_read_so_far >= size_of_file) break;
    }
    file.close();
    return car_list;
}

int main(){
    Car car_one("mazda", 4);
    car_one.list_of_features.push_back("cup holder");
    car_one.list_of_features.push_back("gps");
    Car car_two("honda", 4);
    Car car_three("sazuki", 2);
    car_three.list_of_features.push_back("leather seat");
    write_to_binary_file(car_one);
    write_to_binary_file(car_two);
    write_to_binary_file(car_three);
    std::vector<Car> list = read_bin_of_cars("cars.bin");
    for(auto car : list){
        std::cout << "**************Car Object**************\n";
        std::cout << car.model << std::endl;
        std::cout << car.num_of_wheels << std::endl;
        for (auto feature : car.list_of_features) {
            std::cout << feature << std::endl;
        };
    }
    return 0;
}

Here are the results from the first run

**************Car Object**************
mazda
4
cup holder
gps
**************Car Object**************
honda
4
**************Car Object**************
sazuki
2
leather seat

Here are the results from the second run

Segmentation fault (core dumped) (obviously)

EDIT: this working on the first try is undefined behavior and shouldn't work. There are better ways listed in the comments to go about writing and reading to binary.

Answer 1

There are multiple issues with this code. Summarizing few:

In write_to_binary_file :

write(reinterpret_cast<char*>(&car), sizeof(Car)); This is incorrect way of writing object to file. It will just write memory blog of object car and not necessarily will take care of members pointing to allocations outside of it (for example string or vector etc are not static arrays. They are most likely allocating memory outside of the object. Read about serialization and make use of it instead.

In read_bin_of_cars :

You are reading into pointer car without allocating memory to it. Again, even after you allocate memory, make sure you use serialization to read object from file.

Mind you reinterpret_cast is most dangerous. You should use it only when you know what you are doing. Casting object straight to character pointer is blunder.

There could be still other issues with the code. These are just I found at glance.