c++ - What is the problem with appending arbitrary object to the array?

Question

I have a simple program that process strings from a file into arbitrary class Student (provided below) and appends it to self-written container ArraySequence. When error almost always occurs on marked line (marked with *****).

Class Student:

class Student {
public:
    int year;
    std::string name;
    std::string surname;
    Student(int _year, std::string _name, std::string _surname) {
        this->year = _year;
        this->name = _name;
        this->surname = _surname;
    }
    Student(const Student& s) {
        this->year = s.year;
        this->name = s.name;
        this->surname = s.surname;
    }
    Student& operator=(const Student& s) {
        this->year = s.year;
        this->name = s.name;
        this->surname = s.surname;
        return *this;
    }
    bool operator==(const Student& s) const {
        if (this->year == s.year && this->name == s.name && this->surname == s.surname)
            return true;
        return false;
    }
};

main:

int main() {
    std::ifstream data("filename");
    std::string buffer;
    ArraySequence<Student>* entities = new ArraySequence<Student>;

    getline(data, buffer, '
');
    while (buffer.size() != 0) {
        std::cout << buffer << std::endl;
        int len = static_cast<int>(buffer.find(" "));
        std::string surname = buffer.substr(0, len);
        buffer = buffer.substr(len + 1);
        len = static_cast<int>(buffer.find(" "));
        std::string name = buffer.substr(0, len);
        int year = std::stoi(buffer.substr(len + 1));
        Student b(year, name, surname);
        entities->append(b); *****
        getline(data, buffer, '
');
    }

    std::cout << count << std::endl;

Append:

template <typename T>
void ArraySequence<T>::append(const T& item) {
    ISequence<T>::length++;
    if (!(ISequence<T>::length - 1)) {
        data = (T*)malloc(sizeof(T));
    }
    else {
        data = (T*)realloc(data, sizeof(T) * ISequence<T>::length);
    }
    *(data + ISequence<T>::length - 1) = item;
}

Errors: Thread 1: EXC_BAD_ACCESS (code=EXC_I386_GPFLT); Thread 1: EXC_BAD_ACCESS (code=1, address=0x3);

Sometimes it would just work, but other times it crashes with errors.

I'm using Xcode 11.1

In Thread 1 it says that problem with operator= of Student on the line c++ this->surname = s.surname;

What can I do in a situation like this? Any ideas?

Answer 1

T consists of a non-trivial type (Student contains std::string, plus non-trivial copy/ assignment and destructor), using malloc and realloc will not work correctly.

malloc and realloc do not create viable objects. They only allocate memory. Your program likely crashes due to these invalid Student objects.

The easiest solution is to rewrite ArraySequence<T>::append to only use new[] and delete[], not malloc and realloc.

Note: malloc can be used when using placement-new, but you were not using malloc for this purpose.