How can I do dynamic assignment to a struct inside a struct in C++?

Question

Currently I am making dynamic allocation of structs within structs in C++. So, I made a code that can be dynamically allocated that I think. Here is the code that I made it :

#include<iostream>
using namespace std;
int cnt = 0;
struct num{
    int x;
    int y;
};
struct name {
    char Stu[30] = { 0 };
    num *number;
};

int input(int& a) {
    if (cnt == 0) {
        cout << "입력할 수 : ";
        cin >> a;
        cnt++;
        return a;
    }
    if (cnt == 1) {
        cout << "입력할 수 : ";
        cin >> a;
        cnt++;
    }
}

void input(char *pt) {
    if (cnt == 2) {
        cout << "입력할 글자 : ";
        cin >> pt;
        cnt++;
    }
}

int inputnum(int i) {
    cout << "할당할 숫자를 쓰시오 : ";
    cin >> i;
    return i;
}

void printdata(name *pSt, int i, int j) {
    cout << pSt[i].number[j].x << endl;
    cout << pSt[i].number[j].y << endl;
    cout << pSt[i].Stu << endl;
}

int main() {
    int num1 = 0;
    int num2 = 0;
    num1 = inputnum(num1);
    num2 = inputnum(num2);
    name* student = new name[num1];
    for (int i = 0; i < num1; i++) {
        for (int j = 0; j < num2; j++) {
            student[i].number = new num[num2];
            input(student[i].number[j].x);
            input(student[i].number[j].y);
            input(student[i].Stu);
            cnt = 0;
        }
    }

    for (int i = 0; i < num1; i++) {
        for (int j = 0; j < num2; j++) {
            printdata(student, i, j);
        }
    }

}

But if I input more than 2 in num2, output number is weird like -842150451. How can I do dynamic assignment to a struct inside a struct in C++?

Answer 1

There are several issues with your code.

First, use std::string instead of char[30]. Second, don't allocate memory unless is necessarily. There is no need for num* number. Use a std::vector.

You can put your code as follows:

struct num {
    int x;
    int y;
};
struct name {
    std::string      Stu;
    std::vector<num> number;
};

-842150451 is not a weird number. It's decimal for 0xCDCDCDCD. This is a value used by Microsoft debug CRT to represent uninitialized heap memory. See here.

Concerning the input functions, why don't you write some simpler functions?

void input_x(int& x)
{
    cout << "입력할 수 : ";
    cin >> x;
}

void input_y(int& y)
{
    cout << "입력할 수 : ";
    cin >> y;
}

void input_s(char *pt)
{
    cout << "입력할 글자 : ";
    cin >> pt;
}

Which you can use as follows:

    for (int i = 0; i < num1; i++) {
        for (int j = 0; j < num2; j++) {
            student[i].number = new num[num2];
            input_x(student[i].number[j].x);
            input_y(student[i].number[j].y);
            input_s(student[i].Stu);
        }
    }

The input_s should use std::string, though, if you use what I suggest above:

void input_s(std::string& pt)
{
    cout << "입력할 글자 : ";
    cin >> pt;
}

As for name* student you can replace that with std::vector<name>:

    std::vector<name> student;
    for (int i = 0; i < num1; i++) {
        for (int j = 0; j < num2; j++) {
            name s;
            s.number.resize(num2);
            input(s.number[j].x);
            input(s.number[j].y);
            input(s.Stu);
            student.push_back(s);
        }
    }