By using in() , you can read any size data you want.

const int MAXN = 6e4 + 7;
unsigned int image[MAXN][30][30];
unsigned int num, magic, rows, cols;
unsigned int label[MAXN];
unsigned int in(ifstream& icin, unsigned int size) {
    unsigned int ans = 0;
    for (int i = 0; i < size; i++) {
        unsigned char x;
        icin.read((char*)&x, 1);
        unsigned int temp = x;
        ans <<= 8;
        ans += temp;
    }
    return ans;
}
void input() {
    ifstream icin;
    icin.open("train-images.idx3-ubyte", ios::binary);
    magic = in(icin, 4), num = in(icin, 4), rows = in(icin, 4), cols = in(icin, 4);
    for (unsigned int i = 0; i < num; i++) {
        for (unsigned int x = 0; x < rows; x++) {
            for (unsigned int y = 0; y < cols; y++) {
                image[i][x][y] = in(icin, 1);
            }
        }
    }
    icin.close();
    icin.open("train-labels.idx1-ubyte", ios::binary);
    magic = in(icin, 4), num = in(icin, 4);
    for (unsigned int i = 0; i < num; i++) {
        label[i] = in(icin, 1);
    }
}

By using in() , you can read any size data you want.

const int MAXN = 6e4 + 7;
unsigned int image[MAXN][30][30];
unsigned int num, magic, rows, cols;
unsigned int label[MAXN];
unsigned int in(ifstream& icin, unsigned int size) {
    unsigned int ans = 0;
    for (int i = 0; i < size; i++) {
        unsigned char x;
        icin.read((char*)&x, 1);
        unsigned int temp = x;
        ans <<= 8;
        ans += temp;
    }
    return ans;
}
void input() {
    ifstream icin;
    icin.open("train-images.idx3-ubyte", ios::binary);
    magic = in(icin, 4), num = in(icin, 4), rows = in(icin, 4), cols = in(icin, 4);
    for (int i = 0; i < num; i++) {
        for (int x = 0; x < rows; x++) {
            for (int y = 0; y < cols; y++) {
                image[i][x][y] = in(icin, 1);
            }
        }
    }
    icin.close();
    icin.open("train-labels.idx1-ubyte", ios::binary);
    magic = in(icin, 4), num = in(icin, 4);
    for (int i = 0; i < num; i++) {
        label[i] = in(icin, 1);
    }
}

You can change the size of buf in function in() to read any size data you want

const int MAXN = 6e4 + 7;
unsigned int image[MAXN][30][30];
unsigned int num, magic, rows, cols;
unsigned int label[MAXN];
 
unsigned int in(ifstream& icin, unsigned int size) {
    unsigned char buf[5];
    icin.read((char*)&buf, size);
    unsigned int ans = 0;
    for (int i = 0; i < size; i++) {
        unsigned int temp = buf[i];
        ans <<= 8;
        ans += temp;
    }
    return ans;
}
void input() {
    ifstream icin;
    icin.open("train-images.idx3-ubyte", ios::binary);
    magic = in(icin, 4), num = in(icin, 4), rows = in(icin, 4), cols = in(icin, 4);
    for (unsigned int i = 0; i < num; i++) {
        for (unsigned int x = 0; x < rows; x++) {
            for (unsigned int y = 0; y < cols; y++) {
                image[i][x][y] = in(icin, 1);
            }
        }
    }
    icin.close();
    icin.open("train-labels.idx1-ubyte", ios::binary);
    magic = in(icin, 4), num = in(icin, 4);
    for (unsigned int i = 0; i < num; i++) {
        label[i] = in(icin, 1);
        cout << label[i] << endl;
    }
}

By using in() , you can read any size data you want.

const int MAXN = 6e4 + 7;
unsigned int image[MAXN][30][30];
unsigned int num, magic, rows, cols;
unsigned int label[MAXN];
unsigned int in(ifstream& icin, unsigned int size) {
    unsigned int ans = 0;
    for (int i = 0; i < size; i++) {
        unsigned char x;
        icin.read((char*)&x, 1);
        unsigned int temp = x;
        ans <<= 8;
        ans += temp;
    }
    return ans;
}
void input() {
    ifstream icin;
    icin.open("train-images.idx3-ubyte", ios::binary);
    magic = in(icin, 4), num = in(icin, 4), rows = in(icin, 4), cols = in(icin, 4);
    for (unsigned int i = 0; i < num; i++) {
        for (unsigned int x = 0; x < rows; x++) {
            for (unsigned int y = 0; y < cols; y++) {
                image[i][x][y] = in(icin, 1);
            }
        }
    }
    icin.close();
    icin.open("train-labels.idx1-ubyte", ios::binary);
    magic = in(icin, 4), num = in(icin, 4);
    for (unsigned int i = 0; i < num; i++) {
        label[i] = in(icin, 1);
    }
}