C++测验代码

/*
    返回字符串前n位和返回整数前n位
*/

#include <iostream>

unsigned long left(unsigned long num, int n);
char * left(char * str, int n);

using namespace std;

int main()
{
    unsigned long  num;
    int n;
    cout << "请输入一个整数:";
    cin >> num;
    cout << "请输入要截取的位数:";
    cin >> n;
    cout << "前" << n << "位为" << left(num, n) << endl;

    char * str = new char(20);
    cout << "请输入一个字符串:";
    cin >> str;
    cout << "请输入要截取的位数:";
    cin >> n;
    cout << "前" << n << "位为" << left(str, n) << endl;

    //delete[] str;

    cin.get();
    cin.get();
}


unsigned long left(unsigned long num, int n)
{
    unsigned long len = 1;    //表示这个数一共有几位
    unsigned long temp_num = num;
    while (temp_num /= 10)
    {
        len++;
    }
    unsigned long cut_num_len = len - (unsigned long(n) >= len ? len : unsigned long(n));    //当n小于整数的长度时，直接取回原来的整数
    while (cut_num_len--)
    {
        num /= 10;
    }
    return num;
}

char * left(char * str, int n)
{
    int len = strlen(str);
    n = n >= len ? len : n;
    char * temp_char = new char(n + 1);

    for (int i = 0; i < n; i++) {
        *(temp_char + i) = *(str + i);
    }
    temp_char[n] = '';
    return temp_char;
}

大家看下，有一个问题，当我取消//delete[] str;注释后，程序会报错，不是鼓励如果是用new自成的指针最好要delete掉的吗？求正确答案。