1 std::string str = "56789";
2 int n = atoi(str.c_str());
3 cout<<n; //56789

#include<iostream>
enum ret { kvalid=0,kinvalid };    // 是否输入非法的标记
int status = kvalid;
long long Strtointcode(const char* digit, bool minus)
{long long num = 0;while (*digit != '\0'){if (*digit >= '0'&&*digit <= '9'){int flag = minus ? -1 : 1;num = num * 10 + flag*(*digit - '0');if ((!minus&&num > 0x7FFFFFFF) ||( minus&&num < (signed int)0x80000000)){num = 0;break;}digit++;}else{num = 0;break;}}if (*digit == '\0')status = kvalid;return num;
}
int Strtoint(const char* str)
{status = kinvalid;long long num = 0;if (str != NULL&&*str != '\0'){bool minus = false;if (*str == '+')str++;else if (*str == '-'){str++;minus = true;}if (*str != '\0')num = Strtointcode(str, minus);}return (int)num;
}int main()
{char arr[20];int ret = 0;printf("请输入您要转化的字符串：\n");scanf("%s", arr);ret = Strtoint(arr);if (kvalid == status){printf("%d\n", ret);}else{printf("输入非法\n");printf("%d\n", ret);}system("pause");return 0;
}

1 istringstream is("56"); //构造输入字符串流，流的内容初始化为“56”的字符串
2 int i;
3 is >> i; //从is流中读入一个int值存入i中  

 #include <iostream>   // std::cout  #include <string>     // std::string, std::to_string  int main ()  {  std::string pi = "pi is " + std::to_string(3.1415926);  std::string perfect = std::to_string(1+2+4+7+14+8) + " is a perfect number";  std::cout << pi << std::endl;  std::cout << perfect << std::endl;  return 0;  } 输出结果：pi is 3.141593  36 is a perfect number 

 1 //实现to_string函数2 #include<iostream>3 #include<string>4 using namespace std;5 #define max 1006 string to_String(int n)7 {8     int m = n;9     char s[max];
10     char ss[max];
11     int i=0,j=0;
12     if (n < 0)// 处理负数
13     {
14         m = 0 - m;
15         j = 1;
16         ss[0] = '-';
17     }
18     while (m>0)
19     {
20         s[i++] = m % 10 + '0';
21         m /= 10;
22     }
23     s[i] = '\0';
24     i = i - 1;
25     while (i >= 0)
26     {
27         ss[j++] = s[i--];
28     }
29     ss[j] = '\0';
30     return ss;
31 }
32
33 int main()
34 {
35     cout << "请输入整数:";
36     int m;
37     cin >> m;
38     string s = to_String(m) + "abc";
39     cout << s << endl;
40     system("pause");
41     return 0;
42 }

1 int tem = 30;
2 stringstream srt;
3 srt<<tem;
4 string s1 = srt.str();
5 cout<<s1<<endl; // 30