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 100
 6 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