how to convert string to int in c 2b 2b

1#include <iostream>
2#include <string>
3
4int main() {
5
6    std::string str = "123";
7    int num;
8
9    // using stoi() to store the value of str1 to x
10    num = std::stoi(str);
11
12    std::cout << num;
13
14    return 0;
15}

1int thing = std::stoi(string);

1// For C++11 and later versions
2string str1 = "45"; 
3string str2 = "3.14159"; 
4string str3 = "31337 geek"; 
5
6int myint1 = stoi(str1); 
7int myint2 = stoi(str2); 
8int myint3 = stoi(str3); 
9
10// Output
11stoi("45") is 45
12stoi("3.14159") is 3
13stoi("31337 geek") is 31337 

1#include <iostream>
2#include <string>
3using namespace std;
4int main() {
5 
6    string s = "10";
7 
8    try
9    {
10        int i = stoi(s);
11        cout << i << '\n';
12    }
13    catch (invalid_argument const &e)
14    {
15        cout << "Bad input: std::invalid_argument thrown" << '\n';
16    }
17    catch (out_of_range const &e)
18    {
19        cout << "Integer overflow: std::out_of_range thrown" << '\n';
20    }
21 
22    return 0;
23}

1atoi( str.c_str() )

1string s = "123";
2int n = s.size();
3int num = 0;
4 for(int i = 0 ; i<n;i++)//this what stoi built function do XD
5 {
6  num = num*10+(s[i]-'0');
7 }