Check out example codes for "c++ string to int". It will help you in understanding the concepts better.

Code Example 1

int thing = std::stoi(string);

Code Example 2

// EXAMPLE
std::string sStringAsString = "789";
int iStringAsInt = atoi( sStringAsString.c_str() );

/* SYNTAX
atoi( <your-string>.c_str() )
*/

/* HEADERS
#include <cstring>
#include <string>
*/

Code Example 3

atoi( str.c_str() )

Code Example 4

// stoi example
#include <iostream>   // std::cout
#include <string>     // std::string, std::stoi

int main ()
{
  std::string str_dec = "2001, A Space Odyssey";
  std::string str_hex = "40c3";
  std::string str_bin = "-10010110001";
  std::string str_auto = "0x7f";

  std::string::size_type sz;   // alias of size_t

  int i_dec = std::stoi (str_dec,&sz);
  int i_hex = std::stoi (str_hex,nullptr,16);
  int i_bin = std::stoi (str_bin,nullptr,2);
  int i_auto = std::stoi (str_auto,nullptr,0);

  std::cout << str_dec << ": " << i_dec << " and [" << str_dec.substr(sz) << "]\n";
  std::cout << str_hex << ": " << i_hex << '\n';
  std::cout << str_bin << ": " << i_bin << '\n';
  std::cout << str_auto << ": " << i_auto << '\n';

  return 0;
}

Code Example 5

// Both functions work identically though you'll need to use "#include <string>" 
atoi( str.c_str() );
stoi( str );

Code Example 6

#include <sstream>

int i = 123;
std::string out_string;
std::stringstream ss;
ss << i;
out_string = ss.str();

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