https://github.com/PointCloudLibrary/pcl/blob/pcl-1.8.0/io/src/openni2_grabber.cpp#L329-L330

atoi_sample.cpp

#include <iostream>
#include <string>

int main()
{
  const std::string id = "#3";
  std::cout << "id                          // => " << id << std::endl;
  std::cout << "id[0]                       // => " << id[0] << std::endl;
  std::cout << "id.c_str()                  // => " << id.c_str() << std::endl;
  std::cout << "(id.c_str() + 1)            // => " << (id.c_str() + 1) << std::endl;
  std::cout << "(id.c_str() + 2)            // => " << (id.c_str() + 2) << std::endl;
  std::cout << "atoi(id.c_str())            // => " << atoi(id.c_str()) << std::endl;
  std::cout << "atoi(id.c_str() + 1)        // => " << atoi(id.c_str() + 1) << std::endl;
  std::cout << "(atoi(id.c_str()) - 1)      // => " << (atoi(id.c_str()) - 1) << std::endl;
  std::cout << "(atoi(id.c_str() + 1) - 1)  // => " << (atoi(id.c_str() + 1) - 1) << std::endl
  
  /**
   * id                          // => #3
   * id[0]                       // => #
   * id.c_str()                  // => #3
   * (id.c_str() + 1)            // => 3
   * (id.c_str() + 2)            // => 
   * atoi(id.c_str())            // => 0
   * atoi(id.c_str() + 1)        // => 3
   * (atoi(id.c_str()) - 1)      // => -1
   * (atoi(id.c_str() + 1) - 1)  // => 2
   */
}