This complicated function illustrates how to use strtod properly,
It converts an string that looks like a floating point number
(a double) into that number:

  #include <string>
  #include <cstdlib>

  using namespace std;

// if you want spaces after the number to be acceptable (spaces
// before the number always are, for some reason) you need this
// larger version of the function

  double convert(string s, bool & ok)
  { char * x;
    double d = strtod(s.c_str(), & x);
    int i = 0;
    while (true)
    { char c = x[i];
      if (c == '\0')
        break;
      if (c != ' ')
      { ok = false;
        return 0.0; }
      i += 1; }
    ok = true;
    return d; }

// if you know there will not be spaces after the number, or you
// want them to be considered incorrect, all you need is this

  double convert(string s, bool & ok)
  { char * x;
    double d = strtod(s.c_str(), & x);
    ok = * x == 0;
    return d; }

The best thing is to just use convert exactly as it is. Here's an example
use:

  { string s1 = "   3.251e-7    ";   // spaces not needed, just demonstrating
    string s2 = "3.215e-7x";         // that they are allowed.
    bool ok;

    double d1 = convert(s1, ok);
    if (ok)                                  // you will see "The value is 3.251e-07"
      cout << "The value is " << d1 << "\n";
    else
      cout << s1 << " is not a valid number\n";

    double d2 = convert(s2, ok);
    if (ok)                                  // you will see "3.215e-7x is not a valid number"
      cout << "The value is " << d2 << "\n";
    else
      cout << s2 << " is not a valid number\n"; }


It is annoying because the function that does all the work is a C function,
it has not been updated to C++. That is why it has some odd things in it.

If you read about a "better" way, it isn't. The official C++ version called
stod does not work properly.