stringstream >> asignment

Dear advanced C++ programers:

   I copied a piece simple template and stringstream program from book
(c++ cookbook) on chapter 3, section 5: Parsing a String Containing a
Number in Scientific Notation.
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#include <iostream>
#include <sstream>
#include <string>

using namespace std;

//double sciToDub(const string& str) {
  template<typename T>
  T strToNum(const string& str) {

  stringstream ss(str);
  T tmp;
  ss >> tmp;

// double d = 0;
// ss >> d;
  if (ss.fail()){
    string s = "Unable to format ";
    s += str;
    s += " as a number!";
    throw(s);
  }
  return(tmp);
}
int main() {
  try {
   /*
    cout << sciToDub("1.23456789e5") << endl;
    cout << sciToDub("6.02987654e-5") << endl;
    cout << sciToDub("asdf") << endl;
   */
   double d = strToNum<double>("7.0"); cout << d << endl;
   float f = strToNum<float>("7.0"); cout << f << endl;
   int i = strToNum<int>("7.0"); cout << i << endl;
   char c = strToNum<char>("7.0"); cout << c << endl;
  }
  catch (string& e) {
    cerr << "Whoops: " << e << endl;
  }
}
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on my test of g++,
all 4 show 7
why , especially first 2 (double and float)?
thanks your help in advance a lot
Eric