Question

is there a way to tweak std::stod() in order to increase the number of decimal digits in the (string to double) conversion and to force it to use the US locale?

I have a Qt application that can be run in both console or gui mode:

if (opt->getFlag( 'c' ) || opt->getFlag( "console" ) ){
  ThreadManager  modelMainThread;
  modelMainThread.runFromConsole(inputFileName,scenarioName);
}
else {
  QApplication app(argc, argv);
  MainWindow mainWin;
  mainWin.show();
  return app.exec();
}

Within this application I have a string to double method that wraps the new C++11 stod:

double s2d ( const string &string_h) const {
  try {
    return stod(string_h);
  } catch (...) {
    if (string_h == "") return 0;
    else {
      cout << "error!" << endl;
    }
  }
  return 0;
}

Odd enough, while in the console mode the string to double conversion expects a string with dot as decimal separator, in the gui mode it instead expects a string with comma.
Furthermore, as I was previously using istringstream:

istringstream totalSString( valueAsString );
totalSString >> valueAsDouble;

I noticed that stod truncates the resulting double to just 3 decimal digits, much less than istringstream.

So is there a way to increase the number of decimal digits and to force std::stod to use the US locale for the conversion ?

Thanks 🙂

EDITED:

If I try this script:

// testing stod() ..
vector<string> numbers;
numbers.push_back("123.1234567890");
numbers.push_back("123.1234");
numbers.push_back("123,1234567890");
numbers.push_back("123,1234");
double outd;
for(uint i=0;i<numbers.size();i++){
    try {
        outd =  stod(numbers[i]);
        cout << "Conversion passed: " << numbers[i] << "  -  " << outd << endl;
    } catch (...) {
        cout << "Conversion DID NOT passed: " << numbers[i] << "  -  " <<endl;
    }
}

I got these results:

“console” mode:

Conversion passed: 123.1234567890  -  123.123
Conversion passed: 123.1234  -  123.123
Conversion passed: 123,1234567890  -  123
Conversion passed: 123,1234  -  123

“gui” mode:

Conversion passed: 123.1234567890  -  123
Conversion passed: 123.1234  -  123
Conversion passed: 123,1234567890  -  123.123
Conversion passed: 123,1234  -  123.123

So clearly there is something influencing stod() behaviour !

Answer 1

std::stod and its kin were designed to provide a simple, quick conversion from a string to a numeric type. (full disclosure: it’s my design) So, no, no locales; what you see is what you get.