converting a timestring to a duration - c++

Question

At the moment I am trying to read in a timestring formatted and create a duration from that. I am currently trying to use the boost date_time time_duration class to read and store the value.
boost date_time provides a method time_duration duration_from_string(std::string) that allows a time_duration to be created from a time string and it accepts strings formatted appropriately ("[-]h[h][:mm][:ss][.fff]".).
Now this method works fine if you use a correctly formatted time string. However if you submit something invalid like "ham_sandwich" or "100" then you will instead be returned a time_duration that is not valid. Specifically if you try to pass it to a standard output stream then an assertion will occur.
My question is: Does anyone know how to test the validity of the boost time_duration? and failing that can you suggest another method of reading a timestring and getting a duration from it?
Note: I have tried the obvious testing methods that time_duration provides; is_not_a_date_time(), is_special() etc and they don't pick up that there is an issue.
Using boost 1.38.0

Answer 1

From the documentation, it looks like you may want to try using the stream operators (operator<<, operator>>); error conditions are described at Date Time Input/Output.
Alternately, I suppose you could validate the string before passing it in. Right offhand, it doesn't look like that particular method has any error handling.
Edit:
I'm not sure I would have thought to check the return value like this if it weren't for Brian's answer, but for completeness here's a full example that takes a string as input. You can either check the return value or have it throw an exception (I believe you'd want to catch std::ios_base_failure):
#include <iostream>
#include <sstream>
#include <string>
#include <boost/date_time/posix_time/posix_time.hpp>
using namespace std;
using namespace boost::posix_time;
int main(int argc, char **argv) {
if (argc < 2) {
cout << "Usage: " << argv[0] << " TIME_DURATION" << endl;
return 2;
}
// No exception
stringstream ss_noexcept(argv[1]);
time_duration td1;
if (ss_noexcept >> td1) {
cout << "Valid time duration: " << td1 << endl;
} else {
cout << "Invalid time duration." << endl;
}
// Throws exception
stringstream ss2;
time_duration td2;
ss2.exceptions(ios_base::failbit);
ss2.str(argv[1]);
try {
ss2 >> td2;
cout << "Time duration: " << td2 << endl;
} catch (ios_base::failure e) {
cout << "Invalid time duration (exception caught). what():\n"
<< e.what() << endl;
}
}

Answer 2

Use the stream operators.
time_duration td;
if (std::cin >> td)
{
// it's valid
}
else
{
// it isn't valid
}