How to parse and validate a date in std::string in C++?

Question

I'm working on a project where I have to read a date to make sure that it's a valid date. For example, February 29th is only a valid date on leap years, or June 31st is not a valid date, so the computer would output that information based on the input. My issue is that I can't figure out how to parse the string so that the user can enter "05/11/1996" as a date (for example) and then take that and put it into seperate integers. I was thinking about trying to do something with a while loop and string stream, but I'm a little stuck. If someone could help me with this, I would really appreciate it.

Answer 1

A possible solution might be also based on strptime, however note that this function only validates whether the day is from the interval <1;31> and month from <1;12>, i.e. "30/02/2013" is valid still:

#include <iostream>
#include <ctime>

int main() {
    struct tm tm;
    std::string s("32/02/2013");
    if (strptime(s.c_str(), "%d/%m/%Y", &tm))
        std::cout << "date is valid" << std::endl;
    else
        std::cout << "date is invalid" << std::endl;
}

But since strptime is not always available and additional validation would be nice, here's what you could do:

1. extract day, month, year
2. fill struct tm
3. normalize it
4. check whether normalized date is still the same as retrieved day, month, year

i.e.:

#include <iostream>
#include <sstream>
#include <ctime>

// function expects the string in format dd/mm/yyyy:
bool extractDate(const std::string& s, int& d, int& m, int& y){
    std::istringstream is(s);
    char delimiter;
    if (is >> d >> delimiter >> m >> delimiter >> y) {
        struct tm t = {0};
        t.tm_mday = d;
        t.tm_mon = m - 1;
        t.tm_year = y - 1900;
        t.tm_isdst = -1;

        // normalize:
        time_t when = mktime(&t);
        const struct tm *norm = localtime(&when);
        // the actual date would be:
        // m = norm->tm_mon + 1;
        // d = norm->tm_mday;
        // y = norm->tm_year;
        // e.g. 29/02/2013 would become 01/03/2013

        // validate (is the normalized date still the same?):
        return (norm->tm_mday == d    &&
                norm->tm_mon  == m - 1 &&
                norm->tm_year == y - 1900);
    }
    return false;
}

used as:

int main() {

    std::string s("29/02/2013");
    int d,m,y;

    if (extractDate(s, d, m, y))
        std::cout << "date " 
                  << d << "/" << m << "/" << y
                  << " is valid" << std::endl;
    else
        std::cout << "date is invalid" << std::endl;
}

which in this case would output date is invalid since normalization would detect that 29/02/2013 has been normalized to 01/03/2013.

Answer 2

Another option is to use std::get_time from the <iomanip> header (available since C++11). A good example of its use can be found here.

Answer 3

I'd prefer to use Boost DateTime:

See it Live on Coliru

#include <iostream>
#include <boost/date_time/local_time/local_time.hpp>

struct dateparser
{
    dateparser(std::string fmt)
    {
        // set format
        using namespace boost::local_time;
        local_time_input_facet* input_facet = new local_time_input_facet();
        input_facet->format(fmt.c_str());
        ss.imbue(std::locale(ss.getloc(), input_facet));
    }

    bool operator()(std::string const& text)
    {
        ss.clear();
        ss.str(text);

        bool ok = ss >> pt;

        if (ok)
        {
            auto tm = to_tm(pt);
            year    = tm.tm_year;
            month   = tm.tm_mon + 1; // for 1-based (1:jan, .. 12:dec)
            day     = tm.tm_mday;
        }

        return ok;
    }

    boost::posix_time::ptime pt;
    unsigned year, month, day;

  private:
    std::stringstream ss;
};

int main(){
    dateparser parser("%d/%m/%Y"); // not thread safe

    // parse
    for (auto&& txt : { "05/11/1996", "30/02/1983", "29/02/2000", "29/02/2001" })
    {
        if (parser(txt))
            std::cout << txt << " -> " << parser.pt << " is the " 
                 << parser.day      << "th of "
                 << std::setw(2)    << std::setfill('0') << parser.month
                 << " in the year " << parser.year       << "\n";
        else
            std::cout << txt << " is not a valid date\n";
    }
}

Outputs:

05/11/1996 -> 1996-Nov-05 00:00:00 is the 5th of 11 in the year 96
30/02/1983 is not a valid date
29/02/2000 -> 2000-Feb-29 00:00:00 is the 29th of 02 in the year 100
29/02/2001 is not a valid date