How do I convert a std::string to a boost::gregorian::date?

Question

I'm trying to convert a std::string to a boost::gregorian::date like this:

using namespace boost::gregorian;

std::string str = "1 Mar 2012";
std::stringstream ss(str);
date_input_facet *df = new date_input_facet("%e %b %Y");
ss.imbue(std::locale(ss.getloc(), df));
date d;

ss >> d;  //conversion fails to not-a-date-time

std::cout << "'" << d << "'" << std::endl;  //'not-a-date-time'

But if the string contains "01 Mar 2012", the conversion succeeds.

How can I convert strings like "1 Mar 2012" to the equivalent boost::gregorian::date?

score 10 · Answer 1 · edited Jun 20 '20 at 09:12

This would seem to be a problem with the use of %e in your input facet.

The Boost.Gregorian documentation specifies that:

%d Day of the month as decimal 01 to 31

%e # Like %d, the day of the month as a decimal number, but a leading zero is replaced by a space

The problem is that if you look at the top part of the documentation, you will notice this warning:

The flags marked with a hash sign (#) are implemented by system locale and are known to be missing on some platforms

I've tried the following cases:

input_string = " 1"
date_format = "%e"
result = failed

input_string = "01"
date_format = "%e"
result = success

input_string = "2000 Mar 1"
date_format = "%Y %b %e"
result = failed

input_string = "2000 Mar  1"
date_format = "%Y %b %e"
result = success

input_string = "2000 Mar 01"
date_format = "%Y %b %e"
result = success

It seems thus that this is a limitation of the Boost implementation (or at least, the fact that it relies on a specific locale for the parsing of %e): The parsing fails when %e is the first item in the input string, and a space is used instead of a leading 0.

My (blind) guess would be that the issue comes from stringstream's tendency to skip whitespaces. I've tried to find a solution with the std::noskipws, however, could not find something that worked.

As a workaround, I would recommend adding a leading zero, or if possible, use a different date format.

Another workaround would be to manually add the space, and reverse the order of the "words" in the string. I've accomplished a working solution like this:

#include "boost/date_time/gregorian/gregorian.hpp"
#include <iostream>
#include <string>

int main(void) {
    using namespace boost::gregorian;

    std::string input_date("1 Mar 2000");

    { // local scope to remove temporary variables as soon as possible
        std::stringstream tmp_ss(input_date);
        std::string tmp;
        input_date.clear(); // empty the initial string
        while (tmp_ss >> tmp) {
            input_date.insert(0, tmp); // insert word at beginning of string
            if(tmp.size() == 1) // if word is one char long, add extra space
                input_date.insert(0, " ");
            input_date.insert(0, " "); // add space to separate words
        }
    }

    std::stringstream ss(input_date);

    // The order of the date is reversed.
    date_input_facet *df = new date_input_facet("%Y %b %e");
    ss.imbue(std::locale(ss.getloc(), df));
    
    date d;  //conversion works

    ss >> d;

    std::cout << "'" << d << "'" << std::endl; // ouputs date correctly.

    return 0;
}

Good luck,

How do I convert a std::string to a boost::gregorian::date?

1 Answers1