xlnt library : date to std::string conversion error

With xlnt library:

How to convert a date value to std::string ?

const auto vdate = cell.value<xlnt::date>(); 
    
std::string vdates = std::string(vdate);

I get :

error: no matching function for call to ‘std::__cxx11::basic_string<char>::basic_string(const xlnt::date&)’
 4259 |                         std::string vdates = std::string(vdate);

    
error: no matching function for call to ‘std::__cxx11::basic_string<char>::basic_string(const xlnt::date&)’
 4259 |                         std::string vdates = static_cast<std::string>(vdate);

Solution

Although xlnt::cell has a number of formatting options

cell.number_format(xlnt::number_format::date_yyyymmdd2()); // example

they seem to only affect the visual appearance in Excel and does not affect the outcome when using for example

std::string vdates = cell.to_string();
// or
std::cout << cell << '\n';

but instead shows an integer number representing the date.

xlnt::date is a rather simple placeholder for three ints and doesn't seem to support any kind of formatting either directly or by helper functions.

I would therefore convert xlnt::date to std::chrono::year_month_day and do all the work in the chrono domain. If needed, convert the result back to xlnt::date.

Conversion functions could look like this:

#include <xlnt/xlnt.hpp>
#include <chrono>

std::chrono::year_month_day xlnt2chrono(const xlnt::date& in) {
    if(in.is_null()) {
        // Extracting year/month/day from xlnt::date
        // will throw an exception if you do that when is_null()
        // is true. If you want that behavior, just remove this:
        return {}; // 0000-00-00
    }
    return std::chrono::year_month_day(
        std::chrono::year(in.get_year()),
        std::chrono::month(static_cast<unsigned>(in.get_month())),
        std::chrono::day(static_cast<unsigned>(in.get_day())));
}

xlnt::date chrono2xlnt(const std::chrono::year_month_day& in) {
    return xlnt::date(static_cast<int>(in.year()),
                      static_cast<int>(static_cast<unsigned>(in.month())),
                      static_cast<int>(static_cast<unsigned>(in.day())));
}

^{Note: The cast to unsigned in chrono2xlnt is to extract the numeric month and day and the cast to int is to provide it to xlnt::date without getting warnings.}

When you then have a std::chrono::year_month_day formatting it to your pleasure is just a matter of specifying the format to std::format which will then use the formatter std::formatter<std::chrono::year_month_day>.

Example:

#include <format>
#include <string>

//...

const auto vdate = cell.value<xlnt::date>();

std::string vdates = std::format("{:%Y-%m-%d}", xlnt2chrono(vdate)); // YYYY-mm-dd