#include <ctime>
#include <iostream>
#include <iterator>
#include <locale>
#include <sstream>
#include <string>
using namespace std;

int main() {
    const locale loc{locale::classic()};
    const auto& tmget{use_facet<time_get<char>>(loc)};
    ios_base::iostate err{ios_base::goodbit};
    tm when{};
    const string fmt{"%X"};
    istringstream iss{"3:04"}; // "3:04:05" would succeed
    istreambuf_iterator<char> first{iss};
    const istreambuf_iterator<char> last{};

    tmget.get(first, last, iss, err, &when, fmt.data(), fmt.data() + fmt.size());

    if (err == ios_base::goodbit) {
        cout << "when.tm_hour: " << when.tm_hour << "\n";
        cout << "when.tm_min:  " << when.tm_min << "\n";
        cout << "when.tm_sec:  " << when.tm_sec << "\n";
    } else {
        cout << "time_get::get() failed.\n";
    }
}