Using Strftime to format DateTime - c++

Question

I am working on a function to format the input timestamp to the input format.
std::string 1stformat = "dd - MM - yyyy HH 'Hours' mm 'Minutes' ss 'Seconds' SSS 'Miliseconds –' a '– Time Zone: ' Z '-' zzzz";//will not print anything
std::string 2ndformat = "'This took about' h 'minutes and' s 'seconds.'";//will print out
After format
char date_string[100];
strftime(date_string, 50, format.c_str(), curr_tm);
My problem is that there will be sometimes the input format too long which made the buffer date_string not enough to content. I am just getting into C++ for the past 3 weeks so I don't have much ex about this.

Answer 1

A wrapper for strftime() that grows a buffer as needed until it's big enough to fit the desired time string:
#include <ctime>
#include <iostream>
#include <memory>
#include <string>
std::string safe_strftime(const char *fmt, const std::tm *t) {
std::size_t len = 10; // Adjust initial length as desired. Maybe based on the length of fmt?
auto buff = std::make_unique<char[]>(len);
while (std::strftime(buff.get(), len, fmt, t) == 0) {
len *= 2;
buff = std::make_unique<char[]>(len);
}
return std::string{buff.get()};
}
int main() {
std::time_t now;
std::time(&now);
std::cout << safe_strftime("The date is %Y-%m-%d", std::localtime(&now))
<< '\n';
return 0;
}

Answer 2

Unfortunately, the interface to std::strftime() is less helpful than that of std::snprintf() in that it returns 0 if the buffer is too small, rather than the number of characters that would be written. We need to increase the buffer size heuristically and retry, perhaps like this:
#include <ctime>
#include <string>
#include <vector>
std::string time_to_string(const char *format, const std::tm* time)
{
// first try with an on-stack buffer (fast path)
char buf[200];
auto written = std::strftime(buf, sizeof buf, format, time);
if (written > 0) {
return buf;
}
// now, iterate with an allocated buffer
auto len = sizeof buf;
std::vector<char> v;
do {
v.resize(len *= 2);
written = std::strftime(v.data(), v.size(), format, time);
} while (written == 0);
return {v.data(), written};
}