c++ to_string error with date - c++

Question

I am trying to write a function that returns the date as a string. I went about this like so:
string date() // includes not listed for sake of space, using namespace std
{
tm timeStruct;
int currentMonth = timeStruct.tm_mon + 1;
int currentDay = timeStruct.tm_mday;
int currentYear = timeStruct.tm_year - 100;
string currentDate = to_string(currentMonth) + "/" + to_string(currentDay) + "/" + to_string(currentYear);
return currentDate;
}
this is giving four compile time errors.
1 of these:
to_string was not declared in this scope
and 3 of these:
Function to_string could not be resolved
one for each use of to_string.
According to everywhere else on the internet, this code should work. Can someone please shed some light on the subject?

Answer 1

As has been mentioned in the comments, what you're trying to use requires C++11. This means both a compiler that supports C++11 (E.g. GCC 4.7+), AND possibly manually enabling C++11 (E.g. flag -std=c++11), so check both of those if you believe it should be working for you.
If you're stuck with a compiler that does not support C++11, you can use the following to achieve what you want, with regular C++:
string date()
{
tm timeStruct;
int currentMonth = timeStruct.tm_mon + 1;
int currentDay = timeStruct.tm_mday;
int currentYear = timeStruct.tm_year - 100;
char currentDate[30];
sprintf(currentDate, "%02d/%02d/%d", currentMonth, currentDay, currentYear);
return currentDate; // it will automatically be converted to string
}
Note that for the Day and Month parameters, I used %02d to force it to display at least 2 digits, so 5/1 will actually be represented as 05/01. If you don't want that, you can just use %d instead, which will behave like your original to_string. (I'm not sure what format you're using for currentYear, but you'll probably also want to use either %02d or %04d for that parameter)

Answer 2

std::to_string is part of C++11 and is in the <string> header. The following code works with g++ 4.7, as well as recent versions of clang and VC++. If compiling a file with these contents does not work for you, you are either invoking the compiler incorrectly for C++11 or using a compiler version with insufficient support for C++11.
#include <string>
int main() {
int i;
auto s = std::to_string(i);
}
However there is a better way to print dates in C++11. Here's a program that prints the current date (in ISO 8601 format).
#include <ctime> // time, time_t, tm, localtime
#include <iomanip> // put_time
#include <iostream> // cout
#include <sstream> // stringstream
#include <stdexcept> // runtime_error
#include <string> // string
std::string date() {
static constexpr char const *date_format = "%Y-%m-%d"; // ISO 8601 format
auto t = std::time(nullptr);
if (static_cast<std::time_t>(-1) == t) {
throw std::runtime_error{"std::time failed"};
}
auto *cal = std::localtime(&t);
if (nullptr == cal) {
throw std::runtime_error{"std::localetime failed"};
}
std::stringstream ss;
ss << std::put_time(cal, date_format);
return ss.str();
}
int main() { std::cout << date() << '\n'; }
Unfortunately gcc 4.8 appears to lack put_time (and of course VC++ currently lacks constexpr and universal initializers, but that is easily worked around. VC++ has put_time).