Easiest way to print timestamp to ostream - c++

Question

I'd like to add a timestamp to certain outputs to the std::cout / std::cerr ostreams, without using modified standard streams, like so:
std::cerr << timestamp << "Warning!\n";
or so:
std::cerr << timestamp() << "Warning!\n";
The output should look like this:
[2020-01-23 17:40:15 CET] Warning!
But I'm really not happy with what I've come up with:
class TimeStamp {};
std::ostream &operator<<(std::ostream &stream, const TimeStamp &ts)
{
std::time_t t = std::time(nullptr);
stream << "[" << std::put_time(std::localtime(&t), "%F %T %Z") << "] ";
return stream;
}
TimeStamp ts;
int main()
{
std::cerr << ts << "Warning!\n";
std::cerr << ts << "Another warning!\n";
}
So I'm basically defining an empty class, using a global declaration and overloading the '<<' operator. This feels wrong. A static function like timestamp() is probably better suited, but I'm not quite sure how to go on about this. All the examples I've found online used the overloaded '<<' operator, but it usually made more sense to do so, because some class state was output. Can I locally create an ostream and return that in the function?

Answer 1

There's nothing wrong with the way you've done it. But if you're looking for alternatives, you could create an ostream wrapper:
class Logger {
private:
std::ostream &stream;
void print_time() {
std::time_t t = std::time(nullptr);
stream << "[" << std::put_time(std::localtime(&t), "%F %T %Z") << "] ";
}
public:
//Maybe also take options for how to log?
Logger(std::ostream &stream) : stream(stream) { }
template <typename T>
std::ostream &operator<<(const T &thing) {
print_time();
return stream << thing;
}
};
int main()
{
Logger log(std::cerr);
log << "Warning!" << std::endl;
log << "Another warning!" << std::endl;
}
See it run here: https://ideone.com/YRawuQ

Answer 2

If you're just looking for a standalone function which is what I understood from a "static function like timestamp()" you can just return the date as a string:
std::string timeStamp(){
std::ostringstream strStream;
std::time_t t = std::time(nullptr);
strStream<< "[" << std::put_time(std::localtime(&t), "%F %T %Z") << "] ";
return strStream.str();
}
int main(){
std::cout<<timeStamp()<<" Testing!";
return 0;
}
Remember to include sstream

Answer 3

You can use standard std::chrono::time_point class from the date and time library to represent the timestamp. Then, you need to convert that timestamp to std::time_t type and, eventually, convert the date and time information from a given calendar time to a character string according to the format string.
auto const now = std::chrono::system_clock::now();
auto now_time = std::chrono::system_clock::to_time_t(now);
std::cout << std::put_time(std::localtime(&now_time), "%F %T") << std::endl;
For those who want to know more...
You can use the source_location class that represents certain information about the source code, such as file names, line numbers, and function names. It is being merged into ISO C++ and is available for use.
Full code
#include <ctime>
#include <chrono>
#include <iomanip>
#include <iostream>
#include <string_view>
#include <experimental/source_location>
void error(std::string_view const& message,
std::ostream& os = std::cout,
std::experimental::source_location const& location = std::experimental::source_location::current()) {
auto const now = std::chrono::system_clock::now();
auto now_time = std::chrono::system_clock::to_time_t(now);
os << "[" << std::put_time(std::localtime(&now_time), "%F %T") << "] "
<< "[INFO] "
<< location.file_name() << ":"
<< location.line() << " "
<< message << '\n';
}
void info(std::string_view const& message,
std::ostream& os = std::cout,
std::experimental::source_location const& location = std::experimental::source_location::current()) {
auto const now = std::chrono::system_clock::now();
auto now_time = std::chrono::system_clock::to_time_t(now);
os << "[" << std::put_time(std::localtime(&now_time), "%F %T") << "] "
<< "[INFO] "
<< location.file_name() << ":"
<< location.line() << " "
<< message << '\n';
}
int main() {
error("Some error");
info("Some info");
// or
error("Some error 2", std::cerr);
info("Some info 2", std::cerr);
return 0;
}
Check it out live