How to show time from a time stamp and show current time when I have no argument?

I need to make a function which takes a timestamp (the time in milliseconds, type long) and transforms it into readable time (Y-M-D H:M:S); however, after that, I have to overload the function such that if the function doesn't get a parameter it will return the current date.

I know how to make the function to transform from the given long parameter to readable time, but I don't know how to overload the function.

#include <iostream>
#include <cstdio>
#include <ctime>
#include <string>

using namespace std;

string timeToString(long  timestamp)
{
    const time_t rawtime = (const time_t)timestamp; 

    struct tm * dt;
    char timestr[30];
    char buffer [30];

    dt = localtime(&rawtime);
    strftime(timestr, sizeof(timestr), "%Y-%m-%d %X", dt);
    sprintf(buffer,"%s", timestr);
    string stdBuffer(buffer); 
    return stdBuffer;
}

int main()
{
    cout << timeToString(1538123990) << "\n";
}

Solution

First of all (as mentioned in the comments) you should really be using the Modern C++ library's std::chrono functions for your time operations. However, sticking with the basic code that you have, you can just provide a default value for the parameter of your timeToString function; this should be a value that is meaningless in a real sense, and one that you would never actually pass. I have chosen -1 in the example below, as it is unlikely that you would be using a negative time stamp.

If the function is called with a parameter, then that value is used; otherwise, the function gets called with the given default value. We can then adjust the code to check for that value, as shown below:

#include <iostream>
#include <ctime>
#include <string>

std::string timeToString(long timestamp = -1)
{
    time_t rawtime;                                // We cannot (now) have this as "const"
    if (timestamp < 0) rawtime = time(nullptr);    // Default parameter:- get current time
    else rawtime = static_cast<time_t>(timestamp); // Otherwise convert the given argument

    struct tm* dt = localtime(&rawtime);
    char timestr[30];
//  char buffer[30]; // Redundant (see below)
    strftime(timestr, sizeof(timestr), "%Y-%m-%d %X", dt);
//  sprintf(buffer, "%s", timestr); // This just makes a copy of the same string!
    return std::string(timestr);
}

int main()
{
    std::cout << timeToString(1538123990) << "\n";
    std::cout << timeToString() << "\n"; // No parameter - function will get "-1" instead!
    return 0;
}