Naming a log file using date and time in C++ - c++

Question

So, I want to create a log file for an app I am trying to create and I don't know how to name the log to something like "log/date&time"
Anyway, here is my code:
#include <iostream>
#include <fstream>
#include <time.h>
#include <stdio.h>
#include <sstream>
using namespace std;
int main (int argc, char *argv[])
{
time_t t = time(0);
struct tm * now = localtime( & t );
char buffer [80];
strftime (buffer,80,"%Y-%m-%d.",now); //i think i can't just put "log/%Y-%m-%d." there.
ofstream myfile;
myfile.open ("log/" + buffer); // this is my problem, i can't put the ' "log/" + ' part there
if(myfile.is_open())
{
cout<<"Success"<<std::endl;
}
return 0;
}

Answer 1

You should use std::string which supports concatenation via the overloaded operator+.
std::string buffer(80, '\0');
strftime( &buffer[0], buffer.size(), "some format string", now);
/* ... */
std::ofstream myfile( ("log/" + buffer).c_str() );
// Remove the (..).c_str() part when working with a C++11 conforming
// standard library implementation

Answer 2

you actual question is "why doesnt this work"
myfile.open ("log/" + buffer);
answer - because c++ doesnt support what you want - concatenate a string literal with a char * and return another char *.
do
std::string filetime(buffer);
std::string filename = "log/" + filetime;
open(filename.c_str());

Answer 3

Consider using std:: facilities instead (std::string and std::ostringstream come to mind):
std::ostream& time_digits(std::ostream& out, unsigned int digits)
{ // convenience function: apply width and fill for the next input
return out << std::setw(digits) << std::setfill('0');
}
std::string unique_log_name()
{ // generate unique log name, depending on local time
// example output: "log/2014-04-19.log"
auto now = time(0);
tm *ltm = localtime(&now);
std::ostringstream buffer;
buffer
<< "log/" << time_digits(4) << ltm.tm_year
<< "-" << time_digits(2) << ltm.tm_mon
<< "-" << time_digits(2) << ltm.tm_day;
// could also add these to the name format:
// buffer
// << "-" << time_digits(2) << ltm.dm_hour
// << "-" << time_digits(2) << ltm.tm_min
// << "-" << time_digits(2) << ltm.tm_sec;
buffer << ".log"; // add extension
return buffer.str();
}
void client_code()
{ // construct log stream on unique file name
ofstream myfile{ unique_log_name() };
if(myfile)
{
cout << "Success" << std::endl;
}
}