How to store template variadic arguments into std::ofstream? - c++

Question

I'm implementing a small Log class in c++
class Log
{
public:
enum LogLevel
{
TRACE,
DEBUG,
INFO,
WARNING,
ERROR,
FATAL
};
Log(): m_log(DEBUG) {} ;
~Log() {};
int SetLogFilePath(const std::string& p_directory, const std::string& p_prefix);
void SetLogLevel(const LogLevel p_logLvl);
LogLevel& GetLogLevel();
template<typename... Args>
int Logging(const std::string& p_logLevel, const std::string& p_srcFile, const std::string& p_function, const int& p_line,
const Args&& ... p_args);
private:
LogLevel m_log;
std::string m_logFilePath;
};
This is my Log class, I store directory and prefix into std::string m_logFilePath
template <typename ... Args>
int Log::Logging(const std::string& p_logLevel, const std::string& p_srcFile, const std::string& p_function, const int& p_line,
const Args&&... p_args)
{
std::ofstream log;
log.open(m_logFilePath.c_str(), std::ios::out);
if(!log.good())
{
std::cout << "Couldn't create file : " << m_logFilePath.c_str() << std::endl;
return 0;
}
switch(p_logLevel)
{
case "TRACE":
log << "(TRACE) ";
break;
case "DEBUG":
log << "(DEBUG) ";
break;
case "INFO":
log << "(INFO) ";
break;
case "WARNING":
log << "(WARNING) ";
break;
case "ERROR":
log << "(ERROR) ";
break;
case "FATAL":
log << "(FATAL) ";
break;
default: break;
}
log << "Log Created at (Local Time): " << std::put_time(std::localtime(std::chrono::system_clock::to_time_t(std::chrono::system_clock::now())), "%c") << " ";
log << "File Name: " << p_srcFile.c_str() << " " << "Function Name: " << p_function.c_str() << " " << "Line: " << p_line << " " << std::forward<Args>(p_args) << std::endl;
log.close();
return 1;
}
Above is my implementation of int Logging(...) function.
Basically, I store the logging level, current time, source file path, function name , line from code, and some variadic arguments.
I've been searching for a way to store multiple variadic arguments into my std::ostream log but couldn't find any answer.
Please Help!

Answer 1

If I understand correctly, your problem is that you don't know how to manage (in this case: send to an output stream) a variadic list (variadic template based) of arguments.
I propose an example of use that use the power of comma operator in the context of an (unused) array initialization
using unused = int[];
int cnt { -1 };
(void)unused { 0, (oss << ", extra val "
<< ++cnt << " [" << args << ']', 0)... };
The following is a simplified, but full working, example
#include <sstream>
#include <iostream>
template <typename ... Args>
std::string logging (std::string const & valA, std::string const & valB,
std::string const & valC, Args const & ... args)
{
std::ostringstream oss;
using unused = int[];
oss << "valA [" << valA << "], "
<< "valB [" << valB << "], "
<< "valC [" << valC << ']';
int cnt { -1 };
(void)unused { 0, (oss << ", extra val "
<< ++cnt << " [" << args << ']', 0)... };
return oss.str();
}
int main()
{
auto l = logging("val1", "val2", "val3", 0, 1L, 2LL, 3U, 4UL, 5ULL,
6.0, 7.0f, "eight");
std::cout << l << std::endl;
}