overload << operator and pass parameters to std::cout - c++

Question

I would like to create a class for logging purposes which will behave like std::cout, but will automatically insert additional information to the stream.
a sample usage that I want would be something like (lets not care about object and context type for now, just assume they are std::string) :
Logger l;
l << "Event with object : " << obj << " while in context : " << context;
Then the output would be :
[timestamp] Event with object : [obj_desc] while in context : [this context][eol][flush]
I've been trying with :
template<typename T>
Logger& operator << (const T& msg){
std::cout << timestamp() << msg << std::endl << std::flush;
return *this;
}
but it seems that std::cout cannot resolve the typename T and fails to output a std::string for example while segfaulting.
A possible solution would be to overload this for all types, but this is rather annoying and time consuming.
Is there a better option to decorate std::cout output with more information?
Edit :
I do realize now that endl and flush will be appended to every message which kind of defeat the purpose, but I'm still interested in the general idea. I care more about the monadic syntax to append an arbitrary number of messages than the <<overload

Answer 1

The reason your code does not work is because you have not implemented operator<< for everything you want to pass to it.
This statement:
Logger l;
l << "Event with object : " << obj << " while in context : " << context;
Is basically doing this (assuming operator<< is a member of Logger, which your implementation implies it is):
Logger l;
l.operator<<("Event with object : ").operator<<(obj).operaator<<(" while in context : ").operator<<(context);
So, you need separate overloads of operator<< for string, obj, context, etc. And you need a way to indicate when to flush the complete log message to std::cout.
I would suggest something more like this:
struct LoggerStream
{
std::ostringstream strm;
struct Timestamp
{
};
~LoggerStream()
{
std::string s = strm.str();
if (!s.empty())
std::cout << s << std::flush;
}
LoggerStream& operator<< (const Timestamp &t)
{
strm << "[timestamp] "; // format this however you need
return *this;
}
LoggerStream& operator<< (const object &obj)
{
strm << "[obj_desc]"; // format this however you need
return *this;
}
LoggerStream& operator<< (const context &ctx)
{
strm << "[this context]"; // format this however you need
return *this;
}
LoggerStream& operator<< (std::ostream&(*f)(std::ostream&))
{
if (f == (std::basic_ostream<char>& (*)(std::basic_ostream<char>&)) &std::flush)
{
std::string s = strm.str();
if (!s.empty())
std::cout << s << std::flush;
strm.str("");
strm.clear();
}
else
strm << f;
return *this;
}
template<typename T>
LoggerStream& operator<< (const T& value)
{
strm << value;
return *this;
}
};
class Logger
{
LoggerStream getStream()
{
LoggerStream strm;
strm << Timestamp;
return strm;
}
};
Then you can do things like this:
Logger l;
l.getStream() << "Event with object : " << obj << " while in context : " << context;
...
l.getStream() << "Event with object : " << obj << " while in context : " << context;
...
Logger l;
LoggerStream strm = l.getStream();
strm << "Event with object : " << obj << " while in context : " << context << std::flush;
...
strm << Logger::Timestamp << "Event with object : " << obj << " while in context : " << context << std::flush;
...
Alternatively:
struct Logger
{
std::ostringstream strm;
~Logger()
{
std::string s = strm.str();
if (!s.empty())
std::cout << "[timestamp] " << s << std::flush;
}
Logger& operator<< (const object &obj)
{
strm << "[obj_desc]"; // format this however you need
return *this;
}
Logger& operator<< (const context &ctx)
{
strm << "[this context]"; // format this however you need
return *this;
}
Logger& operator<< (std::ostream&(*f)(std::ostream&))
{
if (f == (std::basic_ostream<char>& (*)(std::basic_ostream<char>&)) &std::flush)
{
std::string s = strm.str();
if (!s.empty())
std::cout << "[timestamp] " << s << std::flush;
strm.str("");
strm.clear();
}
else
strm << f;
return *this;
}
template<typename T>
Logger& operator<< (const T& value)
{
strm << value;
return *this;
}
};
Logger() << "Event with object : " << obj << " while in context : " << context;
...
Logger() << "Event with object : " << obj << " while in context : " << context;
...
Logger l;
l << "Event with object : " << obj << " while in context : " << context << std::flush;
...
l << "Event with object : " << obj << " while in context : " << context << std::flush;
...

Answer 2

You can certainly overload the stream classes if you want, providing operator<< for all the data types you want to support (and that's probably the "correct" way to go) but, if all you're after is a quick way to add logging to a regular stream, there a simpler way:
#include <iostream>
#include <iomanip>
#include <sstream>
#include <ctime>
#include <unistd.h>
#define logcout std::cout << timestamp()
std::string timestamp(void) {
time_t now = time(0);
struct tm *tmx = localtime(&now);
std::ostringstream oss;
oss << '['
<< (tmx->tm_year+1900)
<< '-'
<< std::setfill('0') << std::setw(2) << (tmx->tm_mon+1)
<< '-'
<< std::setfill('0') << std::setw(2) << (tmx->tm_mday)
<< ' '
<< std::setfill('0') << std::setw(2) << (tmx->tm_hour)
<< ':'
<< std::setfill('0') << std::setw(2) << (tmx->tm_min)
<< ':'
<< std::setfill('0') << std::setw(2) << (tmx->tm_sec)
<< "] ";
return oss.str();
}
int main (int argc, char *argv[]) {
logcout << "A slightly\n";
sleep (5);
logcout << "sneaky" << " solution\n";
return 0;
}
which outputs:
[2015-05-26 13:37:04] A slightly
[2015-05-26 13:37:09] sneaky solution
Don't be fooled by the size of the code, I just provided a complete compilable sample for testing. The crux of the matter is the single line:
#define logcout std::cout << timestamp()
where you can then use logcout instead of std::cout, and every occurrence prefixes the stream contents with an arbitrary string (the timestamp in this case, which accounts for the bulk of the code).
It's not what I would call the most pure C++ code but, if your needs are basically what you stated, it'll certainly do the trick.