<< operator overloading in C++ for logging purposes - c++

I have a C++ class where I place many std::cout statements to print informative text messages about a mass of signals that this class is handling. My intentition is to redirect these text messages to a function named log. In this function, I have flag named mVerbose which defines if the log text should be printed. The content of this function is as follows:
void XXXProxy::log(std::stringstream& ss)
{
if(mVerbose)
{
std::cout << ss;
ss << "";
}
}
Then, the caller code snippet to this function is as follows:
std::stringstream logStr;
logStr << "SE"
<< getAddr().toString()
<< ": WAITING on epoll..."
<< std::endl;
log(logStr);
I would like to overload the << operator in my XXXProxy in a way that I can get rid of creating a std::stringstream object and calling the log function. I want to be able to log the text messages as below and let the << operator aggregate everything into:
<< "SE"
<< getAddr().toString()
<< ": WAITING on epoll..."
<< std::endl;
So I wouldlike to have an member << function that looks like:
void XXXProxy::operator << (std::stringstream& ss)
{
if(mVerbose)
{
std::cout << ss;
ss << "";
}
}
QUESTION
I am relatively a novice C++ developer and get lots of compilation errors when attemting to write the above stated like << operator. Could you please make some suggestions or direct me to some links for me to correctly implement this << operator. Thanks.

If you don't want to use std::cout directly and you want to have your own Log class, you could implement a simple wrapper providing the same interface of std::ostream: operator<<:
class Log {
private:
std::ostream& _out_stream;
//Constructor: User provides custom output stream, or uses default (std::cout).
public: Log(std::ostream& stream = std::cout): _out_stream(stream) {}
//Implicit conversion to std::ostream
operator std::ostream() {
return _out_stream;
}
//Templated operator>> that uses the std::ostream: Everything that has defined
//an operator<< for the std::ostream (Everithing "printable" with std::cout
//and its colleages) can use this function.
template<typename T>
Log& operator<< (const T& data)
{
_out_stream << data;
}
}
So if you implement std::ostream& operator>>(std::ostream& os , const YourClass& object) for your classes, you can use this Log class.
The advantage of this approach is that you use the same mechanism to make std::cout << your_class_object work, and to make the class work with the Log.
Example:
struct Foo
{
int x = 0; //You marked your question as C++11, so in class initializers
//are allowed.
//std::ostream::operator<< overload for Foo:
friend std::ostream& operator<<(std::ostream& os , const Foo& foo)
{
os << foo.x;
}
};
int main()
{
Log my_log;
Foo my_foo;
my_foo.x = 31415;
my_log << my_foo << std::endl; //This prints "31415" using std::cout.
}
Possible improvements:
You could write a extern const of class Log, and make the class implement a singleton. This allows you to access the Log everywhere in your program.
It's common in log outputs to have a header, like Log output (17:57): log message. To do that, you could use std::endl as a sentinel and store a flag that says when the next output is the beginning of a line (the beginning of a log message). Checkout the next answer for a complete and working implementation.
References:
std::ostream
operator<< for std::ostream
std::enable_if
std::is_same
decltype specifier

The timestamp of the example was only that, an example :).
But if you like that, we could try to implement it. Thankfully to C++11 and its STL's big improvements, we have an excellent time/date API: std::chrono
std::chronois based in three aspects:
Clocks
Durations
Time points
Also, chrono provides three types of clocks, std::system_clock, std::steady_clock , and std::high_resolution_clock. In our case, we use std::system_clock (We want access to the date-time, not meassuring precise time intervals).
For more info about std::chrono, checkout this awsome Bo Qian's youtube tutorial.
So if we have to implement a time stamp for our log header, we could do this:
EDIT: Like other good things, C++ templates are good tools until you overuse it.
Our problem was that std::endl is a templated function, so we cannot pass it directly to
annother templated function as parammeter (operator<< in our case), because the compiler cannot deduce std::endl template argumments directly. Thats the recurrent error "unresolved overloaded function type".
But there is a much simpler way to do this: Using an explicit overload of operator<< for std::endl only, and other templated for everything else:
class Log
{
private:
std::ostream& _out_stream;
bool _next_is_begin;
const std::string _log_header;
using endl_type = decltype( std::endl ); //This is the key: std::endl is a template function, and this is the signature of that function (For std::ostream).
public:
static const std::string default_log_header;
//Constructor: User passes a custom log header and output stream, or uses defaults.
Log(const std::string& log_header = default_log_header , std::ostream& out_stream = std::cout) : _log_header( log_header ) , _out_stream( out_stream ) , _next_is_begin( true ) {}
//Overload for std::endl only:
Log& operator<<(endl_type endl)
{
_next_is_begin = true;
_out_stream << endl;
return *this;
}
//Overload for anything else:
template<typename T>
Log& operator<< (const T& data)
{
auto now = std::chrono::system_clock::now();
auto now_time_t = std::chrono::system_clock::to_time_t( now ); //Uhhg, C APIs...
auto now_tm = std::localtime( &now_time_t ); //More uhhg, C style...
if( _next_is_begin )
_out_stream << _log_header << "(" << now_tm->tm_hour << ":" << now_tm->tm_min << ":" << now_tm->tm_sec << "): " << data;
else
_out_stream << data;
_next_is_begin = false;
return *this;
}
};
const std::string Log::default_log_header = "Log entry";
This code snippet works perfectly. I have pushed the complete implementation to my github account.
Reference:
std::chrono
std::chrono::system_clock
std::chrono::system_clock::now()
std::time_t
std::chrono::system_clock::to_time_t()
std::tm
std::localtime()

Related

Is it possible to create a C++ class that mimics the std::cout syntax by chaining overloaded insertion operators the same way that easylogging++ does?

The easylogging++ code defines a macro that makes it very simple to use:
LOG(logLevel) << "This mimics std::cout syntax. " << 1 << " + " << 1 << " = " << 2;
I want to make a wrapper class for easylogging++. I can easily create a function with two parameters to wrap the above line. However, is it possible to mimic this syntax in a wrapper class? For example:
Logger logger;
logger(logLevel) << "Line " << 1 << " of log text.";
I know I can easily overload the insertion operator, but that still leaves me with having to write another function to set the log level each time.
UPDATE:
Thanks to Starl1ght's answer I was able to get this working. I figured I would share in case anyone else has a similar need.
I created two overloads. One was for () and the other for <<.
Logger &operator()(logLevelT logLevel) {
mLogLevel = logLevel;
return *this;
}
template <typename T>
Logger &operator<<(T const &value) {
LOG(mLogLevel) << value;
return *this;
}
UPDATE 2:
I wanted to update this post again to give my reasoning and show my final solution.
My reasoning is that my project is a demonstration of abstraction. I'm trying to demonstrate that logging libraries (and many other things) can be abstracted away from the core functionality of your software. This also makes the software components modular. This way I can swap out the easylogging++ library without loosing the syntax because it is implemented in the module interface.
My last update didn't mention how I overcame the obstacle of insertion chaining, so I wanted to post an example to show how I did it. The following code is a simplified example of how to achieve std::cout like syntax for a class.
#include <iostream> // For cout
#include <string> // For strings
#include <sstream> // For ostringstream
enum logLevelT {
INFO_LEVEL,
WARNING_LEVEL,
ERROR_LEVEL,
FATAL_LEVEL
};
class Logger {
private:
std::string logName;
public:
Logger(std::string nameOfLog, std::string pathToLogFile) {
logName = nameOfLog;
//TODO Configure your logging library and instantiate
// an instance if applicable.
}
~Logger(){}
// LogInputStream is instantiated as a temporary object. It is used
// to build the log entry stream. It writes the completed stream
// in the destructor as the object goes out of scope automatically.
struct LogInputStream {
LogInputStream(logLevelT logLevel, std::string nameOfLog) {
currentLogLevel = logLevel;
currentLogName = nameOfLog;
}
// Copy Constructor
LogInputStream(LogInputStream &lis) {
currentLogLevel = lis.currentLogLevel;
currentLogName = lis.currentLogName;
logEntryStream.str(lis.logEntryStream.str());
}
// Destructor that writes the log entry stream to the log as the
// LogInputStream object goes out of scope.
~LogInputStream() {
std::cout << "Logger: " << currentLogName
<< " Level: " << currentLogLevel
<< " logEntryStream = " << logEntryStream.str()
<< std::endl;
//TODO Make a log call to your logging library. You have your log level
// and a completed log entry stream.
}
// Overloaded insertion operator that adds the given parameter
// to the log entry stream.
template <typename T>
LogInputStream &operator<<(T const &value) {
logEntryStream << value;
return *this;
}
std::string currentLogName;
logLevelT currentLogLevel;
std::ostringstream logEntryStream;
};
// Overloaded function call operator for providing the log level
Logger::LogInputStream operator()(logLevelT logLevel) {
LogInputStream logInputStream(logLevel, logName);
return logInputStream;
}
// Overloaded insertion operator that is used if the overloaded
// function call operator is not used.
template <typename T>
Logger::LogInputStream operator<<(T const &value) {
LogInputStream logInputStream(INFO_LEVEL, logName);
logInputStream << value;
return logInputStream;
}
};
int main(int argc, char *argv[]) {
Logger logger1 = Logger("Logger1", "/path/to/log.log");
Logger logger2 = Logger("Logger2", "/path/to/log.log");
logger1(INFO_LEVEL) << "This is the " << 1 << "st test";
logger2(ERROR_LEVEL) << "This is the " << 2 << "nd test";
logger2 << "This is the " << 3 << "rd test";
return 0;
}
I feel like I could have done a better job with the naming and the comments but I am pressed for time. I'm definitely open to any comments or critiques.
You must overload operator() so, it will set internal log level and return *this as type Logger&, so, overloaded operator<< will work on returned reference with necessary log-level set.
Something like this:
Logger& Logger::operator()(LogLevel level) {
// set internal log level
return *this;
}

Operator << in a user defined method

I was trying to write a method that would take parameters using the >> operator as parameters similar to std::cin but I don't know how. Is it possible to create this kind of method that would take this stream like parameter, convert it properly (for example convert all ints to strings etc) and then save to an std::string variable?
Here is an example of how I would like to run the function:
int i = 0;
myMethod << "some text" << i << "moar text";
Inside that method I would like to take those parameters and store in a string.
Edit
I will try to explain exactly what this application is about: I Am trying to make a Clogger singleton class which will be used to save logs to a file. With this construction, I can call *CLogger::instance() << "log stuff"; from anywhere in the code and that's OK. Thanks to answers from this topic I have come to this. The problem is that each operator<< I use, then the object is going to be called. So if I do *CLogger::instance() << "log stuff " << " more stuff " << " even more";` this method(?) is going to be called 3 times:
template<typename T>
CLogger& operator<<(const T& t)
{
...
return *this;
}
That's not good for me as I intend to add some text before and after each log line. For example I would always like to add time before and std::endl after. Following the example I gave instead of getting:
[00:00] log stuff more stuff even more
I would get:
[00:00] log stuff
[00:00] more stuff
[00:00] even more
So I made an attempt to remove this behaviour by changing the method like this:
template<typename T>
CLogger& operator<<(const T& t)
{
ostringstream stream;
stream << t;
m_catString += stream.str();
if (stream.str() == "\n")
{
push_back(m_catString);
m_catString.clear();
}
return *this;
}
This way the program knows when to push new log line if I add "\n" at the end. Its nearly ok, as I bet I will forget to add this. Is there any more clever way?
You can't pass parameters to a method using <<, you need an object.
Something like this:
struct A
{
template<typename T>
A& operator<<(const T& t)
{
std::ostringstream stream;
stream << t;
data += stream.str();
return *this;
}
std::string data;
};
// ...
A a;
a << "Hello " << 34 << " World";
std::cout << a.data;
Regarding your update:
The most obvious thing is to implement the operator in CLogger instead and get rid of the Pusher class; that would let you write *CLogger::instance() << "sample text" << 10;
If you can't do that for some reason (you're giving out information piecemeal, so it's hard to tell), you can declare Pusher a friend and use the same method as everywhere else:
struct Pusher
{
template<typename T>
Pusher& operator<<(const T& t)
{
std::ostringstream stream;
stream << t;
CLogger::instance()->push_back(stream.str());
return *this;
}
};
The only way I know is creating a class class Method and then overload the operator<<, operators overloading
template<class T>
Method &operator<<(const T &x)
{
// Do whatever you like
return *this;
}
Then you can use it like :
Method myMethod;
myMethod << ... ;
you can look at this question about creating a cout-like class
std::cin and std::cout are not functions by the way
EDIT
class CLogger
{
...
template<typename T>
CLogger& operator<<(const T& t)
{
push_back(std::to_string(t));
return *this;
}
};
You don't have to create a class Pusher, just overload the operator in your first class, now you can use it with your object :
myCLogger << t; // this would call the function push back

C++ Using stringstream after << as parameter

Is it possible to write a method that takes a stringstream and have it look something like this,
void method(string str)
void printStringStream( StringStream& ss)
{
method(ss.str());
}
And can be called like this
stringstream var;
printStringStream( var << "Text" << intVar << "More text"<<floatvar);
I looked up the << operator and it looks like it returns a ostream& object but I'm probably reading this wrong or just not implementing it right.
Really all I want is a clean way to concatenate stuff together as a string and pass it to a function. The cleanest thing I could find was a stringstream object but that still leaves much to be desired.
Notes:
I can't use much of c++11 answers because I'm running on Visual Studio 2010 (against my will, but still)
I have access to Boost so go nuts with that.
I wouldn't be against a custom method as long as it cleans up this mess.
Edit:
With #Mooing Duck's answer mixed with #PiotrNycz syntax I achieved my goal of written code like this,
try{
//code
}catch(exception e)
{
printStringStream( stringstream() << "An exception has occurred.\n"
<<" Error: " << e.message
<<"\n If this persists please contact "<< contactInfo
<<"\n Sorry for the inconvenience");
}
This is as clean and readable as I could have hoped for.
Hopefully this helps others clean up writing messages.
Ah, took me a minute. Since operator<< is a free function overloaded for all ostream types, it doesn't return a std::stringstream, it returns a std::ostream like you say.
void printStringStream(std::ostream& ss)
Now clearly, general ostreams don't have a .str() member, but they do have a magic way to copy one entire stream to another:
std::cout << ss.rdbuf();
Here's a link to the full code showing that it compiles and runs fine http://ideone.com/DgL5V
EDIT
If you really need a string in the function, I can think of a few solutions:
First, do the streaming seperately:
stringstream var;
var << "Text" << intVar << "More text"<<floatvar;
printStringStream(var);
Second: copy the stream to a string (possible performance issue)
void printStringStream( ostream& t)
{
std::stringstream ss;
ss << t.rdbuf();
method(ss.str());
}
Third: make the other function take a stream too
Make your wrapper over std::stringstream. In this new class you can define whatever operator << you need:
class SSB {
public:
operator std::stringstream& () { return ss; }
template <class T>
SSB& operator << (const T& v) { ss << v; return *this; }
template <class T>
SSB& operator << (const T* v) { ss << v; return *this; }
SSB& operator << (std::ostream& (*v)(std::ostream&)) { ss << v; return *this; }
// Be aware - I am not sure I cover all <<'s
private:
std::stringstream ss;
};
void print(std::stringstream& ss)
{
std::cout << ss.str() << std::endl;
}
int main() {
SSB ssb;
print (ssb << "Hello" << " world in " << 2012 << std::endl);
print (SSB() << "Hello" << " world in " << 2012 << std::endl);
}
For ease of writing objects that can be inserted into a stream, all these classes overload operator<< on ostream&. (Operator overloading can be used by subclasses, if no closer match exists.) These operator<< overloads all return ostream&.
What you can do is make the function take an ostream& and dynamic_cast<> it to stringstream&. If the wrong type is passed in, bad_cast is thrown.
void printStringStream(ostream& os) {
stringstream &ss = dynamic_cast<stringstream&>(os);
cout << ss.str();
}
Note: static_cast<> can be used, it will be faster, but not so bug proof in the case you passed something that is not a stringstream.
Since you know you've got a stringstream, just cast the return value:
stringstream var;
printStringStream(static_cast<stringstream&>(var << whatever));
Just to add to the mix: Personally, I would create a stream which calls whatever function I need to call upon destruction:
#include <sstream>
#include <iostream>
void someFunction(std::string const& value)
{
std::cout << "someFunction(" << value << ")\n";
}
void method(std::string const& value)
{
std::cout << "method(" << value << ")\n";
}
class FunctionStream
: private virtual std::stringbuf
, public std::ostream
{
public:
FunctionStream()
: std::ostream(this)
, d_function(&method)
{
}
FunctionStream(void (*function)(std::string const&))
: std::ostream(this)
, d_function(function)
{
}
~FunctionStream()
{
this->d_function(this->str());
}
private:
void (*d_function)(std::string const&);
};
int main(int ac, char* av[])
{
FunctionStream() << "Hello, world: " << ac;
FunctionStream(&someFunction) << "Goodbye, world: " << ac;
}
It is worth noting that the first object sent to the temporary has to be of a specific set of types, namely one of those, the class std::ostream knows about: Normally, the shift operator takes an std::ostream& as first argument but a temporary cannot be bound to this type. However, there are a number of member operators which, being a member, don't need to bind to a reference! If you want to use a user defined type first, you need to extract a reference temporary which can be done by using one of the member input operators.

How to create a type-tag for template specialization

I have a custom logging class that supports iostream-syntax via a templated operator <<:
template< class T >
MyLoggingClass & operator <<(MyLoggingClass &, const T &) {
// do stuff
}
I also have a specialized version of this operator that is supposed to be called when a log-message is complete:
template< >
MyLoggingClass & operator <<(MyLoggingClass &, consts EndOfMessageType &){
// build the message and process it
}
EndOfMessageType is defined like this:
class EndOfMessageType {};
const EndOfMessageType eom = EndOfMessageType( );
The global constant eom is defined so that users can use it just like std::endl at the end of their log-messages. My question is, are there any pitfalls to this solution, or is there some established pattern to do this?
Thanks in advance!
std::endl is a function, not an object, and operator<< is overloaded for accepting a pointer to a function taking and returning a reference to ostream. This overload just calls the function and passes *this.
#include <iostream>
int main()
{
std::cout << "Let's end this line now";
std::endl(std::cout); //this is the result of cout << endl, or cout << &endl ;)
}
Just an alternative to consider.
By the way, I don't think there is any need to specialize the operator: a normal overload does just as well, if not better.
I think your solution is acceptable. If you wanted to do it differently, you could create a class Message, that would be used instead of the your MyLoggingClass and provided automatic termination.
{
Message m;
m << "Line: " << l; // or m << line(l)
m << "Message: foo"; // or m << message("foo");
log << m; // this would automatically format the message
}
I have done it this way, like some other people did. Have a function Error / Log / Warning / etc that could look like this
DiagnosticBuilder Error( ErrType type, string msg, int line );
This will return a temporary builder object, whose class is basically defined like
struct DiagnosticBuilder {
DiagnosticBuilder(std::string const& format)
:m_emit(true), m_format(format)
{ }
DiagnosticBuilder(DiagnosticBuilder const& other)
:m_emit(other.m_emit), m_format(other.m_format), m_args(other.m_args) {
other.m_emit = false;
}
~DiagnosticBuilder() {
if(m_emit) {
/* iterate over m_format, and print the next arg
everytime you hit '%' */
}
}
DiagnosticBuilder &operator<<(string const& s) {
m_args.push_back(s);
return *this;
}
DiagnosticBuilder &operator<<(int n) {
std::ostringstream oss; oss << n;
m_args.push_back(oss.str());
return *this;
}
// ...
private:
mutable bool m_emit;
std::string m_format;
std::vector<std::string> m_args;
};
So if you are building a log message in a loop, be it so
DiagnosticBuilder b(Error("The data is: %"));
/* do some loop */
b << result;
As soon as the builder's destructor is called automatically, the message is emitted. Mostly you would use it anonymously
Error("Hello %, my name is %") << "dear" << "litb";

custom stream manipulator for class

I am trying to write a simple audit class that takes input via operator << and writes the audit after receiving a custom manipulator like this:
class CAudit
{
public:
//needs to be templated
CAudit& operator << ( LPCSTR data ) {
audittext << data;
return *this;
}
//attempted manipulator
static CAudit& write(CAudit& audit) {
//write contents of audittext to audit and clear it
return audit;
}
private:
std::stringstream audittext;
};
//to be used like
CAudit audit;
audit << "Data " << data << " received at " << time << CAudit::write;
I recognise that the overloaded operator in my code does not return a stream object but was wondering if it was still possible to use a manipulator like syntax. Currently the compiler is seeing the '<<' as the binary right shift operator.
Thanks for any input,
Patrick
To make it work you have to add overload of operator << for functions,
than call the function from it:
class CAudit
{
//...other details here as in original question
CAudit& operator << (CAudit & (*func)(CAudit &))
{
return func(*this);
}
};
CAudit audit;
audit << "some text" << CAudit::write;
Binary shift operator and stream operator is the same operator. It is completely legal to overload operator+ for your class to write "Hello world" on std::cout (although it is a very bad idea). The same way C++ standard authors decided to overload operator<< for streams as writing to the stream.
You didn't write clearly what is your problem. My guess is compilation error. The best thing in this case is to quote the error message. If I am right, the problem is, that you defined only operator<< for LPCSTR, and then you want it to work function object on the right side.
You use word "manipulator", but you misunderstand something. Manipulator for a stream (stream from STL) is a function that performs some actions on the stream it is written to. And it works only because of this overload:
ostream& operator<< (ostream& ( *pf )(ostream&));
which takes a function and applies it to a stream.
Similarly you need:
CAudit& operator<< (CAudit& ( *pf )(CAudit& audit))
{
return (*pf)(audit);
}
Wouldn't this
class CAudit
{
public:
template< typename T >
CAudit& operator<<( const T& data )
{
audittext << data;
return *this;
}
class write {};
void operator<<( const write& data )
{
/* whatever */
}
private:
std::stringstream audittext;
};
do what you want?
I do something very similar for tracing, but use a stringstream. This ensures that all 3rd party operator << () and manipulators work. I also use the desctructor instead of the customer write manipulator.
class DebugStream
{
public:
DebugStream(short level, const char * file, int line) {
sstream << "L" << level << "\t" << file << "\t" << line << "\t";
}
~DebugStream() { write(sstream.str()); }
std::ostream & stream() { return sstream; }
private:
std::stringstream sstream;
DebugStream(const DebugStream &);
DebugStream & operator=(const DebugStream &);
};
This is then made available with some macros:
#define DBG_ERROR if (1<=dbg_level()) DebugStream(1, __FILE__, __LINE__).stream()
#define DBG_INFO if (2<=dbg_level()) DebugStream(2, __FILE__, __LINE__).stream()
And the code just uses the macros
DBG_INFO << "print some debug information";
You don't need a specific write manipulator to flush the data to the log file. When the anonymous DebugStream object goes out of scope (once control leaves the line) the the contents are automatically written.
Although I usually avoid macros in this case the use of the if statement means you don't have the overhead of building the trace line unless you actually require it.
Returning the ostream via the stream() method enables this to work for global member functions, as anonymous objects cannot be passed as non-const reference parameters.