Error reporting only once - c++

Question

I'm writing some code which handles a lot of data. When an error occures, it usually happens that the error will occur many times, so I want to report it only once. The problem I have is, that I want to have individual error messages. In C I would use a method with variadic arguments, bit of course this is not really typesafe, so I wonder how I can achieve the same in C++ with typesafe output. I know that I can stringstream and create the indvidual string, but that would mean that I have to create the full error message, even if it is discarded, because it was already printed, and stringstream is not exactly fast either.
So currently I use code like this:
std::string key = "BT:EMPTY";
if(mErrorReport.find(key) == mErrorReport.end())
{
std::cerr << "ERROR [" << Name<< "] Type is empty! " << std::endl;
mErrorReport.insert(key);
}
std::string key = "UI:"+Unitcode;
if(mErrorReport.find(key) == mErrorReport.end())
{
std::cerr << "ERROR [" << Name<< "] Room with the id " << Unitcode << " doesn't exist! " << std::endl;
mErrorReport.insert(key);
}
...
In C I would have written a variadic function like this:
void ErrorLog(const char *key, int nLogLevel, const char fmt, ...)
{
// Check if this error was already reported before.
if(mErrorLog.find(key) == mErrorLog.end())
{
fprintf(stderr, fmt, ...);
mErrorLog.insert(key);
}
}
So I wonder if there is some best practice for something like that.

Answer 1

Why don't you just use
void ErrorLog(const std::string& key, const std::string& name, const std::string& what)
{
if (mErrorLog.find(key) == mErrorLog.end())
{
std::cerr << "ERROR[" << name << "]" << what << std::endl;
mErrorLog.insert(key);
}
}
and call it like
ErrorLog("BT:EMPTY", Name, "Type is empty!");
ErrorLog("UI:" + Unitcode, Name, std::string("Room with the id ") + Unitcode + " doesn't exist!");
If Name doesn't change you could remove the parameter and just add it to the std::err call.
Update: Alternative solution
class ErrorLogWriter
{
public:
ErrorLogWriter(const std::string& name, const std::string& key, std::set<std::string>& log)
:m_name(name)
, m_key(key)
, m_log(log)
{}
ErrorLogWriter& operator<<(const std::string& msg)
{
if (m_log.find(m_key) == m_log.end())
{
std::cerr << "ERROR[" << m_name << "]" << msg << std::endl;
m_log.insert(m_key);
}
return *this;
}
private:
std::string m_name;
std::string m_key;
std::set<std::string>& m_log;
};
class ErrorLog
{
public:
ErrorLog(const std::string& name, std::set<std::string>& log)
:m_name(name)
,m_log(log)
{}
ErrorLogWriter operator()(const std::string& key)
{
return ErrorLogWriter(m_name, key, m_log);
}
private:
std::string m_name;
std::set<std::string>& m_log;
};
int main()
{
std::string Name = "NAME";
std::string Unitcode = "UNITCODE";
std::set<std::string> mErrorLog;
ErrorLog log(Name, mErrorLog);
log("BT:EMPTY") << "Type is empty!";
log("UI:" + Unitcode) << "Room with the id " << Unitcode << " doesn't exist!";
}