Question

I’m looking at making a logging class which has members like Info, Error etc that can configurably output to console, file, or to nowhere.

For efficiency, I would like to avoid the overhead of formatting messages that are going to be thrown away (ie info messages when not running in a verbose mode). If I implement a custom std::streambuf that outputs to nowhere, I imagine that the std::ostream layer will still do all the formatting. Can anyone suggest a way to have a truly ‘null’ std::ostream that avoids doing any work at all on the parameters passed to it with <<?

Answer 1

To prevent the operator<<() invocations from doing formatting, you should know the streamtype at compile-time. This can be done either with macros or with templates.

My template solution follows.

class NullStream { public:     void setFile() { /* no-op */ }     template<typename TPrintable>     NullStream& operator<<(TPrintable const&)     { return *this; } /* no-op */ }  template<class TErrorStream> // add TInfoStream etc class Logger { public:     TErrorStream& errorStream() {         return m_errorStream;     }  private:     TErrorStream m_errorStream; };  //usage int main() {     Logger<std::ofstream> normal_logger; // does real output     normal_logger.errorStream().open('out.txt');     normal_logger.errorStream() << 'My age is ' << 19;      Logger<NullStream> null_logger; // does zero output with zero overhead     null_logger.errorStream().open('out.txt'); // no-op     null_logger.errorStream() << 'My age is ' << 19; // no-op } 

Since you have to do this at compile-time, it is of course quite inflexible.

For example, you cannot decide the logging level at runtime from a configuration file.