Question

I’m working on a simple logger wrapper for my projects which will let me to easily swap out the backend.
This is my ideal interface:

log::error << "some" << " log " << "message";

The way I implemented it was:

1. log::error#operator<< returns a temporary Sink object.

2. Sink#operator<< returns *this and defines a move constructor.

3. The complete message can be utilized in Sink‘s destructor which is called at the end of the invocation chain.

Contrived Implementation:

#include <iostream>
#include <string>

struct Sink {

  Sink (std::string const& msg) : m_message(msg) {}

  // no copying
  Sink (Sink const& orig) = delete;

  // move constructor
  Sink (Sink && orig) : m_message(std::move(orig.m_message)) {};

  // use the complete string in the destructor
  ~Sink() { std::cerr << m_message << std::endl;}

  Sink operator<< (std::string const& msg) {
    m_message.append(msg);
    return std::move(*this);
  }

  std::string m_message;
};

struct Level {
  Sink operator<< (std::string const& msg) { return Sink(msg); }
};

int main() {
  Level log;

  log << "this" << " is " << "a " << "test";
}

This works fine except I need a clean way of disabling logging.
If I wasn’t using chaining, my log function could use a pre-processor directive to remove the function’s content

void log (std::string) {
  #ifdef LOGGING_ENABLED
    // log message
  #endif
}

The compiler would then optimize and remove the empty function call. But I don’t know how I’d do that with the api I’m trying to achieve. I know it’s possible because glog does it somehow.

Using directives like this defeats the purpose of having a nice api.

#ifdef LOGGING_ENABLED
  log << "this" << " is " << "a " << "test";
#endif

What is a clean way of disabling these types of chained calls?
Any help is appreciated.

Answer 1

You have to imlement another Sink which does nothing when logging. Glog calls this a null-stream:

// A class for which we define operator<<, which does nothing.
class GOOGLE_GLOG_DLL_DECL NullStream : public LogMessage::LogStream {
 public:
  // Initialize the LogStream so the messages can be written somewhere
  // (they'll never be actually displayed). This will be needed if a
  // NullStream& is implicitly converted to LogStream&, in which case
  // the overloaded NullStream::operator<< will not be invoked.
  NullStream() : LogMessage::LogStream(message_buffer_, 1, 0) { }
  NullStream(const char* /*file*/, int /*line*/,
             const CheckOpString& /*result*/) :
      LogMessage::LogStream(message_buffer_, 1, 0) { }
  NullStream &stream() { return *this; }
 private:
  // A very short buffer for messages (which we discard anyway). This
  // will be needed if NullStream& converted to LogStream& (e.g. as a
  // result of a conditional expression).
  char message_buffer_[2];
};

// Do nothing. This operator is inline, allowing the message to be
// compiled away. The message will not be compiled away if we do
// something like (flag ? LOG(INFO) : LOG(ERROR)) << message; when
// SKIP_LOG=WARNING. In those cases, NullStream will be implicitly
// converted to LogStream and the message will be computed and then
// quietly discarded.
template<class T>
inline NullStream& operator<<(NullStream &str, const T &) { return str; }

In your case, a simple implementation would look like

#ifdef LOGGING_ENABLED
  /* your sink */
#else
  struct Sink {
    Sink (std::string)  {}
    Sink (Sink const& orig) {};
  };
  template <typename T> Sink operator<<(Sink s, T) { return s; }
#endif

This is very simple and can be optimized away from the compiler.