Standard no-op output stream

You need a custom streambuf.

class NullBuffer : public std::streambuf
{
public:
  int overflow(int c) { return c; }
};

You can then use this buffer in any ostream class

NullBuffer null_buffer;
std::ostream null_stream(&null_buffer);
null_stream << "Nothing will be printed";

streambuf::overflow is the function called when the buffer has to output data to the actual destination of the stream. The NullBuffer class above does nothing when overflow is called so any stream using it will not produce any output.

If this is to disable logging output, your dummyStream would still cause arguments to be evaluated. If you want to minimize impact when logging is disabled, you can rely on a conditional, such as:

#define debugStream \
    if (debug_disabled) {} \
    else std::cerr

So if you have code like:

debugStream << "debugging output: " << foo() << std::endl;

No arguments will be evaluated if debug_disabled is true.

The basic method voor new stream classes is:

1. Derive a class from std::streambuf;
2. Override the virtual functions in that class. This is where the real work is done. In your case, empty implementations should be good enough.
3. Derive a class from std::ostream with one member, your streambuf class.
4. The constructor of your streamclass should forward the pointer to that member to the base constructor of std::ostream.

I'm afraid you won't get rid of the formatting step, though.

Hopefully this gives you some pointers; I don't have the time to expand this into a full answer, sorry.

Update: See john's answer for details.

For runtime-controllable redirection of log messages, a self-contained solution combining the ideas of john and Sjoerd:

class DebugStream {
private:
    class NullStream : public std::ostream {
    private:
        class NullBuffer : public std::streambuf {
        public:
            int overflow(int c) override { return c; }
        } buffer_;
    public:
        NullStream() : std::ostream(&buffer_) {}
    } null_;

    std::ostream &output_;
    bool enabled_;

public:
    DebugStream(std::ostream &output = std::cout) : output_(output), enabled_(false) {}
    void enable(const bool enable) { enabled_ = enable; }

    template <typename T> std::ostream& operator<<(const T &arg) {
        if (enabled_) return output_ << arg;
        else return null_ << arg;
    }
};

extern DebugStream debug_stream;
#define TRACE_ENABLE(x) debug_stream.enable(x)
#define TRACELN(x) debug_stream << x << std::endl
#define TRACE(x) debug_stream << x

Then you can do stuff like:

TRACELN("The value of x is " << x " and the value of y is " << y);

It would also be easy to just remove the trace statements from a release version completely with #define the trace macros to empty statements.

You still need to define debug_stream somewhere global, though.