C++ - Passing std::ostream to a function

Question

I thought of a small debug inline function in C++:

void inline debug( int debug_level, ostream& out ) {
    if ( debug_level <= verbosity ) {
        out.flush();
    }
    else {
        ostream tmp;
        tmp << out;
    }
}

This is an example of how I wanted to use it:

_debug( 7, cout << "Something something" << someint << endl );

However it does not work in the way I planned - I wanted it to print the message only if the verbosity level is higher or equal then the debug level passed to function, but it seems it prints everytime regardless of the debug level, so data stays in the cout buffer. By now I think this function's not the best idea I've had lately, but still I want to know if there's a way to clear the buffer associated with cout, cerr etc. Is it possible to get this kind of function to work properly?

Answer 1

Either using a macro as shown above, or like this:

struct nullstream : ostream {
    nullstream() : ostream(0) { }
};

ostream& dout(int debug_level, ostream& out = cerr) {
    static nullstream dummy;
    return debug_level <= verbosity ? dummy : out;
}

// …

dout(level) << "foo" << endl;
dout(level, cout) << "IMPORTANT" << endl;

(Using endl also triggers flushing, no need to flush manually!)