Short circuit `operator<<` output in C++

Question

I have some code that is sprinkled with constructs like this

if(debug) {
    Output << "f1: " << f1() << "\n";
}

Now what I want to do is write a stream class Debug where I could write it like this

Debug << "f1: " << f1() << "\n";

If some global flag is set then this would generate output, otherwise not.

Now: this can be quite easily done by making Debug return a stream that goes to /dev/null which would swallow the output. The problem is that f1() still gets evaluated (and 'rendered' into a textual representation which might be even more expensive) which might be quite bad for the performance.

Now my question: is there any trick that allows the skipping of the 'evaluation' of

"f1: " << f1() << "\n"

completely if Debug decides that no output should be done? Similar to the short circuiting that C++ does for f() && g() where g() is not evaluated if f()is false (I seriously considered writing a stream class that uses && as the output operator but from what I read short-circuiting is not done for overloaded operator&&)

Answer 1

What you can do is define this macro:

#define Debug_Stream \
if(!debug); else Output

This would make this:

Debug_Stream << "f1: " << f1() << "\n";

become equivalent to this:

if(debug) {
    Output << "f1: " << f1() << "\n";
}

But literally (plus whitespace for readability)

if(!debug);
else
    Output << "f1: " << f1() << "\n";