Question

I’m using a logging module that can have reporting enabled/disabled at runtime. Calls generally go something like:

WARN(      'Danger Will Robinson! There are '      + boost::lexical_cast<string>(minutes)      + ' minutes of oxygen left!' ); 

I’m using an inline function for WARN, but I’m curious as to how much optimization is going on behind the scenes — evaluation of the arguments throughout the entire program would be costly. The WARN function goes something like this:

bool WARNINGS_ENABLED = false; inline void WARN(const string &message) {     if (!WARNINGS_ENABLED) {        return;     }     // ... } 

Given that constructing the string argument has no side-effects, will the compiler optimize it out? Is a certain level of optimization required (-Ox in g++ for some x)?

Answer 1

If you need to be able to selectively enable and disable the warnings at run-time, the compiler will not be able to optimize out the call.

What you need is to rename your function to WARN2 and add a macro something like:

#define WARN(s) do {if (WARNINGS_ENABLED) WARN2(s);} while (false) 

This will prevent the evaluation of s at run-time unless you have warnings enabled.

The do-while stuff is a trick that allows it to be used anywhere in the code (naked statement, statement within a braced if-block, statement within an unbraced if-block, braced and unbraced while statements and so on).