Linux上的C ++-如何#define #ifdef / #endif调试条件

Question

I want to trace my code if DEBUG is defined, as in the following #ifdef code block:

#ifdef DEBUG
    OP_LOG(debug) << "SEQUENCE:  " __FILE__ << "::" << __FUNCTION__;
#endif

Someone said I could use a #define so that the above three lines would only be one line instead. This is the closest I could get to figuring it out, but it gives tracing output unconditionally, whether or not DEBUG is defined:

#define DEBUG_TRACE(note) OP_LOG(debug) << "SEQUENCE :  " << __FILE__ <<"::" <<__FUNCTION__ << note

I don't know how to make a conditional #define . Is this possible, and if so, how to do it?

Answer 1

You can't put #ifdef in the replacement part of a #define , but you can use #define inside an #ifdef / #endif block, like this:

#ifdef DEBUG
#define DEBUG_TRACE(note) ... // put your debug code here
#else
#define DEBUG_TRACE(note)
#endif

In the #else case, this defines DEBUG_TRACE to expand to nothing if DEBUG isn't defined.

Answer 2

Someone said I could create a #define, so that the above three lines would only be one line instead.

Not that I have heard of. My favorite way to do this would be

#ifdef DEBUG
#define DEBUG_LOG OP_LOG(debug) << "SEQUENCE:  " __FILE__ << "::" << __FUNCTION__;
#else
#define DEBUG_LOG
#endif

This way you have to only write DEBUG_LOG where ever you want to log only if -DDEBUG is set.

---

The way it works:

Look at the #ifdef , #else , #endif statement. We define DEBUG_LOG to be expanded to nothing when -DDEBUG isn't set and as OP_LOG(debug) << "SEQUENCE: " __FILE__ << "::" << __FUNCTION__; if it is. That way you don't have to worry about -DDEBUG later on, you just have to use DEBUG_LOG when you want to place a trace call.