plans for macros

Steven Schveighoffer schveiguy at
Wed May 14 08:31:25 PDT 2008

I just found a very good use for macros, and I was wondering how they could 
be used to help in this situation.

If I have a log object, and that log object is supposed to evaluate its 
arguments only if the logging level allows it, checked at runtime.

So this is the ideal usage in the calling function:


This is ideal because it only outputs at the appropriate level, and it only 
evaluates the expensive function if the log level is enabled.

However, this is very verbose, and is prone to errors.  Many log systems use 
the following method:


Which does the if-statement for you.  However, they warn you to write your 
logging code in the first form if the code to build the output is expensive 
to avoid building the output even when it is not output.  But D has a better 

class Log
void outputInformation(lazy string x)

Now, we can still use the second form, even when building the string is 
expensive.  But there are issues with this solution.  For one, lazy 
evaluation adds delegate functions wherever the logging is required, adding 
to runtime and code bloat.  Second, variadic functions would be nice for 
logging, especially with formatting, but the only way to do lazy variadic 
functions is with template tuples, so there is another lot of generated 
code, and is even more inefficient.

But a macro would solve the problem quite nicely.  A macro would evaluate 
the if statement in the calling function, and so would prevent evaluation of 
the expensive string building unless necessary, AND would require no 
delegates to do it.

The question I have is, when macros are implemented, can I have a 'class 
scoped' macro?  That is, a macro that knows what context it is supposed to 
be in, and is passed a 'this' pointer?  And will macros support variadic 

For example, I'd like to have a macro to output formatted log information 
only if the log is enabled, but I want to call it like a member function of 
the log.


More information about the Digitalmars-d-learn mailing list