C++ Preprocessor: The Code in the Middle - Predefined Macros
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Most compilers come standard with a number of macros predefined: __DATE__ , __TIME__ , __LINE__ and __FILE__ . These macros are very useful when you have to debug different builds. The date and time macros are translated to the date and moment the source was compiled. The line macro translates to the line number it appeared on in the source file and the file macro is translated to the actual filename of the source.
You will see that these macros come in handy when we start customizing some assert functionality.
Macro Troubles
Hopefully you are aware that macros are evil and can cause you a lot of headaches. Even though they have their clear benefits, they can cause a lot of trouble as well. There are four types of problems you can expect when using them.
Macros are not type-safe. When you find that typesafety might be an issue when you are implementing a macro function, you might want to consider replacing it with a templated function.
Macros have to be defined on one line. Things can become pretty complex and hard to read if your macro function extends over several lines. (You can make your macro span several lines by adding a backslash character (\) at the end of every line, except for the last).
Macros perform simple string substitution. This means that there is no such thing as a macro function call; the code defined in the macro is going to be pasted wherever that macro is used. This can cause considerable bloat in your source.
Because macros are expanded inline, not all debuggers can display the contents of the macro during a debug session. This means that you have no means to verify the way your macro function is being executed. This can lead to a lot of pain, only to discover you’ve forgotten a couple of parentheses!
As rough and coarse they may be, there are many nice tricks you can perform with preprocessor macros. Just take a look at the excellent crafted BOOST_FOREACH [Niebler] macro.
You will find a whole new dimension when you are able to combine macros and templates in meaningful ways. Soon I will be able to present to you the Active Object pattern and a nice macro that simplifies the implementation of this pattern using the ACE [ACE] library.
But first things first. Next time we’ll create a custom assert.
References
[STL] – The Standard Template Library
< comes standard with your compiler but this one is very portable>
http://www.stlport.org
[BOOST] – Boost C++ Libraries
http://www.boost.org
[ACE] – The ADAPTIVE Communication Environment
http://www.cs.wustl/edu/~schmidt/ACE.html
[Niebler] – Eric Niebler
“Conditional Love: FOREACH Redux”
http://www.artima.com/cppsource/foreach.html
[Alexandrescu] – Andrei Alexandrescu
Assertions
http://www.cuj.com/documents/s=8248/cujcexp2104alexandr/
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