C++ Preprocessor: Always Assert Your Code Is Right - Refining Assert
(Page 4 of 4 )
The simplest solution is to just expand the assert macro and make it accept a message as well as a condition:
#ifndef NDEBUG
# define ASSERT( isOK, message ) \
if ( !(isOK) ) { \
(void)printf(“ERROR!! Assert ‘%s’ failed on line %d “ \
“in file ‘%s’\n%s\n”, \
#isOK, __LINE__, __FILE__, #message); \
__asm { int 3 } \
}
#else
# define ASSERT( unused, message ) do {} while ( false )
#endif
Again the stringize operator helps us to stuff the message we want into the printf statement. We could call it a day now, but I am a very lazy coder… I do not want to be forced to put messages into the assert, sometimes I just want to assert.
Of course it is possible to write an ASSERT(condition) and an ASSERTm(condition, message) macro, but did I mention I am a forgetful coder too? I’d much rather have a single ASSERT statement that can do both.
The first thing that comes to mind is the fact I could do this easily with a function:
void MyAssert(bool isOK, char const *message=””) {
if ( !isOK ) {
(void)printf(“ERROR!! Assert ‘%s’ failed on line %d “
“in file ‘%s’\n%s\n”,
__LINE__, __FILE__, message);
__asm { int 3 } \
}
}
So maybe if I declared another function:
void NoAssert(bool isOK, char const *message=””) {}
And then defined assert as:
#ifndef NDEBUG
# define ASSERT MyAssert
#else
# define ASSERT NoAssert
#endif
While this seems like a quick solution, I have completely lost my extra debug information! The line information is the same now for every assert… oh… the compiler substitutes __LINE__ with the actual line number it is compiling at that moment, and since we are making a function call – all line numbers lead to the MyAssert function!
Alexandrescu demonstrates a great way around this problem [Alexandrescu]. (It is also a great article showing how you can take assertions to a higher level after this one, by making them throw exceptions!)
#ifndef NDEBUG
#define ASSERT \
struct MyAssert { \
MyAssert(bool isOK, char const *message=””) { \
if ( !isOK ) { \
(void)printf(“ERROR!! Assert failed in “ \
“file ‘%s’\n%s\n”, __FILE__, message); \
__asm { int 3 } \
} \
} \
} myAsserter = MyAssert
#endif
For some reason my Visual C++ 7.1 compiler will not accept the __LINE__ macro next to the __FILE__ macro in the code above. The strange thing is that the __FILE__ macro works fine, but with __LINE__ it complains:
error C2065: '__LINE__Var' : undeclared identifier
It is never easy, but I do not want to give up at this point. Since the macro is expanded as a single line into the place where we are calling it, and since the compiler apparently has no objection to me assigning __LINE__ as a default parameter in a constructor, let's try again:
#ifndef NDEBUG
#define ASSERT \
struct MyAssert { \
int mLine; \
MyAssert(int line=__LINE__) : mLine(line) {} \
MyAssert(bool isOK, char const *message=””) { \
if ( !isOK ) { \
(void)printf(“ERROR!! Assert failed on “ \
“line %d in file ‘%s’\n%s\n”, \
MyAssert().mLine, __FILE__, message); \
__asm { int 3 } \
} \
}\
} myAsserter = MyAssert
#endif
Now that we have our line information back, we are nearly there; as soon as we add a second assert, the compiler complains that we are redefining the struct MyAssert! If only we could keep the struct declaration local… and again Alexandrescu shows us how [Alexandrescu]:
#ifndef NDEBUG
#define ASSERT \
if ( false ) {} else struct LocalAssert { \
int mLine; \
LocalAssert(int line=__LINE__) : mLine(line) {} \
LocalAssert(bool isOK, char const *message=””) { \
if ( !isOK ) { \
(void)printf(“ERROR!! Assert failed on “ \
“line %d in file ‘%s’\n%s\n”, \
LocalAssert().mLine, __FILE__, message); \
__asm { int 3 } \
} \
} myAsserter = LocalAssert
#else
#define ASSERT \
if ( true ) {} else struct NoAssert { \
NoAssert(bool isOK, char const *message=””) {} \
} myAsserter = NoAssert
#endif
There is a lot of fun to be had with macros and sometimes it is possible to create the wildest incantations with them [Niebler/Alexandrescu]. I hope you are convinced that despite the fact that they can be considered evil, there is something magical about them as well.
As a final example I will show you how to create personal and customizable debug streams in the next article… all with macros.
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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