Let's say we'll ignore obvious things like header guards.
Sometimes, you want to generate code that needs to be copy/pasted by the precompiler:
#define RAISE_ERROR_STL(p_strMessage) \
do \
{ \
try \
{ \
std::tstringstream strBuffer ; \
strBuffer << p_strMessage ; \
strMessage = strBuffer.str() ; \
raiseSomeAlert(__FILE__, __FUNCSIG__, __LINE__, strBuffer.str().c_str()) \
} \
catch(...){} \
{ \
} \
} \
while(false)
which enables you to code this:
RAISE_ERROR_STL("Hello... The following values " << i << " and " << j << " are wrong") ;
And can generate messages like:
Error Raised:
====================================
File : MyFile.cpp, line 225
Function : MyFunction(int, double)
Message : "Hello... The following values 23 and 12 are wrong"
Note that mixing templates with macros can lead to even better results (i.e. automatically generating the values side-by-side with their variable names)
Other times, you need the __FILE__ and/or the __LINE__ of some code, to generate debug info, for example. The following is a classic for Visual C++:
#define WRNG_PRIVATE_STR2(z) #z
#define WRNG_PRIVATE_STR1(x) WRNG_PRIVATE_STR2(x)
#define WRNG __FILE__ "("WRNG_PRIVATE_STR1(__LINE__)") : ------------ : "
As with the following code:
#pragma message(WRNG "Hello World")
it generates messages like:
C:\my_project\my_cpp_file.cpp (225) : ------------ Hello World
Other times, you need to generate code using the # and ## concatenation operators, like generating getters and setters for a property (this is for quite a limited cases, through).
Other times, you will generate code than won't compile if used through a function, like:
#define MY_TRY try{
#define MY_CATCH } catch(...) {
#define MY_END_TRY }
Which can be used as
MY_TRY
doSomethingDangerous() ;
MY_CATCH
tryToRecoverEvenWithoutMeaningfullInfo() ;
damnThoseMacros() ;
MY_END_TRY
(still, I only saw this kind of code rightly used once)
Last, but not least, the famous boost::foreach !!!
#include <string>
#include <iostream>
#include <boost/foreach.hpp>
int main()
{
std::string hello( "Hello, world!" );
BOOST_FOREACH( char ch, hello )
{
std::cout << ch;
}
return 0;
}
(Note: code copy/pasted from the boost homepage)
Which is (IMHO) way better than std::for_each.
So, macros are always useful because they are outside the normal compiler rules. But I find that most the time I see one, they are effectively remains of C code never translated into proper C++.
