C/C++ Macro expansion vs. Code generation

Question

Both Macro expansion & Code generation have pros & cons. What's your favorite approach and why? When should we choose one over the other? Please kindly advise. Thank you!

Macro expansion can be very handy & helpful: http://dtemplatelib.sourceforge.net/table.htm

vs

While Code generation gives you plenty of nice code: http://code.google.com/p/protobuf/ http://incubator.apache.org/thrift/

score 9 · Answer 1 · answered Nov 27 '09 at 09:52

9

For c++ I prefer either template metaprogramming or code generation over macros, but macros still have their uses.

The example you have given with dbtemplatelib could be covered with c++0x Variadic Templates, with additional benefits like type checking etc.

answered Nov 27 '09 at 09:52

Gunther Piez

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Thanks for your suggestion. Templates have their virtues too. Variadic macros are there but variadic templates are yet to be popular as C++0x is still new. – Viet Nov 27 '09 at 09:55
Well, to be exact, it isn't even out yet, and it will rather be a "c++1x". But variadic templates are implemented in some compilers, for example, to use them with gcc, you have to add "-std=c++0x" as a compiler switch. – Gunther Piez Nov 27 '09 at 14:19

score 5 · Answer 2 · answered Nov 27 '09 at 09:39

5

In C or C++, macro expansion is notoriously difficult to debug. On the other hand, writing a code generator is easier to debug because it's a separate program in itself.

However, you should be aware that this is merely a limitation of the C preprocessor. For example, in the Lisp family of languages, macro expansion is code generation, they're exactly the same thing. To write a macro, you write a program (in Lisp) to transform S-expression input into another S-expression, which is then passed to the compiler.

answered Nov 27 '09 at 09:39

Greg Hewgill

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Thanks for your reply, Greg. It takes more time to write a decent code generator and macros look like a quick hack to get things done. – Viet Nov 27 '09 at 09:48
++ My sentiments exactly. It would be nice if there were a way to step through the preprocessing. Even so, I prefer macros if the code generation is simple enough. – Mike Dunlavey Nov 30 '09 at 15:48

score 3 · Answer 3 · answered Nov 30 '09 at 16:15

3

Both have their problems. Unlike macros, code generation can produce readable and debuggable (is that even a word?) code, but it is less flexible and harder to change.

answered Nov 30 '09 at 16:15

Nemanja Trifunovic

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score 2 · Accepted Answer · edited May 23 '17 at 12:25

It's a tradeoff. Let me give an example. I stumbled on the technique of differential execution around 1985, and I think it's a really good tool for programming user interfaces. Basically, it takes simple structured programs like this:

void Foo(..args..){
  x = y;
  if (..some test..){
    Bar(arg1, ...)
  }
  while(..another test..){
    ...
  }
  ...
}

and mucks with the control structure like this:

void deFoo(..args..){
  if (mode & 1){x = y;}
  {int svmode = mode; if (deIf(..some test..)){
    deBar(((mode & 1) arg1 : 0), ...)
  } mode = svmode;}
  {int svmode = mode; while(deIf(..another test..)){
    ...
  } mode = svmode;}
  ...
}

Now, a really good way to do that would have been to write a parser for C or whatever the base language is, and then walk the parse tree, generating the code I want. (When I did it in Lisp, that part was easy.)

But who wants to write a parser for C, C++, or whatever?

So, instead, I just write macros so that I can write the code like this:

void deFoo(..args..){
  PROTECT(x = y);
  IF(..some test..)
    deBar(PROTECT(arg1), ...)
  END
  WHILE(..another test..)
    ...
  END
  ...
}

However, when I do this in C#, somebody in their wisdom decided macros were bad, and I don't want to write a C# parser, so I gotta do the code generation by hand. This is a royal pain, but it's still worth it compared to the usual way of coding these things.

C/C++ Macro expansion vs. Code generation

4 Answers4