Preprocessor tricks

Let's suppose that we have defined following macros:

Code:

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#define AAA 'a'
#define BBB 8

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Is it possible somehow to define a macro CCC which will be the string of AAA duplicated BBB times?

Code:

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#define CCC "aaaaaaaa"

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I'm loooking the equivalent to above mentioned CCC macro definition, which
will be changed automatically whenever AAA or BBB is changed.
CCC should be constructed from AAA and BBB in compile-time.

Thank You.

No, that's not how preprocessor works. Preprocess is done at essentially copy and paste, you can't have logic like that.

It's trivial to implement, albeit the code can become quite bulky (if done in a reusable manner).
Luckily, most of the legwork has been written for you already (see the Boost.Preprocessor library):

Code:

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#define AAA "a"
#define BBB 8
#define REPEAT_VERBATIM(z, n, aux) aux
#define CCC BOOST_PP_REPEAT(BBB, REPEAT_VERBATIM, AAA)

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CCC should effectively expand to "a" * BBB. During compilation, these adjacent strings will be concatenated and you'll end up with what you asked for in your OP.

Preprocessor tricks might just be the one thing less intuitive than template metaprogramming.

Quote:

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CCC should effectively expand to "a" * BBB. During compilation, these adjacent strings will be concatenated and you'll end up with what you asked for in your OP.

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It won't work. Preprocessing is done in single phase, not in two phases.

Quote:

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Originally Posted by Ajay Vijay

It won't work.

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Yes, it will.

Quote:

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Originally Posted by Ajay Vijay

Preprocessing is done in single phase, not in two phases.

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You clearly don't understand how BOOST_PP_REPEAT works, then.

Quote:

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You clearly don't understand how BOOST_PP_REPEAT works, then.

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Actually I did not. And I still couldn't understand how it works. But I see this macro is using templates.

Quote:

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Originally Posted by Ajay Vijay

But I see this macro is using templates.

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What templates?

Quote:

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Originally Posted by Ajay Vijay

And I still couldn't understand how it works.

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Simulated recursion.

Quite easily generated:

Code:

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#!/usr/bin/perl
use strict;
use warnings;

foreach(2 .. $ARGV[0])
{
  my $i = $_ - 1;

  print "#define REPEAT_$_(m, d) REPEAT_$i(m, d)m($i, d)\n";
}

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Code:

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#define EXPAND_CONCAT(a, b) a##b
#define REPEAT(n, m, d) EXPAND_CONCAT(REPEAT_, n)(m, d)

#define REPEAT_0(m, d)
#define REPEAT_1(m, d) m(0, d)
#define REPEAT_2(m, d) REPEAT_1(m, d)m(1, d)
#define REPEAT_3(m, d) REPEAT_2(m, d)m(2, d)
#define REPEAT_4(m, d) REPEAT_3(m, d)m(3, d)
#define REPEAT_5(m, d) REPEAT_4(m, d)m(4, d)
#define REPEAT_6(m, d) REPEAT_5(m, d)m(5, d)
#define REPEAT_7(m, d) REPEAT_6(m, d)m(6, d)
#define REPEAT_8(m, d) REPEAT_7(m, d)m(7, d)
#define REPEAT_9(m, d) REPEAT_8(m, d)m(8, d)
//...
//#define REPEAT_{LIM}(m, d) REPEAT_{LIM-1}(m, d)m({LIM-1}, d)

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This particular implementation can be affected by your preprocessor's recursive depth limitations.
You can work around this limitation by expanding each REPEAT_N call yourself (i.e., while generating the chain), at the expense of your source file's size, of course.

That said, considering the OP's original requirements:

Code:

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#define ELEM "a"
#define COUNT 8
#define REPEAT_VERBATIM(i, d) d
#define EXPRESSION REPEAT(COUNT, REPEAT_VERBATIM, ELEM)

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1. EXPRESSION will expand to "a""a""a""a""a""a""a""a" in translation phase 4.
2. "a""a""a""a""a""a""a""a" will be converted to "aaaaaaaa" in translation phase 6.

Not confusing at all. :wave:

Quote:

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Originally Posted by Lindley

Preprocessor tricks might just be the one thing less intuitive than template metaprogramming.

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LOL :D
Both are great techniques for stopping a maintenance programmer in his tracks.