[Boost-users] Closures emulation made easy

Subject: [Boost-users] Closures emulation made easy
From: Alexander Gutenev (gutenev_at_[hidden])
Date: 2008-09-05 04:43:35

Hi.

Here's a trick.

Use the idea of Boost.ScopeExit. But don't implement capturing of variables 
like in Boost.ScopeExit - use Boost.Bind which does it.

Have easy to write and debug closure body - don't spend too much time on 
making Boost.Lamda work.
And a compact implementation with very small proprocessor overhead, compared 
to Boost.ScopeExit.

// Implementation

#define CONCATENATE_DIRECT(s1, s2) s1##s2
#define CONCATENATE(s1, s2) CONCATENATE_DIRECT(s1, s2)
#define ANONYMOUS_VARIABLE(str) CONCATENATE(str, __LINE__)

#define LOCAL(R, P) \
  typedef struct ANONYMOUS_VARIABLE(xg1) { \
    typedef R(*F) P; static R body P {

#define LOCAL_END(name) \
  } } ANONYMOUS_VARIABLE(xg2); \
  const ANONYMOUS_VARIABLE(xg2)::F name \
    = ANONYMOUS_VARIABLE(xg2)::body;

// Example

#include <algorithm>

int main()
{
  LOCAL(bool, (int i, int j, bool reverse))
    return reverse ? (j < i) : (i < j);
  LOCAL_END(myFcn)
  int a[] = { 2, 5, 1, 6 };
  std::sort(
    a,
    a + sizeof(a)/sizeof(a[0]),
    boost::bind(myFcn, _1, _2, true));
}