Improve the performance of Boost.Function's swap. Thanks to Niels Dekker for the original patch. Fixes #1910

[SVN r48615]

test/Jamfile.v2

@@ -58,6 +58,7 @@ import testing ;
    
   [ run libs/function/test/contains2_test.cpp : : : : ]
    
+  [ run libs/function/test/nothrow_swap.cpp :  :  :  :  ]
  ;
 }
       

test/nothrow_swap.cpp

@@ -0,0 +1,51 @@
+#include <boost/test/minimal.hpp>
+#include <boost/function.hpp>
+
+struct tried_to_copy { };
+
+struct MaybeThrowOnCopy {
+  MaybeThrowOnCopy(int value = 0) : value(value) { }
+
+  MaybeThrowOnCopy(const MaybeThrowOnCopy& other) : value(other.value) {
+    if (throwOnCopy)
+      throw tried_to_copy();
+  }
+
+  MaybeThrowOnCopy& operator=(const MaybeThrowOnCopy& other) {
+    if (throwOnCopy)
+      throw tried_to_copy();
+    value = other.value;
+    return *this;
+  }
+
+  int operator()() { return value; }
+
+  int value;
+
+  // Make sure that this function object doesn't trigger the
+  // small-object optimization in Function.
+  float padding[100];
+
+  static bool throwOnCopy;
+};
+
+bool MaybeThrowOnCopy::throwOnCopy = false;
+
+int test_main(int, char* [])
+{
+  boost::function0<int> f;
+  boost::function0<int> g;
+
+  MaybeThrowOnCopy::throwOnCopy = false;
+  f = MaybeThrowOnCopy(1);
+  g = MaybeThrowOnCopy(2);
+  BOOST_CHECK(f() == 1);
+  BOOST_CHECK(g() == 2);
+
+  MaybeThrowOnCopy::throwOnCopy = true;
+  f.swap(g);
+  BOOST_CHECK(f() == 2);
+  BOOST_CHECK(g() == 1);
+  
+  return 0;
+}