have a single union ubuf definition in lmptype.h and remove others

src/atom_vec.h

@@ -209,25 +209,6 @@ class AtomVec : protected Pointers {
 
   bool *threads;
 
-  // union data struct for packing 32-bit and 64-bit ints into double bufs
-  // this avoids aliasing issues by having 2 pointers (double,int)
-  //   to same buf memory
-  // constructor for 32-bit int prevents compiler
-  //   from possibly calling the double constructor when passed an int
-  // copy to a double *buf:
-  //   buf[m++] = ubuf(foo).d, where foo is a 32-bit or 64-bit int
-  // copy from a double *buf:
-  //   foo = (int) ubuf(buf[m++]).i;, where (int) or (tagint) match foo
-  //   the cast prevents compiler warnings about possible truncation
-
-  union ubuf {
-    double d;
-    int64_t i;
-    ubuf(double arg) : d(arg) {}
-    ubuf(int64_t arg) : i(arg) {}
-    ubuf(int arg) : i(arg) {}
-  };
-
   // local methods
 
   void grow_nmax();

src/compute.h

@@ -161,17 +161,6 @@ class Compute : protected Pointers {
     return j >> SBBITS & 3;
   }
 
-  // union data struct for packing 32-bit and 64-bit ints into double bufs
-  // see atom_vec.h for documentation
-
-  union ubuf {
-    double d;
-    int64_t i;
-    ubuf(double arg) : d(arg) {}
-    ubuf(int64_t arg) : i(arg) {}
-    ubuf(int arg) : i(arg) {}
-  };
-
   // private methods
 
   void adjust_dof_fix();

src/fix.h

@@ -242,17 +242,6 @@ class Fix : protected Pointers {
   void v_tally(int, int *, double, double *);
   void v_tally(int, double *);
   void v_tally(int, int, double);
-
-  // union data struct for packing 32-bit and 64-bit ints into double bufs
-  // see atom_vec.h for documentation
-
-  union ubuf {
-    double d;
-    int64_t i;
-    ubuf(double arg) : d(arg) {}
-    ubuf(int64_t arg) : i(arg) {}
-    ubuf(int arg) : i(arg) {}
-  };
 };
 
 namespace FixConst {

src/lmptype.h

@@ -171,6 +171,46 @@ typedef int bigint;
 
 #endif
 
+  /// Data structe for packing 32-bit and 64-bit integers
+  /// into double (communication) buffers
+  ///
+  /// Using this union avoids aliasing issues by having member types
+  /// (double, int) referencing the same buffer memory location.
+  ///
+  /// The explicit constructor for 32-bit integers prevents compilers
+  /// from (incorrectly) calling the double constructor when storing
+  ///  an int into a double buffer.
+  /*
+\verbatim embed:rst
+**Usage:**
+
+To copy an integer into a double buffer:
+
+.. code-block: c++
+
+   double buf[2];
+   int    foo =   1;
+   tagint bar = 2<<40;
+   buf[1] = ubuf(foo).d;
+   buf[2] = ubuf(bar).d;
+
+To copy from a double buffer back to an int:
+
+.. code-block: c++
+
+   foo = (int)    ubuf(buf[1]).i;
+   bar = (tagint) ubuf(buf[2]).i;
+
+The typecast prevents compiler warnings about possible truncations.
+\endverbatim
+  */
+  union ubuf {
+    double  d;
+    int64_t i;
+    ubuf(const double  &arg) : d(arg) {}
+    ubuf(const int64_t &arg) : i(arg) {}
+    ubuf(const int     &arg) : i(arg) {}
+  };
 }
 
 // preprocessor macros for compiler specific settings

src/pair.h

@@ -258,17 +258,6 @@ class Pair : protected Pointers {
                       double, double, double, double, double, double);
   void virial_fdotr_compute();
 
-  // union data struct for packing 32-bit and 64-bit ints into double bufs
-  // see atom_vec.h for documentation
-
-  union ubuf {
-    double d;
-    int64_t i;
-    ubuf(double arg) : d(arg) {}
-    ubuf(int64_t arg) : i(arg) {}
-    ubuf(int arg) : i(arg) {}
-  };
-
   inline int sbmask(int j) const {
     return j >> SBBITS & 3;
   }

src/reset_ids.h

@@ -53,17 +53,6 @@ class ResetIDs : protected Pointers {
   static int sort_bins(int, char *, int &, int *&, char *&, void *);
   
   void sort();
-  
-  // union data struct for packing 32-bit and 64-bit ints into double bufs
-  // see atom_vec.h for documentation
-
-  union ubuf {
-    double d;
-    int64_t i;
-    ubuf(double arg) : d(arg) {}
-    ubuf(int64_t arg) : i(arg) {}
-    ubuf(int arg) : i(arg) {}
-  };
 };
 
 }