Make the cxx callback conversion work. Addresses #226.

git-svn-id: file:///svn/tokudb@1404 c7de825b-a66e-492c-adef-691d508d4ae1

cxx/db.cpp

@@ -105,16 +105,22 @@ int Db::set_bt_compare(bt_compare_fcn_type bt_compare_fcn) {
     return the_Env->maybe_throw_error(ret);
 }
 
-// open questions:
-// how to convert callback types in the most simple way?
-
-static int Db_associate_callback(DB *secondary, const DBT *key, const DBT *data, DBT *result) {
-    Db *dbs = Db::get_Db(secondary);
-    return EINVAL; // Dbs->get_associate_callback(Dbs, (const Dbt *) key, (const Dbt *) data, (Dbt *) result);
+// Q: How to convert callback types in the most simple way?
+// A: (Bradley) I see three ways to do this:  The issue is that we have a C++ callback function, and we want to pass it to a C function.
+//    The fastest way is wrong:  You cannot just pass the C++ function pointer since you cannot mix function pointers to C and C++.
+//    The "right" way is to declare an "extern C" function and do all the conversions.  Create a Dbt from a DBT, and then call the C function.  For returned data we would have do something too.  But it turns out that DBT and Dbt pointers are interchangable so that leads to
+//    The "fast" way.  Declare an "extern C" function, and then use Dbt::get_const_Dbt() to do the conversion quickly.
+
+extern "C" int associate_callback_c (DB*db_c, const DBT *k, const DBT *d, DBT *result) {
+    assert(db_c!=0);
+    Db *db_cxx=Db::get_Db(db_c);
+    assert(db_cxx);
+    return (*db_cxx->associate_callback_cxx)(db_cxx, Dbt::get_const_Dbt(k), Dbt::get_const_Dbt(d), Dbt::get_Dbt(result));
 }
 
 int Db::associate(DbTxn *txnid, Db *secondary, int (*callback)(Db *secondary, const Dbt *key, const Dbt *data, Dbt *result), u_int32_t flags) {
     // secondary->set_associate_callback(callback);
-    int ret = the_db->associate(the_db, txnid->get_DB_TXN(), secondary->get_DB(), Db_associate_callback, flags);
+    secondary->associate_callback_cxx = callback;
+    int ret = the_db->associate(the_db, txnid->get_DB_TXN(), secondary->get_DB(), associate_callback_c, flags);
     return the_Env->maybe_throw_error(ret);
 }

include/db_cxx.h

@@ -63,6 +63,10 @@ class Dbt : private DBT
     void      set_ulen(u_int32_t p)    { ulen = p; }
 
     DBT *get_DBT(void)                              { return (DBT*)this; }
+    const DBT *get_const_DBT(void) const            { return (const DBT*)this; }
+
+    static Dbt* get_Dbt(DBT *dbt)                   { return (Dbt *)dbt; }
+    static const Dbt* get_const_Dbt(const DBT *dbt) { return (const Dbt *)dbt; }
 
     Dbt(void */*data*/, u_int32_t /*size*/);
     Dbt(void);
@@ -109,6 +113,9 @@ class Db {
     int set_bt_compare(bt_compare_fcn_type bt_compare_fcn);
     int associate(DbTxn *, Db *, int (*)(Db *, const Dbt *, const Dbt *, Dbt *), u_int32_t);
 
+    /* the cxx callbacks must be public so they can be called by the c callback.  But it's really private. */
+    int (*associate_callback_cxx)(Db *, const Dbt *, const Dbt *, Dbt*);
+
  private:
     DB *the_db;
     DbEnv *the_Env;