diff --git a/libgnucash/engine/CMakeLists.txt b/libgnucash/engine/CMakeLists.txt
index 630d8fd1fb6..40c2742410c 100644
--- a/libgnucash/engine/CMakeLists.txt
+++ b/libgnucash/engine/CMakeLists.txt
@@ -174,23 +174,23 @@ set (engine_SOURCES
   gnc-rational.cpp
   gnc-session.c
   gnc-timezone.cpp
-  gnc-uri-utils.c
+  gnc-uri-utils.cpp
   engine-helpers.c
   guid.cpp
   policy.cpp
-  gncAddress.c
-  gncBillTerm.c
+  gncAddress.cpp
+  gncBillTerm.cpp
   gncBusiness.c
-  gncCustomer.c
-  gncEmployee.c
-  gncEntry.c
+  gncCustomer.cpp
+  gncEmployee.cpp
+  gncEntry.cpp
   gncIDSearch.c
-  gncInvoice.c
-  gncJob.c
-  gncOrder.c
-  gncOwner.c
-  gncTaxTable.c
-  gncVendor.c
+  gncInvoice.cpp
+  gncJob.cpp
+  gncOrder.cpp
+  gncOwner.cpp
+  gncTaxTable.cpp
+  gncVendor.cpp
   kvp-frame.cpp
   kvp-value.cpp
   qof-backend.cpp
diff --git a/libgnucash/engine/gnc-uri-utils.c b/libgnucash/engine/gnc-uri-utils.cpp
similarity index 99%
rename from libgnucash/engine/gnc-uri-utils.c
rename to libgnucash/engine/gnc-uri-utils.cpp
index fd962dc2bb1..164860e6b6e 100755
--- a/libgnucash/engine/gnc-uri-utils.c
+++ b/libgnucash/engine/gnc-uri-utils.cpp
@@ -66,7 +66,7 @@ gboolean gnc_uri_is_known_scheme (const gchar *scheme)
 
     for ( node = known_scheme_list; node != NULL; node = node->next )
     {
-        gchar *known_scheme = node->data;
+        gchar *known_scheme = (char*)node->data;
         if ( !g_ascii_strcasecmp (scheme, known_scheme) )
         {
             is_known_scheme = TRUE;
diff --git a/libgnucash/engine/gncAddress.c b/libgnucash/engine/gncAddress.cpp
similarity index 99%
rename from libgnucash/engine/gncAddress.c
rename to libgnucash/engine/gncAddress.cpp
index 27e249f658d..98d6622f884 100644
--- a/libgnucash/engine/gncAddress.c
+++ b/libgnucash/engine/gncAddress.cpp
@@ -312,7 +312,7 @@ gncAddressCreate (QofBook *book, QofInstance *prnt)
 
     if (!book) return NULL;
 
-    addr = g_object_new (GNC_TYPE_ADDRESS, NULL);
+    addr = GNC_ADDRESS(g_object_new (GNC_TYPE_ADDRESS, NULL));
     qof_instance_init_data(&addr->inst, GNC_ID_ADDRESS, book);
     addr->book = book;
     addr->dirty = FALSE;
@@ -631,7 +631,7 @@ static QofObject GncAddressDesc =
     DI(.interface_version = ) QOF_OBJECT_VERSION,
     DI(.e_type            = ) GNC_ID_ADDRESS,
     DI(.type_label        = ) "Address",
-    DI(.create            = ) (gpointer)qofAddressCreate,
+    DI(.create            = ) (void* (*)(QofBook*))qofAddressCreate,
     DI(.book_begin        = ) NULL,
     DI(.book_end          = ) NULL,
     DI(.is_dirty          = ) qof_collection_is_dirty,
diff --git a/libgnucash/engine/gncBillTerm.c b/libgnucash/engine/gncBillTerm.cpp
similarity index 95%
rename from libgnucash/engine/gncBillTerm.c
rename to libgnucash/engine/gncBillTerm.cpp
index 9be71330dc2..bc554254727 100644
--- a/libgnucash/engine/gncBillTerm.c
+++ b/libgnucash/engine/gncBillTerm.cpp
@@ -95,14 +95,14 @@ static inline void maybe_resort_list (GncBillTerm *term)
     struct _book_info *bi;
 
     if (term->parent || term->invisible) return;
-    bi = qof_book_get_data (qof_instance_get_book(term), _GNC_MOD_NAME);
+    bi = static_cast<_book_info*>(qof_book_get_data (qof_instance_get_book(term), _GNC_MOD_NAME));
     bi->terms = g_list_sort (bi->terms, (GCompareFunc)gncBillTermCompare);
 }
 
 static inline void addObj (GncBillTerm *term)
 {
     struct _book_info *bi;
-    bi = qof_book_get_data (qof_instance_get_book(term), _GNC_MOD_NAME);
+    bi = static_cast<_book_info*>(qof_book_get_data (qof_instance_get_book(term), _GNC_MOD_NAME));
     bi->terms = g_list_insert_sorted (bi->terms, term,
                                       (GCompareFunc)gncBillTermCompare);
 }
@@ -110,7 +110,7 @@ static inline void addObj (GncBillTerm *term)
 static inline void remObj (GncBillTerm *term)
 {
     struct _book_info *bi;
-    bi = qof_book_get_data (qof_instance_get_book(term), _GNC_MOD_NAME);
+    bi = static_cast<_book_info*>(qof_book_get_data (qof_instance_get_book(term), _GNC_MOD_NAME));
     bi->terms = g_list_remove (bi->terms, term);
 }
 
@@ -250,7 +250,7 @@ GncBillTerm * gncBillTermCreate (QofBook *book)
     GncBillTerm *term;
     if (!book) return NULL;
 
-    term = g_object_new (GNC_TYPE_BILLTERM, NULL);
+    term = GNC_BILLTERM(g_object_new (GNC_TYPE_BILLTERM, NULL));
     qof_instance_init_data(&term->inst, _GNC_MOD_NAME, book);
     term->name = CACHE_INSERT ("");
     term->desc = CACHE_INSERT ("");
@@ -293,7 +293,7 @@ static void gncBillTermFree (GncBillTerm *term)
     /* disconnect from the children */
     for (list = term->children; list; list = list->next)
     {
-        child = list->data;
+        child = GNC_BILLTERM(list->data);
         gncBillTermSetParent(child, NULL);
     }
     g_list_free(term->children);
@@ -334,7 +334,18 @@ void gncBillTermSetType (GncBillTerm *term, GncBillTermType type)
 }
 
 /** \brief Convert bill term types from text. */
-FROM_STRING_FUNC(GncBillTermType, ENUM_TERMS_TYPE)
+GncBillTermType GncBillTermTypefromString (const char *str)
+{
+    if (str)
+    {
+        if (!strcmp (str, "GNC_TERM_TYPE_DAYS"))
+            return GNC_TERM_TYPE_DAYS;
+        else if (!strcmp (str, "GNC_TERM_TYPE_PROXIMO"))
+            return GNC_TERM_TYPE_PROXIMO;
+    }
+    PERR ("str cannot be %s", str);
+    return GNC_TERM_TYPE_DAYS;
+}
 
 static
 void qofBillTermSetType (GncBillTerm *term, const char *type_label)
@@ -497,9 +508,9 @@ GncBillTerm *gncBillTermLookupByName (QofBook *book, const char *name)
 
     for ( ; list; list = list->next)
     {
-        GncBillTerm *term = list->data;
+        GncBillTerm *term = GNC_BILLTERM(list->data);
         if (!g_strcmp0 (term->name, name))
-            return list->data;
+            return term;
     }
     return NULL;
 }
@@ -509,7 +520,7 @@ GList * gncBillTermGetTerms (QofBook *book)
     struct _book_info *bi;
     if (!book) return NULL;
 
-    bi = qof_book_get_data (book, _GNC_MOD_NAME);
+    bi = static_cast<_book_info*>(qof_book_get_data (book, _GNC_MOD_NAME));
     return bi->terms;
 }
 
@@ -527,7 +538,11 @@ const char *gncBillTermGetDescription (const GncBillTerm *term)
 
 GncBillTermType gncBillTermGetType (const GncBillTerm *term)
 {
-    if (!term) return 0;
+    if (!term)
+    {
+        PERR ("term cannot be null");
+        return GNC_TERM_TYPE_DAYS;
+    }
     return term->type;
 }
 
@@ -856,7 +871,7 @@ static void _gncBillTermDestroy (QofBook *book)
 
     if (!book) return;
 
-    bi = qof_book_get_data (book, _GNC_MOD_NAME);
+    bi = static_cast<_book_info*>(qof_book_get_data (book, _GNC_MOD_NAME));
 
     col = qof_book_get_collection (book, GNC_ID_BILLTERM);
     qof_collection_foreach (col, destroy_billterm_on_book_close, NULL);
@@ -870,7 +885,7 @@ static QofObject gncBillTermDesc =
     DI(.interface_version = ) QOF_OBJECT_VERSION,
     DI(.e_type            = ) _GNC_MOD_NAME,
     DI(.type_label        = ) "Billing Term",
-    DI(.create            = ) (gpointer)gncBillTermCreate,
+    DI(.create            = ) (void* (*)(QofBook*))gncBillTermCreate,
     DI(.book_begin        = ) _gncBillTermCreate,
     DI(.book_end          = ) _gncBillTermDestroy,
     DI(.is_dirty          = ) qof_collection_is_dirty,
diff --git a/libgnucash/engine/gncCustomer.c b/libgnucash/engine/gncCustomer.cpp
similarity index 99%
rename from libgnucash/engine/gncCustomer.c
rename to libgnucash/engine/gncCustomer.cpp
index 9786b3dd7a9..ca4e7e81221 100644
--- a/libgnucash/engine/gncCustomer.c
+++ b/libgnucash/engine/gncCustomer.cpp
@@ -306,7 +306,7 @@ GncCustomer *gncCustomerCreate (QofBook *book)
 
     if (!book) return NULL;
 
-    cust = g_object_new (GNC_TYPE_CUSTOMER, NULL);
+    cust = GNC_CUSTOMER(g_object_new (GNC_TYPE_CUSTOMER, NULL));
     qof_instance_init_data (&cust->inst, _GNC_MOD_NAME, book);
 
     cust->id = CACHE_INSERT ("");
@@ -700,7 +700,7 @@ GList * gncCustomerGetJoblist (const GncCustomer *cust, gboolean show_all)
         GList *list = NULL, *iterator;
         for (iterator = cust->jobs; iterator; iterator = iterator->next)
         {
-            GncJob *j = iterator->data;
+            auto j = GNC_JOB(iterator->data);
             if (gncJobGetActive (j))
                 list = g_list_prepend (list, j);
         }
@@ -913,7 +913,7 @@ static QofObject gncCustomerDesc =
     DI(.interface_version = ) QOF_OBJECT_VERSION,
     DI(.e_type            = ) _GNC_MOD_NAME,
     DI(.type_label        = ) "Customer",
-    DI(.create            = ) (gpointer)gncCustomerCreate,
+    DI(.create            = ) (void* (*)(QofBook*))gncCustomerCreate,
     DI(.book_begin        = ) NULL,
     DI(.book_end          = ) gnc_customer_book_end,
     DI(.is_dirty          = ) qof_collection_is_dirty,
diff --git a/libgnucash/engine/gncEmployee.c b/libgnucash/engine/gncEmployee.cpp
similarity index 98%
rename from libgnucash/engine/gncEmployee.c
rename to libgnucash/engine/gncEmployee.cpp
index fd9518f88d4..e070dba38b6 100644
--- a/libgnucash/engine/gncEmployee.c
+++ b/libgnucash/engine/gncEmployee.cpp
@@ -205,13 +205,13 @@ gnc_employee_set_property (GObject         *object,
         gncEmployeeSetLanguage(emp, g_value_get_string(value));
         break;
     case PROP_CURRENCY:
-        gncEmployeeSetCurrency(emp, g_value_get_object(value));
+        gncEmployeeSetCurrency(emp, GNC_COMMODITY(g_value_get_object(value)));
         break;
     case PROP_ACL:
         gncEmployeeSetAcl(emp, g_value_get_string(value));
         break;
     case PROP_ADDRESS:
-        qofEmployeeSetAddr(emp, g_value_get_object(value));
+        qofEmployeeSetAddr(emp, QOF_INSTANCE(g_value_get_object(value)));
         break;
     case PROP_WORKDAY:
         gncEmployeeSetWorkday(emp, *(gnc_numeric*)g_value_get_boxed(value));
@@ -220,7 +220,7 @@ gnc_employee_set_property (GObject         *object,
         gncEmployeeSetRate(emp, *(gnc_numeric*)g_value_get_boxed(value));
         break;
     case PROP_CCARD:
-        gncEmployeeSetCCard(emp, g_value_get_object(value));
+        gncEmployeeSetCCard(emp, GNC_ACCOUNT(g_value_get_object(value)));
         break;
     case PROP_PDF_DIRNAME:
         qof_instance_set_kvp (QOF_INSTANCE (emp), value, 1, OWNER_EXPORT_PDF_DIRNAME);
@@ -431,7 +431,7 @@ GncEmployee *gncEmployeeCreate (QofBook *book)
 
     if (!book) return NULL;
 
-    employee = g_object_new (GNC_TYPE_EMPLOYEE, NULL);
+    employee = GNC_EMPLOYEE(g_object_new (GNC_TYPE_EMPLOYEE, NULL));
     qof_instance_init_data (&employee->inst, _GNC_MOD_NAME, book);
 
     employee->id = CACHE_INSERT ("");
@@ -809,7 +809,7 @@ gboolean gncEmployeeEqual(const GncEmployee* a, const GncEmployee* b)
 
 static const char * _gncEmployeePrintable (gpointer item)
 {
-    GncEmployee *v = item;
+    auto v = GNC_EMPLOYEE(item);
     if (!item) return NULL;
     return gncAddressGetName(v->addr);
 }
@@ -899,7 +899,7 @@ static QofObject gncEmployeeDesc =
     DI(.interface_version = ) QOF_OBJECT_VERSION,
     DI(.e_type            = ) _GNC_MOD_NAME,
     DI(.type_label        = ) "Employee",
-    DI(.create            = ) (gpointer)gncEmployeeCreate,
+    DI(.create            = ) (void* (*)(QofBook*))gncEmployeeCreate,
     DI(.book_begin        = ) NULL,
     DI(.book_end          = ) gnc_employee_book_end,
     DI(.is_dirty          = ) qof_collection_is_dirty,
diff --git a/libgnucash/engine/gncEntry.c b/libgnucash/engine/gncEntry.cpp
similarity index 98%
rename from libgnucash/engine/gncEntry.c
rename to libgnucash/engine/gncEntry.cpp
index 78e6cd972d6..8e056e293eb 100644
--- a/libgnucash/engine/gncEntry.c
+++ b/libgnucash/engine/gncEntry.cpp
@@ -417,7 +417,7 @@ GncEntry *gncEntryCreate (QofBook *book)
 
     if (!book) return NULL;
 
-    entry = g_object_new (GNC_TYPE_ENTRY, NULL);
+    entry = GNC_ENTRY(g_object_new (GNC_TYPE_ENTRY, NULL));
     qof_instance_init_data (&entry->inst, _GNC_MOD_NAME, book);
 
     entry->desc = CACHE_INSERT ("");
@@ -977,13 +977,21 @@ gnc_numeric gncEntryGetInvDiscount (const GncEntry *entry)
 
 GncAmountType gncEntryGetInvDiscountType (const GncEntry *entry)
 {
-    if (!entry) return 0;
+    if (!entry)
+    {
+        PERR ("entry must not be null");
+        return GNC_AMT_TYPE_VALUE;
+    }
     return entry->i_disc_type;
 }
 
 GncDiscountHow gncEntryGetInvDiscountHow (const GncEntry *entry)
 {
-    if (!entry) return 0;
+    if (!entry)
+    {
+        PERR ("entry must not be null");
+        return GNC_DISC_PRETAX;
+    }
     return entry->i_disc_how;
 }
 
@@ -1069,7 +1077,11 @@ GncOwner * gncEntryGetBillTo (GncEntry *entry)
 
 GncEntryPaymentType gncEntryGetBillPayment (const GncEntry* entry)
 {
-    if (!entry) return 0;
+    if (!entry)
+    {
+        PERR ("entry must not be null");
+        return GNC_PAYMENT_CASH;
+    }
     return entry->b_payment;
 }
 
@@ -1158,7 +1170,7 @@ static void gncEntryComputeValueInt (gnc_numeric qty, gnc_numeric price,
     /* First, compute the aggregate tpercent and tvalue numbers */
     for (node = entries; node; node = node->next)
     {
-        GncTaxTableEntry *entry = node->data;
+        auto entry = static_cast<GncTaxTableEntry*>(node->data);
         gnc_numeric amount = gncTaxTableEntryGetAmount (entry);
 
         switch (gncTaxTableEntryGetType (entry))
@@ -1296,7 +1308,7 @@ static void gncEntryComputeValueInt (gnc_numeric qty, gnc_numeric price,
         PINFO("Computing tax value list");
         for (node = entries; node; node = node->next)
         {
-            GncTaxTableEntry *entry = node->data;
+            auto entry = static_cast<GncTaxTableEntry*>(node->data);
             Account *acc = gncTaxTableEntryGetAccount (entry);
             gnc_numeric amount = gncTaxTableEntryGetAmount (entry);
 
@@ -1436,7 +1448,7 @@ gncEntryRecomputeValues (GncEntry *entry)
     entry->i_tax_value_rounded = gnc_numeric_zero();
     for (tv_iter = entry->i_tax_values; tv_iter; tv_iter=tv_iter->next)
     {
-        GncAccountValue *acc_val = tv_iter->data;
+        auto acc_val = static_cast<GncAccountValue*>(tv_iter->data);
         entry->i_tax_value_rounded = gnc_numeric_add (entry->i_tax_value_rounded, acc_val->value,
                                      denom, GNC_HOW_DENOM_EXACT | GNC_HOW_RND_ROUND_HALF_UP);
     }
@@ -1447,7 +1459,7 @@ gncEntryRecomputeValues (GncEntry *entry)
     entry->b_tax_value_rounded = gnc_numeric_zero();
     for (tv_iter = entry->b_tax_values; tv_iter; tv_iter=tv_iter->next)
     {
-        GncAccountValue *acc_val = tv_iter->data;
+        auto acc_val = static_cast<GncAccountValue*>(tv_iter->data);
         entry->b_tax_value_rounded = gnc_numeric_add (entry->b_tax_value_rounded, acc_val->value,
                                                       denom, GNC_HOW_DENOM_EXACT | GNC_HOW_RND_ROUND_HALF_UP);
     }
@@ -1553,7 +1565,7 @@ AccountValueList * gncEntryGetDocTaxValues (GncEntry *entry, gboolean is_cust_do
     /* Make a copy of the list with negated values if necessary. */
     for (node = int_values; node; node = node->next)
     {
-        GncAccountValue *acct_val = node->data;
+        auto acct_val = static_cast<GncAccountValue*>(node->data);
         values = gncAccountValueAdd (values, acct_val->account,
                                      (is_cn ? gnc_numeric_neg (acct_val->value)
                                       : acct_val->value));
@@ -1589,7 +1601,7 @@ AccountValueList * gncEntryGetBalTaxValues (GncEntry *entry, gboolean is_cust_do
     /* Make a copy of the list with negated values if necessary. */
     for (node = int_values; node; node = node->next)
     {
-        GncAccountValue *acct_val = node->data;
+        auto acct_val = static_cast<GncAccountValue*>(node->data);
         values = gncAccountValueAdd (values, acct_val->account,
                                      (is_cust_doc ? gnc_numeric_neg (acct_val->value)
                                       : acct_val->value));
@@ -1723,7 +1735,7 @@ static QofObject gncEntryDesc =
     DI(.interface_version = ) QOF_OBJECT_VERSION,
     DI(.e_type            = ) _GNC_MOD_NAME,
     DI(.type_label        = ) "Order/Invoice/Bill Entry",
-    DI(.create            = ) (gpointer)gncEntryCreate,
+    DI(.create            = ) (void* (*)(QofBook*))gncEntryCreate,
     DI(.book_begin        = ) NULL,
     DI(.book_end          = ) gnc_entry_book_end,
     DI(.is_dirty          = ) qof_collection_is_dirty,
diff --git a/libgnucash/engine/gncInvoice.c b/libgnucash/engine/gncInvoice.cpp
similarity index 97%
rename from libgnucash/engine/gncInvoice.c
rename to libgnucash/engine/gncInvoice.cpp
index eb4ab1db8da..fb40ad5cb03 100644
--- a/libgnucash/engine/gncInvoice.c
+++ b/libgnucash/engine/gncInvoice.cpp
@@ -47,6 +47,8 @@
 #include "gncOwnerP.h"
 #include "engine-helpers.h"
 
+#include <optional>
+
 struct _gncInvoice
 {
     QofInstance   inst;
@@ -317,7 +319,7 @@ GncInvoice *gncInvoiceCreate (QofBook *book)
 
     if (!book) return NULL;
 
-    invoice = g_object_new (GNC_TYPE_INVOICE, NULL);
+    invoice = GNC_INVOICE(g_object_new (GNC_TYPE_INVOICE, NULL));
     qof_instance_init_data (&invoice->inst, _GNC_MOD_NAME, book);
 
     invoice->id = CACHE_INSERT ("");
@@ -345,7 +347,7 @@ GncInvoice *gncInvoiceCopy (const GncInvoice *from)
     book = qof_instance_get_book (from);
     g_assert (book);
 
-    invoice = g_object_new (GNC_TYPE_INVOICE, NULL);
+    invoice = GNC_INVOICE(g_object_new (GNC_TYPE_INVOICE, NULL));
     qof_instance_init_data (&invoice->inst, _GNC_MOD_NAME, book);
 
     gncInvoiceBeginEdit (invoice);
@@ -378,7 +380,7 @@ GncInvoice *gncInvoiceCopy (const GncInvoice *from)
     // Copy all invoice->entries
     for (node = from->entries; node; node = node->next)
     {
-        GncEntry *from_entry = node->data;
+        auto from_entry = GNC_ENTRY(node->data);
         GncEntry *to_entry = gncEntryCreate (book);
         gncEntryCopy (from_entry, to_entry, FALSE);
 
@@ -703,7 +705,7 @@ void gncInvoiceAddPrice (GncInvoice *invoice, GNCPrice *price)
     commodity = gnc_price_get_commodity (price);
     while (node != NULL)
     {
-        GNCPrice *curr = (GNCPrice*)node->data;
+        auto curr = GNC_PRICE(node->data);
         if (gnc_commodity_equal (commodity, gnc_price_get_commodity (curr)))
             break;
         node = g_list_next (node);
@@ -767,7 +769,7 @@ void gncInvoiceRemoveEntries (GncInvoice *invoice)
     for (GList *next, *node = invoice->entries; node; node = next)
     {
         next = node->next;
-        GncEntry *entry = node->data;
+        auto entry = GNC_ENTRY(node->data);
 
         switch (gncInvoiceGetOwnerType (invoice))
         {
@@ -907,7 +909,7 @@ static gnc_numeric gncInvoiceSumTaxesInternal (AccountValueList *taxes)
         // to the desired denom and addition will just preserve it in that case.
         for (node = taxes; node; node=node->next)
         {
-            GncAccountValue *acc_val = node->data;
+            auto acc_val = static_cast<GncAccountValue*>(node->data);
             tt = gnc_numeric_add (tt, acc_val->value, GNC_DENOM_AUTO,
                                   GNC_HOW_DENOM_EXACT | GNC_HOW_RND_ROUND_HALF_UP);
         }
@@ -917,8 +919,7 @@ static gnc_numeric gncInvoiceSumTaxesInternal (AccountValueList *taxes)
 
 static gnc_numeric gncInvoiceGetNetAndTaxesInternal (GncInvoice *invoice, gboolean use_value,
                                                      AccountValueList **taxes,
-                                                     gboolean use_payment_type,
-                                                     GncEntryPaymentType type)
+                                                     std::optional<GncEntryPaymentType> type)
 {
     GList *node;
     gnc_numeric net_total = gnc_numeric_zero ();
@@ -938,10 +939,10 @@ static gnc_numeric gncInvoiceGetNetAndTaxesInternal (GncInvoice *invoice, gboole
 
     for (node = gncInvoiceGetEntries (invoice); node; node = node->next)
     {
-        GncEntry *entry = node->data;
+        auto entry = GNC_ENTRY(node->data);
         gnc_numeric value;
 
-        if (use_payment_type && gncEntryGetBillPayment (entry) != type)
+        if (type.has_value() && gncEntryGetBillPayment (entry) != type.value())
             continue;
 
         if (use_value)
@@ -970,7 +971,7 @@ static gnc_numeric gncInvoiceGetNetAndTaxesInternal (GncInvoice *invoice, gboole
         // which could otherwise happen when using a tax table with multiple tax rates
         for (node = tv_list; node; node=node->next)
         {
-            GncAccountValue *acc_val = node->data;
+            auto acc_val = static_cast<GncAccountValue*>(node->data);
             acc_val->value = gnc_numeric_convert (acc_val->value,
                                   denom, GNC_HOW_DENOM_EXACT | GNC_HOW_RND_ROUND_HALF_UP);
         }
@@ -983,7 +984,7 @@ static gnc_numeric gncInvoiceGetNetAndTaxesInternal (GncInvoice *invoice, gboole
 
 static gnc_numeric gncInvoiceGetTotalInternal (GncInvoice *invoice, gboolean use_value,
                                                gboolean use_tax,
-                                               gboolean use_payment_type, GncEntryPaymentType type)
+                                               std::optional<GncEntryPaymentType> type)
 {
     AccountValueList *taxes;
     gnc_numeric total;
@@ -991,7 +992,7 @@ static gnc_numeric gncInvoiceGetTotalInternal (GncInvoice *invoice, gboolean use
     if (!invoice) return gnc_numeric_zero ();
 
     ENTER ("");
-    total = gncInvoiceGetNetAndTaxesInternal (invoice, use_value, use_tax? &taxes : NULL, use_payment_type, type);
+    total = gncInvoiceGetNetAndTaxesInternal (invoice, use_value, use_tax? &taxes : NULL, type);
 
     if (use_tax)
     {
@@ -1009,25 +1010,25 @@ static gnc_numeric gncInvoiceGetTotalInternal (GncInvoice *invoice, gboolean use
 gnc_numeric gncInvoiceGetTotal (GncInvoice *invoice)
 {
     if (!invoice) return gnc_numeric_zero ();
-    return gncInvoiceGetTotalInternal (invoice, TRUE, TRUE, FALSE, 0);
+    return gncInvoiceGetTotalInternal (invoice, TRUE, TRUE, {});
 }
 
 gnc_numeric gncInvoiceGetTotalSubtotal (GncInvoice *invoice)
 {
     if (!invoice) return gnc_numeric_zero ();
-    return gncInvoiceGetTotalInternal (invoice, TRUE, FALSE, FALSE, 0);
+    return gncInvoiceGetTotalInternal (invoice, TRUE, FALSE, {});
 }
 
 gnc_numeric gncInvoiceGetTotalTax (GncInvoice *invoice)
 {
     if (!invoice) return gnc_numeric_zero ();
-    return gncInvoiceGetTotalInternal (invoice, FALSE, TRUE, FALSE, 0);
+    return gncInvoiceGetTotalInternal (invoice, FALSE, TRUE, {});
 }
 
 gnc_numeric gncInvoiceGetTotalOf (GncInvoice *invoice, GncEntryPaymentType type)
 {
     if (!invoice) return gnc_numeric_zero ();
-    return gncInvoiceGetTotalInternal (invoice, TRUE, TRUE, TRUE, type);
+    return gncInvoiceGetTotalInternal (invoice, TRUE, TRUE, type);
 }
 
 AccountValueList *gncInvoiceGetTotalTaxList (GncInvoice *invoice)
@@ -1035,7 +1036,7 @@ AccountValueList *gncInvoiceGetTotalTaxList (GncInvoice *invoice)
     AccountValueList *taxes;
     if (!invoice) return NULL;
 
-    gncInvoiceGetNetAndTaxesInternal (invoice, FALSE, &taxes, FALSE, 0);
+    gncInvoiceGetNetAndTaxesInternal (invoice, FALSE, &taxes, {});
     return taxes;
 }
 
@@ -1174,7 +1175,7 @@ GNCPrice * gncInvoiceGetPrice (GncInvoice *invoice, gnc_commodity *commodity)
 
     while (node != NULL)
     {
-        GNCPrice *curr = (GNCPrice*)node->data;
+        auto curr = GNC_PRICE(node->data);
 
         if (gnc_commodity_equal (commodity, gnc_price_get_commodity (curr)))
             return curr;
@@ -1371,7 +1372,7 @@ GHashTable *gncInvoiceGetForeignCurrencies (const GncInvoice *invoice)
 
     for (entries_iter = invoice->entries; entries_iter != NULL; entries_iter = g_list_next(entries_iter))
     {
-        GncEntry *entry = (GncEntry*)entries_iter->data;
+        auto entry = GNC_ENTRY(entries_iter->data);
         Account *this_acc;
         gnc_commodity *account_currency;
         AccountValueList *tt_amts = NULL, *tt_iter;
@@ -1401,7 +1402,7 @@ GHashTable *gncInvoiceGetForeignCurrencies (const GncInvoice *invoice)
 
         for (tt_iter = tt_amts; tt_iter != NULL; tt_iter = g_list_next(tt_iter))
         {
-            GncAccountValue *tt_amt_val = (GncAccountValue*)tt_iter->data;
+            auto tt_amt_val = static_cast<GncAccountValue*>(tt_iter->data);
             Account *tt_acc = tt_amt_val->account;
             gnc_commodity *tt_acc_currency = xaccAccountGetCommodity (tt_acc);
 
@@ -1580,7 +1581,7 @@ Transaction * gncInvoicePostToAccount (GncInvoice *invoice, Account *acc,
         total = gnc_numeric_neg (total);
         for (node = taxes; node; node = node->next)
         {
-            GncAccountValue *acc_val = node->data;
+            auto acc_val = static_cast<GncAccountValue*>(node->data);
             acc_val->value = gnc_numeric_neg (acc_val->value);
         }
     }
@@ -1592,7 +1593,7 @@ Transaction * gncInvoicePostToAccount (GncInvoice *invoice, Account *acc,
     for (iter = gncInvoiceGetEntries (invoice); iter; iter = iter->next)
     {
         gnc_numeric value, tax;
-        GncEntry * entry = iter->data;
+        auto entry = GNC_ENTRY(iter->data);
         Account *this_acc;
 
         /* Stabilize the TaxTable in this entry */
@@ -1695,7 +1696,7 @@ Transaction * gncInvoicePostToAccount (GncInvoice *invoice, Account *acc,
         PINFO ("Processing Split List");
     for (iter = splitinfo; iter; iter = iter->next)
     {
-        GncAccountValue *acc_val = iter->data;
+        auto acc_val = static_cast<GncAccountValue*>(iter->data);
 
         //gnc_numeric amt_rounded = gnc_numeric_convert(acc_val->value,
         //    denom, GNC_HOW_DENOM_EXACT | GNC_HOW_RND_ROUND_HALF_UP);
@@ -1833,7 +1834,7 @@ gncInvoiceUnpost (GncInvoice *invoice, gboolean reset_tax_tables)
         PINFO ("Recreating link transactions for remaining lots");
     for (lot_split_iter = lot_split_list; lot_split_iter; lot_split_iter = lot_split_iter->next)
     {
-        Split *split = lot_split_iter->data;
+        auto split = GNC_SPLIT(lot_split_iter->data);
         GList *other_split_list, *list_iter;
         Transaction *other_txn = xaccSplitGetParent (split);
         GList *lot_list = NULL;
@@ -1847,7 +1848,7 @@ gncInvoiceUnpost (GncInvoice *invoice, gboolean reset_tax_tables)
         other_split_list = xaccTransGetSplitList (other_txn);
         for (list_iter = other_split_list; list_iter; list_iter = list_iter->next)
         {
-            Split *other_split = list_iter->data;
+            auto other_split = GNC_SPLIT(list_iter->data);
             GNCLot *other_lot = xaccSplitGetLot (other_split);
 
             /* Omit the lot we are about to delete */
@@ -1873,7 +1874,7 @@ gncInvoiceUnpost (GncInvoice *invoice, gboolean reset_tax_tables)
          * send it a modified event to reset its paid status */
         for (list_iter = lot_list; list_iter; list_iter = list_iter->next)
         {
-            GNCLot *other_lot = list_iter->data;
+            auto other_lot = GNC_LOT(list_iter->data);
             GncInvoice *other_invoice = gncInvoiceGetInvoiceFromLot (other_lot);
 
             if (!gnc_lot_count_splits (other_lot))
@@ -1905,7 +1906,7 @@ gncInvoiceUnpost (GncInvoice *invoice, gboolean reset_tax_tables)
 
         for (iter = gncInvoiceGetEntries (invoice); iter; iter = iter->next)
         {
-            GncEntry *entry = iter->data;
+            auto entry = GNC_ENTRY(iter->data);
 
             gncEntryBeginEdit (entry);
             if (is_cust_doc)
@@ -1935,7 +1936,7 @@ struct lotmatch
 static gboolean
 gnc_lot_match_owner_balancing (GNCLot *lot, gpointer user_data)
 {
-    struct lotmatch *lm = user_data;
+    struct lotmatch *lm = static_cast<struct lotmatch *>(user_data);
     GncOwner owner_def;
     const GncOwner *owner;
     gnc_numeric balance = gnc_lot_get_balance (lot);
@@ -2185,7 +2186,7 @@ gboolean gncInvoiceEqual(const GncInvoice *a, const GncInvoice *b)
 
 static const char * _gncInvoicePrintable (gpointer obj)
 {
-    GncInvoice *invoice = obj;
+    auto invoice = GNC_INVOICE(obj);
 
     g_return_val_if_fail (invoice, NULL);
 
@@ -2224,7 +2225,7 @@ static QofObject gncInvoiceDesc =
     DI(.interface_version = ) QOF_OBJECT_VERSION,
     DI(.e_type            = ) _GNC_MOD_NAME,
     DI(.type_label        = ) "Invoice",
-    DI(.create            = ) (gpointer)gncInvoiceCreate,
+    DI(.create            = ) (void* (*)(QofBook*))gncInvoiceCreate,
     DI(.book_begin        = ) NULL,
     DI(.book_end          = ) gnc_invoice_book_end,
     DI(.is_dirty          = ) qof_collection_is_dirty,
diff --git a/libgnucash/engine/gncJob.c b/libgnucash/engine/gncJob.cpp
similarity index 99%
rename from libgnucash/engine/gncJob.c
rename to libgnucash/engine/gncJob.cpp
index e1e55a09e4a..ca641a45a14 100644
--- a/libgnucash/engine/gncJob.c
+++ b/libgnucash/engine/gncJob.cpp
@@ -215,7 +215,7 @@ GncJob *gncJobCreate (QofBook *book)
 
     if (!book) return NULL;
 
-    job = g_object_new (GNC_TYPE_JOB, NULL);
+    job = GNC_JOB(g_object_new (GNC_TYPE_JOB, NULL));
     qof_instance_init_data (&job->inst, _GNC_MOD_NAME, book);
 
     job->id = CACHE_INSERT ("");
@@ -556,7 +556,7 @@ static const char * _gncJobPrintable (gpointer item)
 {
     GncJob *c;
     if (!item) return NULL;
-    c = item;
+    c = GNC_JOB(item);
     return c->name;
 }
 
@@ -565,7 +565,7 @@ static QofObject gncJobDesc =
     DI(.interface_version = ) QOF_OBJECT_VERSION,
     DI(.e_type            = ) _GNC_MOD_NAME,
     DI(.type_label        = ) "Job",
-    DI(.create            = ) (gpointer)gncJobCreate,
+    DI(.create            = ) (void* (*)(QofBook*))gncJobCreate,
     DI(.book_begin        = ) NULL,
     DI(.book_end          = ) NULL,
     DI(.is_dirty          = ) qof_collection_is_dirty,
diff --git a/libgnucash/engine/gncOrder.c b/libgnucash/engine/gncOrder.cpp
similarity index 99%
rename from libgnucash/engine/gncOrder.c
rename to libgnucash/engine/gncOrder.cpp
index 21748717a90..1f7c09d5fe7 100644
--- a/libgnucash/engine/gncOrder.c
+++ b/libgnucash/engine/gncOrder.cpp
@@ -284,7 +284,7 @@ GncOrder *gncOrderCreate (QofBook *book)
 
     if (!book) return NULL;
 
-    order = g_object_new (GNC_TYPE_ORDER, NULL);
+    order = GNC_ORDER(g_object_new (GNC_TYPE_ORDER, NULL));
     qof_instance_init_data (&order->inst, _GNC_MOD_NAME, book);
 
     order->id = CACHE_INSERT ("");
@@ -533,7 +533,7 @@ int gncOrderCompare (const GncOrder *a, const GncOrder *b)
 static const char *
 _gncOrderPrintable (gpointer obj)
 {
-    GncOrder *order = obj;
+    GncOrder *order = GNC_ORDER(obj);
 
     g_return_val_if_fail (order, NULL);
 
@@ -554,7 +554,7 @@ static QofObject gncOrderDesc =
     DI(.interface_version = ) QOF_OBJECT_VERSION,
     DI(.e_type            = ) _GNC_MOD_NAME,
     DI(.type_label        = ) "Order",
-    DI(.create            = ) (gpointer)gncOrderCreate,
+    DI(.create            = ) (void* (*)(QofBook*))gncOrderCreate,
     DI(.book_begin        = ) NULL,
     DI(.book_end          = ) NULL,
     DI(.is_dirty          = ) qof_collection_is_dirty,
diff --git a/libgnucash/engine/gncOwner.c b/libgnucash/engine/gncOwner.cpp
similarity index 98%
rename from libgnucash/engine/gncOwner.c
rename to libgnucash/engine/gncOwner.cpp
index 6441f5fbc16..48a103fa6da 100644
--- a/libgnucash/engine/gncOwner.c
+++ b/libgnucash/engine/gncOwner.cpp
@@ -705,7 +705,7 @@ gboolean gncOwnerIsValid (const GncOwner *owner)
 gboolean
 gncOwnerLotMatchOwnerFunc (GNCLot *lot, gpointer user_data)
 {
-    const GncOwner *req_owner = user_data;
+    auto req_owner = static_cast<const GncOwner*>(user_data);
     GncOwner lot_owner;
     const GncOwner *end_owner;
     GncInvoice *invoice = gncInvoiceGetInvoiceFromLot (lot);
@@ -905,7 +905,7 @@ Split *gncOwnerFindOffsettingSplit (GNCLot *lot, gnc_numeric target_amount)
 
     for (ls_iter = gnc_lot_get_split_list (lot); ls_iter; ls_iter = ls_iter->next)
     {
-        Split *split = ls_iter->data;
+        auto split = GNC_SPLIT(ls_iter->data);
 
         if (!split)
             continue;
@@ -1024,7 +1024,7 @@ gncOwnerSetLotLinkMemo (Transaction *ll_txn)
     // Find all splits in the lot link transaction that are also in a document lot
     for (lts_iter = xaccTransGetSplitList (ll_txn); lts_iter; lts_iter = lts_iter->next)
     {
-        Split *split = lts_iter->data;
+        auto split = GNC_SPLIT(lts_iter->data);
         GNCLot *lot;
         GncInvoice *invoice;
         gchar *title;
@@ -1052,10 +1052,10 @@ gncOwnerSetLotLinkMemo (Transaction *ll_txn)
     titles = g_list_sort (titles, (GCompareFunc)g_strcmp0);
 
     // Create the memo as we'd want it to be
-    new_memo = g_strconcat (memo_prefix, titles->data, NULL);
+    new_memo = g_strconcat (memo_prefix, static_cast<const char*>(titles->data), NULL);
     for (titer = titles->next; titer; titer = titer->next)
     {
-        gchar *tmp_memo = g_strconcat (new_memo, " - ", titer->data, NULL);
+        gchar *tmp_memo = g_strconcat (new_memo, " - ", static_cast<const char*>(titer->data), NULL);
         g_free (new_memo);
         new_memo = tmp_memo;
     }
@@ -1064,8 +1064,8 @@ gncOwnerSetLotLinkMemo (Transaction *ll_txn)
     // Update the memos of all the splits we found previously (if needed)
     for (siter = splits; siter; siter = siter->next)
     {
-        if (g_strcmp0 (xaccSplitGetMemo (siter->data), new_memo) != 0)
-            xaccSplitSetMemo (siter->data, new_memo);
+        if (g_strcmp0 (xaccSplitGetMemo (GNC_SPLIT(siter->data)), new_memo) != 0)
+            xaccSplitSetMemo (GNC_SPLIT(siter->data), new_memo);
     }
 
     g_list_free (splits);
@@ -1091,7 +1091,7 @@ get_ll_transaction_from_lot (GNCLot *lot)
      */
     for (ls_iter = gnc_lot_get_split_list (lot); ls_iter; ls_iter = ls_iter->next)
     {
-        Split *ls = ls_iter->data;
+        auto ls = GNC_SPLIT(ls_iter->data);
         Transaction *ll_txn = xaccSplitGetParent (ls);
         SplitList *ts_iter;
 
@@ -1100,7 +1100,7 @@ get_ll_transaction_from_lot (GNCLot *lot)
 
         for (ts_iter = xaccTransGetSplitList (ll_txn); ts_iter; ts_iter = ts_iter->next)
         {
-            Split *ts = ts_iter->data;
+            auto ts = GNC_SPLIT(ts_iter->data);
             GNCLot *tslot = xaccSplitGetLot (ts);
 
             if (!tslot)
@@ -1271,7 +1271,7 @@ void gncOwnerAutoApplyPaymentsWithLots (const GncOwner *owner, GList *lots)
 
     for (left_iter = lots; left_iter; left_iter = left_iter->next)
     {
-        GNCLot *left_lot = left_iter->data;
+        auto left_lot = GNC_LOT(left_iter->data);
         gnc_numeric left_lot_bal;
         gboolean left_lot_has_doc;
         gboolean left_modified = FALSE;
@@ -1305,7 +1305,7 @@ void gncOwnerAutoApplyPaymentsWithLots (const GncOwner *owner, GList *lots)
          */
         for (right_iter = left_iter->next; right_iter; right_iter = right_iter->next)
         {
-            GNCLot *right_lot = right_iter->data;
+            auto right_lot = GNC_LOT(right_iter->data);
             gnc_numeric right_lot_bal;
             gboolean right_lot_has_doc;
 
@@ -1502,7 +1502,7 @@ gncOwnerGetBalanceInCurrency (const GncOwner *owner,
         /* For each account */
         for (acct_node = acct_list; acct_node; acct_node = acct_node->next)
         {
-            Account *account = acct_node->data;
+            auto account = GNC_ACCOUNT(acct_node->data);
             GList *lot_list = NULL, *lot_node;
 
             /* Check if this account can have lots for the owner, otherwise skip to next */
@@ -1520,7 +1520,7 @@ gncOwnerGetBalanceInCurrency (const GncOwner *owner,
             /* For each lot */
             for (lot_node = lot_list; lot_node; lot_node = lot_node->next)
             {
-                GNCLot *lot = lot_node->data;
+                auto lot = GNC_LOT(lot_node->data);
                 gnc_numeric lot_balance = gnc_lot_get_balance (lot);
                 GncInvoice *invoice = gncInvoiceGetInvoiceFromLot(lot);
                 if (invoice)
diff --git a/libgnucash/engine/gncTaxTable.c b/libgnucash/engine/gncTaxTable.cpp
similarity index 95%
rename from libgnucash/engine/gncTaxTable.c
rename to libgnucash/engine/gncTaxTable.cpp
index fec75d7aed9..a74d964ab6a 100644
--- a/libgnucash/engine/gncTaxTable.c
+++ b/libgnucash/engine/gncTaxTable.cpp
@@ -154,7 +154,7 @@ maybe_resort_list (GncTaxTable *table)
     struct _book_info *bi;
 
     if (table->parent || table->invisible) return;
-    bi = qof_book_get_data (qof_instance_get_book(table), _GNC_MOD_NAME);
+    bi = static_cast<_book_info*>(qof_book_get_data (qof_instance_get_book(table), _GNC_MOD_NAME));
     bi->tables = g_list_sort (bi->tables, (GCompareFunc)gncTaxTableCompare);
 }
 
@@ -167,7 +167,7 @@ mod_table (GncTaxTable *table)
 static inline void addObj (GncTaxTable *table)
 {
     struct _book_info *bi;
-    bi = qof_book_get_data (qof_instance_get_book(table), _GNC_MOD_NAME);
+    bi = static_cast<_book_info*>(qof_book_get_data (qof_instance_get_book(table), _GNC_MOD_NAME));
     bi->tables = g_list_insert_sorted (bi->tables, table,
                                        (GCompareFunc)gncTaxTableCompare);
 }
@@ -175,7 +175,7 @@ static inline void addObj (GncTaxTable *table)
 static inline void remObj (GncTaxTable *table)
 {
     struct _book_info *bi;
-    bi = qof_book_get_data (qof_instance_get_book(table), _GNC_MOD_NAME);
+    bi = static_cast<_book_info*>(qof_book_get_data (qof_instance_get_book(table), _GNC_MOD_NAME));
     bi->tables = g_list_remove (bi->tables, table);
 }
 
@@ -322,7 +322,7 @@ impl_refers_to_object(const QofInstance* inst, const QofInstance* ref)
 
         for (node = tt->entries; node != NULL; node = node->next)
         {
-            GncTaxTableEntry* tte = node->data;
+            GncTaxTableEntry* tte = static_cast<GncTaxTableEntry*>(node->data);
 
             if (tte->account == GNC_ACCOUNT(ref))
             {
@@ -408,7 +408,7 @@ gncTaxTableCreate (QofBook *book)
     GncTaxTable *table;
     if (!book) return NULL;
 
-    table = g_object_new (GNC_TYPE_TAXTABLE, NULL);
+    table = GNC_TAXTABLE(g_object_new (GNC_TYPE_TAXTABLE, NULL));
     qof_instance_init_data (&table->inst, _GNC_MOD_NAME, book);
     table->name = CACHE_INSERT ("");
     addObj (table);
@@ -438,9 +438,7 @@ gncTaxTableFree (GncTaxTable *table)
     remObj (table);
 
     /* destroy the list of entries */
-    for (list = table->entries; list; list = list->next)
-        gncTaxTableEntryDestroy (list->data);
-    g_list_free (table->entries);
+    g_list_free_full (table->entries, (GDestroyNotify)gncTaxTableEntryDestroy);
 
     if (!qof_instance_get_destroying(table))
         PERR("free a taxtable without do_free set!");
@@ -452,7 +450,7 @@ gncTaxTableFree (GncTaxTable *table)
     /* disconnect from the children */
     for (list = table->children; list; list = list->next)
     {
-        child = list->data;
+        child = GNC_TAXTABLE(list->data);
         gncTaxTableSetParent(child, NULL);
     }
     g_list_free(table->children);
@@ -550,7 +548,7 @@ void gncTaxTableMakeInvisible (GncTaxTable *table)
     if (!table) return;
     gncTaxTableBeginEdit (table);
     table->invisible = TRUE;
-    bi = qof_book_get_data (qof_instance_get_book(table), _GNC_MOD_NAME);
+    bi = static_cast<_book_info*>(qof_book_get_data (qof_instance_get_book(table), _GNC_MOD_NAME));
     bi->tables = g_list_remove (bi->tables, table);
     gncTaxTableCommitEdit (table);
 }
@@ -670,9 +668,9 @@ GncTaxTable *gncTaxTableLookupByName (QofBook *book, const char *name)
 
     for ( ; list; list = list->next)
     {
-        GncTaxTable *table = list->data;
+        GncTaxTable *table = GNC_TAXTABLE(list->data);
         if (!g_strcmp0 (table->name, name))
-            return list->data;
+            return table;
     }
     return NULL;
 }
@@ -703,7 +701,7 @@ GncTaxTableList * gncTaxTableGetTables (QofBook *book)
     struct _book_info *bi;
     if (!book) return NULL;
 
-    bi = qof_book_get_data (book, _GNC_MOD_NAME);
+    bi = static_cast<_book_info*>(qof_book_get_data (book, _GNC_MOD_NAME));
     return bi ? bi->tables : NULL;
 }
 
@@ -737,7 +735,7 @@ static GncTaxTable *gncTaxTableCopy (const GncTaxTable *table)
     for (list = table->entries; list; list = list->next)
     {
         GncTaxTableEntry *entry, *e;
-        entry = list->data;
+        entry = static_cast<GncTaxTableEntry*>(list->data);
         e = gncTaxTableEntryCopy (entry);
        /* Clang static analyzer thinks we're leaking e, but we're not.
         * We're transferring it to table. */
@@ -800,7 +798,7 @@ Account * gncTaxTableEntryGetAccount (const GncTaxTableEntry *entry)
 
 GncAmountType gncTaxTableEntryGetType (const GncTaxTableEntry *entry)
 {
-    if (!entry) return 0;
+    if (!entry) return GncAmountType(0);
     return entry->type;
 }
 
@@ -950,7 +948,7 @@ GList *gncAccountValueAdd (GList *list, Account *acc, gnc_numeric value)
     /* Try to find the account in the list */
     for (li = list; li; li = li->next)
     {
-        res = li->data;
+        res = static_cast<GncAccountValue*>(li->data);
         if (res->account == acc)
         {
             res->value = gnc_numeric_add (res->value, value, GNC_DENOM_AUTO,
@@ -973,7 +971,7 @@ GList *gncAccountValueAddList (GList *l1, GList *l2)
 
     for (li = l2; li; li = li->next )
     {
-        GncAccountValue *val = li->data;
+        GncAccountValue *val = static_cast<GncAccountValue*>(li->data);
         l1 = gncAccountValueAdd (l1, val->account, val->value);
     }
 
@@ -987,7 +985,7 @@ gnc_numeric gncAccountValueTotal (GList *list)
 
     for ( ; list ; list = list->next)
     {
-        GncAccountValue *val = list->data;
+        GncAccountValue *val = static_cast<GncAccountValue*>(list->data);
         total = gnc_numeric_add (total, val->value, GNC_DENOM_AUTO, GNC_HOW_DENOM_REDUCE | GNC_HOW_RND_ROUND_HALF_UP);
     }
     return total;
@@ -1031,7 +1029,7 @@ static void _gncTaxTableDestroy (QofBook *book)
 
     if (!book) return;
 
-    bi = qof_book_get_data (book, _GNC_MOD_NAME);
+    bi = static_cast<_book_info*>(qof_book_get_data (book, _GNC_MOD_NAME));
 
     col = qof_book_get_collection (book, GNC_ID_TAXTABLE);
     qof_collection_foreach (col, destroy_taxtable_on_book_close, NULL);
@@ -1045,7 +1043,7 @@ static QofObject gncTaxTableDesc =
     DI(.interface_version = ) QOF_OBJECT_VERSION,
     DI(.e_type            = ) _GNC_MOD_NAME,
     DI(.type_label        = ) "Tax Table",
-    DI(.create            = ) (gpointer)gncTaxTableCreate,
+    DI(.create            = ) (void* (*)(QofBook*))gncTaxTableCreate,
     DI(.book_begin        = ) _gncTaxTableCreate,
     DI(.book_end          = ) _gncTaxTableDestroy,
     DI(.is_dirty          = ) qof_collection_is_dirty,
diff --git a/libgnucash/engine/gncVendor.c b/libgnucash/engine/gncVendor.cpp
similarity index 98%
rename from libgnucash/engine/gncVendor.c
rename to libgnucash/engine/gncVendor.cpp
index 7d0340c707b..3ab02210d6c 100644
--- a/libgnucash/engine/gncVendor.c
+++ b/libgnucash/engine/gncVendor.cpp
@@ -218,7 +218,7 @@ gnc_vendor_set_property (GObject         *object,
         gncVendorSetNotes(vendor, g_value_get_string(value));
         break;
     case PROP_CURRENCY:
-        gncVendorSetCurrency(vendor, g_value_get_object(value));
+        gncVendorSetCurrency(vendor, GNC_COMMODITY(g_value_get_object(value)));
         break;
     case PROP_ACTIVE:
         gncVendorSetActive(vendor, g_value_get_boolean(value));
@@ -227,13 +227,13 @@ gnc_vendor_set_property (GObject         *object,
         gncVendorSetTaxTableOverride(vendor, g_value_get_boolean(value));
         break;
     case PROP_BILLTERMS:
-        gncVendorSetTerms(vendor, g_value_get_object(value));
+        gncVendorSetTerms(vendor, GNC_BILLTERM(g_value_get_object(value)));
         break;
     case PROP_TAXTABLE:
-        gncVendorSetTaxTable(vendor, g_value_get_object(value));
+        gncVendorSetTaxTable(vendor, GNC_TAXTABLE(g_value_get_object(value)));
         break;
     case PROP_ADDRESS:
-        qofVendorSetAddr(vendor, g_value_get_object(value));
+        qofVendorSetAddr(vendor, QOF_INSTANCE(g_value_get_object(value)));
         break;
     case PROP_TAX_INCLUDED:
         gncVendorSetTaxIncluded(vendor, (GncTaxIncluded)g_value_get_int(value));
@@ -456,7 +456,7 @@ GncVendor *gncVendorCreate (QofBook *book)
 
     if (!book) return NULL;
 
-    vendor = g_object_new (GNC_TYPE_VENDOR, NULL);
+    vendor = GNC_VENDOR(g_object_new (GNC_TYPE_VENDOR, NULL));
     qof_instance_init_data (&vendor->inst, _GNC_MOD_NAME, book);
 
     vendor->id = CACHE_INSERT ("");
@@ -943,7 +943,7 @@ vend_handle_qof_events (QofInstance *entity, QofEventId event_type,
 
 static const char * _gncVendorPrintable (gpointer item)
 {
-    GncVendor *v = item;
+    GncVendor *v = GNC_VENDOR(item);
     if (!item) return NULL;
     return v->name;
 }
@@ -975,7 +975,7 @@ static QofObject gncVendorDesc =
     DI(.interface_version = ) QOF_OBJECT_VERSION,
     DI(.e_type            = ) _GNC_MOD_NAME,
     DI(.type_label        = ) "Vendor",
-    DI(.create            = ) (gpointer)gncVendorCreate,
+    DI(.create            = ) (void* (*)(QofBook*))gncVendorCreate,
     DI(.book_begin        = ) NULL,
     DI(.book_end          = ) gnc_vendor_book_end,
     DI(.is_dirty          = ) qof_collection_is_dirty,
diff --git a/po/POTFILES.in b/po/POTFILES.in
index 3ce4eb2bf72..7372b1d6f64 100644
--- a/po/POTFILES.in
+++ b/po/POTFILES.in
@@ -626,42 +626,42 @@ libgnucash/engine/cap-gains.cpp
 libgnucash/engine/cashobjects.cpp
 libgnucash/engine/engine-helpers.c
 libgnucash/engine/gnc-accounting-period.c
-libgnucash/engine/gncAddress.c
+libgnucash/engine/gncAddress.cpp
 libgnucash/engine/gnc-aqbanking-templates.cpp
-libgnucash/engine/gncBillTerm.c
+libgnucash/engine/gncBillTerm.cpp
 libgnucash/engine/gnc-budget.cpp
 libgnucash/engine/gncBusiness.c
 libgnucash/engine/gnc-commodity.cpp
 libgnucash/engine/gnc-commodity.h
-libgnucash/engine/gncCustomer.c
+libgnucash/engine/gncCustomer.cpp
 libgnucash/engine/gnc-date.cpp
 libgnucash/engine/gnc-datetime.cpp
-libgnucash/engine/gncEmployee.c
+libgnucash/engine/gncEmployee.cpp
 libgnucash/engine/gnc-engine.cpp
-libgnucash/engine/gncEntry.c
+libgnucash/engine/gncEntry.cpp
 libgnucash/engine/gnc-euro.cpp
 libgnucash/engine/gnc-event.c
 libgnucash/engine/gnc-features.cpp
 libgnucash/engine/gnc-hooks.c
 libgnucash/engine/gncIDSearch.c
 libgnucash/engine/gnc-int128.cpp
-libgnucash/engine/gncInvoice.c
-libgnucash/engine/gncJob.c
+libgnucash/engine/gncInvoice.cpp
+libgnucash/engine/gncJob.cpp
 libgnucash/engine/gnc-lot.cpp
 libgnucash/engine/gnc-numeric.cpp
 libgnucash/engine/gnc-option.cpp
 libgnucash/engine/gnc-option-date.cpp
 libgnucash/engine/gnc-optiondb.cpp
 libgnucash/engine/gnc-option-impl.cpp
-libgnucash/engine/gncOrder.c
-libgnucash/engine/gncOwner.c
+libgnucash/engine/gncOrder.cpp
+libgnucash/engine/gncOwner.cpp
 libgnucash/engine/gnc-pricedb.cpp
 libgnucash/engine/gnc-rational.cpp
 libgnucash/engine/gnc-session.c
-libgnucash/engine/gncTaxTable.c
+libgnucash/engine/gncTaxTable.cpp
 libgnucash/engine/gnc-timezone.cpp
-libgnucash/engine/gnc-uri-utils.c
-libgnucash/engine/gncVendor.c
+libgnucash/engine/gnc-uri-utils.cpp
+libgnucash/engine/gncVendor.cpp
 libgnucash/engine/guid.cpp
 libgnucash/engine/kvp-frame.cpp
 libgnucash/engine/kvp-value.cpp