improve type change error messages

documentation/nt.rst

@@ -14,6 +14,9 @@ Library of common type definitions. ::
 
    ntscalar
 
+.. note::
+    The nt::* factories are expensive.  Avoid repeated use with `pvxs::Value::cloneEmpty`.
+
 .. _time_t:
 
 time_t

src/servermon.cpp

@@ -218,7 +218,7 @@ struct ServerMonitorControl : public server::MonitorControlOp
         Guard G(mon->lock);
 
         if(val && mon->type && mon->type.get()!=Value::Helper::desc(val))
-            throw std::logic_error("Type change not allowed in post()");
+            throw std::logic_error("Type change not allowed in post().  Recommend pvxs::Value::cloneEmpty()");
 
         if((mon->queue.size() < mon->limit) || force || !val) {
             mon->queue.push_back(std::move(val));

src/sharedpv.cpp

@@ -17,6 +17,7 @@
 #include <pvxs/server.h>
 
 #include "utilpvt.h"
+#include "dataimpl.h"
 
 typedef epicsGuard<epicsMutex> Guard;
 typedef epicsGuardRelease<epicsMutex> UnGuard;
@@ -298,6 +299,8 @@ void SharedPV::open(const Value& initial)
 {
     if(!impl)
         throw std::logic_error("Empty SharedPV");
+    else if(!initial || initial.type()!=TypeCode::Struct)
+        throw std::logic_error("Must specify non-empty initial Struct");
 
     decltype (impl->pending) pending;
     decltype (impl->mpending) mpending;
@@ -382,9 +385,16 @@ void SharedPV::post(Value&& val)
 {
     if(!impl)
         throw std::logic_error("Empty SharedPV");
+    else if(!val)
+        throw std::logic_error("Can't post() empty Value");
 
     Guard G(impl->lock);
 
+    if(!impl->current)
+        throw std::logic_error("Must open() before post()ing");
+    else if(Value::Helper::desc(impl->current)!=Value::Helper::desc(val))
+        throw std::logic_error("post() requires the exact type of open().  Recommend pvxs::Value::cloneEmpty()");
+
     impl->current.assign(val);
 
     for(auto& sub : impl->subscribers) {