[Delegator] Improve error messages when return types are missing

include/cantera/base/Delegator.h

@@ -102,6 +102,16 @@ namespace Cantera
 class Delegator
 {
 public:
+    //! Get the name of the user-defined class in the extension language
+    std::string delegatorName() const {
+        return m_delegatorName;
+    }
+
+    //! Set the name of the user-defined class in the extension language
+    void setDelegatorName(const std::string& delegatorName) {
+        m_delegatorName = delegatorName;
+    }
+
     //! Set delegates for member functions with the signature `void()`.
     void setDelegate(const std::string& name, const std::function<void()>& func,
                      const std::string& when)
@@ -215,7 +225,7 @@ class Delegator
             throw NotImplementedError("Delegator::setDelegate",
                 "for function named '{}' with signature 'double(void*)'.", name);
         }
-        *m_funcs_d_vp[name] = makeDelegate(func, when, m_base_d_vp[name]);
+        *m_funcs_d_vp[name] = makeDelegate(name, func, when, m_base_d_vp[name]);
     }
 
     //! Set delegates for member functions with the signature `string(size_t)`
@@ -228,7 +238,7 @@ class Delegator
                 "for function named '{}' with signature "
                 "'string(size_t)'.", name);
         }
-        *m_funcs_s_sz[name] = makeDelegate(func, when, m_base_s_sz[name]);
+        *m_funcs_s_sz[name] = makeDelegate(name, func, when, m_base_s_sz[name]);
     }
 
     //! Set delegates for member functions with the signature `size_t(string)`
@@ -238,10 +248,10 @@ class Delegator
     {
         if (!m_funcs_sz_csr.count(name)) {
             throw NotImplementedError("Delegator::setDelegate",
-                "for function named '{}' with signature "
+                "for function '{}' with signature "
                 "'size_t(const string&)'.", name);
         }
-        *m_funcs_sz_csr[name] = makeDelegate(func, when, m_base_sz_csr[name]);
+        *m_funcs_sz_csr[name] = makeDelegate(name, func, when, m_base_sz_csr[name]);
     }
 
     void holdExternalHandle(const shared_ptr<ExternalHandle>& handle) {
@@ -382,6 +392,7 @@ class Delegator
     //! Create a delegate for a function with a return value
     template <typename ReturnType, class ... Args>
     std::function<ReturnType(Args ...)> makeDelegate(
+        const std::string& name,
         const std::function<int(ReturnType&, Args ...)>& func,
         const std::string& when,
         const std::function<ReturnType(Args ...)>& base)
@@ -413,20 +424,21 @@ class Delegator
                 }
             };
         } else if (when == "replace") {
-            return [base, func](Args ... args) {
+            return [base, name, func, this](Args ... args) {
                 ReturnType ret;
                 int has_ret = func(ret, args ...);
                 if (!has_ret) {
                     throw CanteraError("Lambda generated by Delegator::makeDelegate",
-                        "Delegate for function of type '{}'\ndid not return a value",
-                        demangle(typeid(base)));
+                        "Method '{}' of class '{}' did not return a value of type '{}'.",
+                        name, delegatorName(), demangle(typeid(ret)));
                 }
                 return ret;
             };
         } else {
             throw CanteraError("Delegator::makeDelegate",
+                "For function named '{}':\n"
                 "'when' must be one of 'before', 'after', or 'replace';"
-                " not '{}", when);
+                " not '{}'", name, when);
         }
     }
 
@@ -486,6 +498,9 @@ class Delegator
 
     //! Cleanup functions to be called from the destructor
     std::list<shared_ptr<ExternalHandle>> m_handles;
+
+    //! Name of the class in the extension language
+    std::string m_delegatorName;
 };
 
 }

interfaces/cython/cantera/delegator.pxd

@@ -26,6 +26,8 @@ cdef extern from "cantera/base/Delegator.h" namespace "Cantera":
     cdef cppclass CxxDelegator "Cantera::Delegator":
         Delegator()
 
+        void setDelegatorName(string&)
+
         void setDelegate(string&, function[void()], string&) except +translate_exception
         void setDelegate(string&, function[void(cbool)], string&) except +translate_exception
         void setDelegate(string&, function[void(double)], string&) except +translate_exception

interfaces/cython/cantera/delegator.pyx

@@ -235,6 +235,8 @@ cdef int assign_delegates(obj, CxxDelegator* delegator) except -1:
       if present, indicates that the delegate is required, how it is executed with
       respect to the base class method (that is, ``before``, ``after``, or ``replace``).
     """
+    delegator.setDelegatorName(stringify(obj.__class__.__name__))
+
     # Find all delegate methods, and make sure there aren't multiple
     # conflicting implementations
     cdef string cxx_name