Add AnyMap.keys_str function

include/cantera/base/AnyMap.h

@@ -237,6 +237,10 @@ public:
 
     bool hasKey(const std::string& key) const;
 
+    //! Return a string listing the keys in this AnyMap, e.g. for use in error
+    //! messages
+    std::string keys_str() const;
+
     bool getBool(const std::string& key, bool default_) const;
     long int getInt(const std::string& key, long int default_) const;
     double getDouble(const std::string& key, double default_) const;

src/base/AnyMap.cpp

@@ -565,7 +565,8 @@ const AnyValue& AnyMap::at(const std::string& key) const
     try {
         return m_data.at(key);
     } catch (std::out_of_range& err) {
-        throw CanteraError("AnyMap::at", "Key '{}' not found", key);
+        throw CanteraError("AnyMap::at",
+            "Key '{}' not found.\nExisting keys: {}", key, keys_str());
     }
 }
 
@@ -574,6 +575,21 @@ bool AnyMap::hasKey(const std::string& key) const
     return (m_data.find(key) != m_data.end());
 }
 
+std::string AnyMap::keys_str() const
+{
+    fmt::memory_buffer b;
+    auto iter = this->begin();
+    if (iter != this->end()) {
+        format_to(b, "{}", iter->first);
+        ++iter;
+    }
+    while (iter != this->end()) {
+        format_to(b, ", {}", iter->first);
+        ++iter;
+    }
+    return to_string(b);
+}
+
 template<class T>
 const T& AnyMap::get(const std::string& key, const T& default_,
                      std::function<const T&(const AnyValue*)> getter) const

test/general/test_containers.cpp

@@ -48,11 +48,17 @@ TEST(AnyMap, map_conversion) {
     AnyMap m;
     m["compound"]["first"] = "bar";
     m["compound"]["second"] = "baz";
+    m["empty"] = AnyMap();
 
     auto x = m["compound"].asMap<std::string>();
     EXPECT_EQ(x.size(), (size_t) 2);
     EXPECT_EQ(x["first"], "bar");
     EXPECT_EQ(x["second"], "baz");
+    std::string keys = m["compound"].as<AnyMap>().keys_str();
+    EXPECT_NE(keys.find("first"), npos);
+    EXPECT_NE(keys.find("second"), npos);
+    EXPECT_EQ(keys.size(), (size_t) 13);
+    EXPECT_EQ(m["empty"].as<AnyMap>().keys_str(), "");
 
     std::map<std::string, double> zz{{"a", 9.0}, {"b", 13.5}};
     m["foo"] = zz;