stringid: add query-or-insert indexing operator

stringid.cpp

@@ -13,9 +13,23 @@
 using cruft::stringid;
 
 
-//-----------------------------------------------------------------------------
+///////////////////////////////////////////////////////////////////////////////
 stringid::id_t
-stringid::add (std::string key) {
+stringid::operator[] (std::string_view const &key)
+{
+    if (auto const pos = m_map.find (key); pos != m_map.cend ())
+        return pos->second;
+
+
+    auto const &[pos, success] = m_map.emplace (key, m_map.size ());
+    return pos->second;
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+stringid::id_t
+stringid::add (std::string key)
+{
     auto pos = m_map.find (key);
     if (pos != m_map.end ())
         throw std::invalid_argument ("duplicate stringid key");
@@ -26,9 +40,10 @@ stringid::add (std::string key) {
 }
 
 
-//-----------------------------------------------------------------------------
+///////////////////////////////////////////////////////////////////////////////
 stringid::id_t
-stringid::find (const std::string &key) const {
+stringid::find (const std::string &key) const
+{
     auto pos = m_map.find (key);
     if (pos == m_map.end ())
         throw std::out_of_range ("invalid stringid key");
@@ -37,8 +52,9 @@ stringid::find (const std::string &key) const {
 }
 
 
-//-----------------------------------------------------------------------------
+///////////////////////////////////////////////////////////////////////////////
 void
-stringid::clear (void) {
+stringid::clear (void)
+{
     m_map.clear ();
 }

stringid.hpp

@@ -19,6 +19,11 @@ namespace cruft {
     public:
         typedef size_t id_t;
 
+        ///////////////////////////////////////////////////////////////////////
+        /// Returns the ID for the supplied string, or creates and returns a
+        /// new ID if the string is not present.
+        id_t operator[] (std::string_view const&);
+
 
         ///////////////////////////////////////////////////////////////////////
         id_t add  (std::string);
@@ -59,7 +64,7 @@ namespace cruft {
 
 
     private:
-        std::map<const std::string, id_t> m_map;
+        std::map<const std::string, id_t, std::less<>> m_map;
     };
 }
 

test/stringid.cpp

@@ -23,5 +23,10 @@ main (int, char**) {
     tap.expect_eq (id1 + 1, id2, "monotonically increasing IDs");
     tap.expect_eq (id1, map.find ("first"), "first element still matches");
 
+    tap.expect_eq (map["second"], id2, "index lookup invokes no change");
+
+    auto id3 = map["third"];
+    tap.expect (id3 != id1 && id3 != id2, "index creation adds new value");
+
     return tap.status ();
 }