Symbol: rework initialisation and introduce a "bottom" Symbol

Up to now, we tolerated null pointers in Literal instances.
But we can not tolerate passing a null cString to Symbol initialisation.
Rather, hereby we introduce a dedicated "bottom" Symbol, a valid "null object"

src/lib/symbol-impl.cpp

@@ -59,6 +59,33 @@ namespace lib {
   const size_t STRING_MAX_RELEVANT = LUMIERA_IDSTRING_MAX_RELEVANT;
   
   
+  namespace { // symbol table implementation
+    
+    /** @warning grows eternally, never shrinks */
+    std::unordered_set<string> symbolTable;
+  }
+  
+  
+  /**
+   * create Symbol by symbol table lookup.
+   * @note identical strings will be mapped to
+   *       the same Symbol (embedded pointer)
+   */
+  Symbol::Symbol (string&& definition)
+    : Literal{symbolTable.insert(forward<string> (definition)).first->c_str()}
+    { }
+  
+  
+  /* == predefined marker Symbols == */
+  Symbol Symbol::BOTTOM  = "⟂";
+  Symbol Symbol::FAILURE = "↯";
+  
+  // see also: lib/format-obj.cpp
+  // We can not share these definitions due to undefined static init order
+  
+  
+  
+  
   /** equality on Literal and Symbol values is defined
    *  based on the content, not the address. */
   bool
@@ -90,20 +117,4 @@ namespace lib {
   
   
   
-  namespace { // quick-n-dirty symbol table implementation
-    
-    /** @warning grows eternally, never shrinks */
-    std::unordered_set<string> symbolTable;                  ////////////////////////////////TICKET #158 replace by symbol table class
-  }
-  
-  /** create Symbol by symbol table lookup.
-   * @note identical strings will be mapped to
-   *       the same Symbol (embedded pointer)
-   */
-  Symbol::Symbol (string&& definition)
-   : Literal{symbolTable.insert(forward<string> (definition)).first->c_str()}
-   { }
-  
-  
-  
 } // namespace lib

src/lib/symbol.hpp

@@ -96,25 +96,28 @@ namespace lib {
     : public Literal
     {
     public:
-       explicit
-       Symbol (std::string&& definition);
+      static Symbol BOTTOM;
+      static Symbol FAILURE;
+      
+      Symbol (const char* lit =NULL)
+        : Symbol{std::string(lit? lit : BOTTOM.c())}
+        { }
+      
+      explicit
+      Symbol (std::string&& definition);
+      
+      Symbol (std::string const& str)
+        : Symbol{std::string(str)}
+        { }
+      
+      Symbol (Literal const& base, std::string ext)
+        : Symbol{std::string(base)+"."+ext}
+        { }
+      
+      Symbol (Literal const& base, const char* ext)
+        : Symbol{base, std::string(ext)}
+        { }
        
-       Symbol()
-         : Symbol{""}
-         { }
-       
-       Symbol (const char* lit)
-         : Symbol{std::string(lit)}
-         { }
-      
-       Symbol (Literal const& base, std::string ext)
-         : Symbol{std::string(base)+"."+ext}
-         { }
-      
-       Symbol (Literal const& base, const char* ext)
-         : Symbol{base, std::string(ext)}
-         { }
-      
        Symbol (Symbol const&) = default;
        Symbol (Symbol &&)     = default;
        

tests/library/symbol-test.cpp

@@ -113,7 +113,7 @@ namespace test{
           CHECK (sy1 == sy2);
           CHECK (not isSameObject (l1,sy1));
           CHECK (not isSameObject (sy1,sy2));
-          CHECK (l1.c() == sy1.c());
+          CHECK (l1.c() != sy1.c());
           CHECK (sy1.c() == sy2.c());
           
           Symbol sy3;