C++ Mathematical Expression Library (ExprTk) https://www.partow.net/programming/exprtk/index.html

readme.txt

@@ -885,11 +885,14 @@ version using ExprTk:
    foo.register_symbol_table(symbol_table);
 
    parser_t parser;
-   if (parser.compile(foo_str,foo))
+   if (!parser.compile(foo_str,foo))
    {
-      foo.value();
+      // Error in expression...
+      return;
    }
 
+   T result = foo.value();
+
 
 (3) Parser
 A  component  which  takes  as input  a  string  representation  of an