[C++] The first commit for C++ version

16 tests out of 169 are passing.
diff --git a/ast.cc b/ast.cc
new file mode 100644
index 0000000..fdd4588
--- /dev/null
+++ b/ast.cc
@@ -0,0 +1,67 @@
+#include "ast.h"
+
+#include "eval.h"
+#include "stringprintf.h"
+#include "value.h"
+
+AST::AST() {}
+
+AST::~AST() {}
+
+string RuleAST::DebugString() const {
+  return StringPrintf("RuleAST(expr=%s term=%d after_term=%s)",
+                      expr->DebugString().c_str(),
+                      term,
+                      after_term->DebugString().c_str());
+}
+
+string AssignAST::DebugString() const {
+  const char* opstr = "???";
+  switch (op) {
+    case AssignOp::EQ: opstr = "EQ"; break;
+    case AssignOp::COLON_EQ: opstr = "COLON_EQ"; break;
+    case AssignOp::PLUS_EQ: opstr = "PLUS_EQ"; break;
+    case AssignOp::QUESTION_EQ: opstr = "QUESTION_EQ"; break;
+  }
+  const char* dirstr = "???";
+  switch (directive) {
+    case AssignDirective::NONE: dirstr = ""; break;
+    case AssignDirective::OVERRIDE: dirstr = "override"; break;
+    case AssignDirective::EXPORT: dirstr = "export"; break;
+  }
+  return StringPrintf("AssignAST(lhs=%s rhs=%s opstr=%s dir=%s)",
+                      lhs->DebugString().c_str(),
+                      rhs->DebugString().c_str(),
+                      opstr, dirstr);
+}
+
+string CommandAST::DebugString() const {
+  return StringPrintf("CommandAST(%s)",
+                      expr->DebugString().c_str());
+}
+
+RuleAST::~RuleAST() {
+  delete expr;
+  delete after_term;
+}
+
+void RuleAST::Eval(Evaluator* ev) const {
+  ev->EvalRule(this);
+}
+
+AssignAST::~AssignAST() {
+  delete lhs;
+  delete rhs;
+}
+
+void AssignAST::Eval(Evaluator* ev) const {
+  ev->EvalAssign(this);
+}
+
+CommandAST::~CommandAST() {
+  delete expr;
+}
+
+void CommandAST::Eval(Evaluator* ev) const {
+  ev->EvalCommand(this);
+}