libexpr: pass Exprs as references, not pointers

almost all places where Exprs are passed as pointers expect the pointers
to be non-null. pass them as references to encode this constraint in the
type system as well (and also communicate that Exprs must not be freed).

Change-Id: Ia98f166fec3c23151f906e13acb4a0954a5980a2

1 files changed, 9 insertions, 0 deletions

diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 8631445c9..91516c092 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -42,12 +42,21 @@ std::string showAttrPath(const SymbolTable & symbols, const AttrPath & attrPath)
 
 struct Expr
 {
+protected:
+    Expr(Expr &&) = default;
+    Expr & operator=(Expr &&) = default;
+
+public:
     struct AstSymbols {
         Symbol sub, lessThan, mul, div, or_, findFile, nixPath, body;
     };
 
 
+    Expr() = default;
+    Expr(const Expr &) = delete;
+    Expr & operator=(const Expr &) = delete;
     virtual ~Expr() { };
+
     virtual void show(const SymbolTable & symbols, std::ostream & str) const;
     virtual void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env);
     virtual void eval(EvalState & state, Env & env, Value & v);