A working but clunky solution for expressions

cclox_src/main.cc

@@ -2,6 +2,7 @@
 #include "error_handler.hh"
 #include "parser.hh"
 #include <iostream>
+#include <memory>
 #include <sysexits.h>
 
 ErrorHandler error_handler{};
@@ -16,11 +17,15 @@ int main(int argc, char *argv[]) {
   //
   // run_interpreter(argc, argv);
 
-  Unary left {Token{TokenType::MINUS, "-", Object{}, 1}, Literal{123}};
-  Token op{TokenType::STAR, "*", Object{}, 1};
-  Grouping right{Literal{45.67}};
+  ExprPtr literal = std::make_shared<Expr>(ExprType::LITERAL, Literal{123});
+  ExprPtr left = std::make_shared<Expr>(ExprType::UNARY, Unary{Token{TokenType::MINUS, "-", Object{}, 1}, literal});
 
-  Binary expr {left, op, right};
+  Token op{TokenType::STAR, "*", Object{}, 1};
+
+  ExprPtr gliteral = std::make_shared<Expr>(ExprType::LITERAL, Literal{45.67});
+  ExprPtr right = std::make_shared<Expr>(ExprType::GROUPING, gliteral);
+
+  Expr expr = {ExprType::BINARY, Binary{left, op, right}};
 
   AstPrinter printer{};
   std::cout << printer.print(expr) << '\n';

cclox_src/parser/expr.cc

@@ -1,20 +1,24 @@
 #include "expr.hh"
 #include "../tokenizer.hh"
+#include <cassert>
 #include <string>
-#include <type_traits>
-#include <variant>
 #include <vector>
 #include <sstream>
 
-std::string AstPrinter::print(const Expression &expr) {
-  return std::visit([this](auto &&node) -> std::string {
-    using T = std::decay_t<decltype(node)>;
-
-    if constexpr(std::is_same_v<T, Binary>) {
-      return parenthesize(node.op.lexeme, {node.left, node.right});
-    } else if constexpr(std::is_same_v<T, Grouping>) {
-      return parenthesize("group", {node.expression});
-    } else if constexpr(std::is_same_v<T, Literal>) {
+std::string AstPrinter::print(const Expr &expr) {
+  switch (expr.type) {
+    case ExprType::BINARY: {
+      Binary node = std::get<Binary>(expr.value);
+      assert(node.left != NULL && node.right != NULL);
+      return parenthesize(node.op.lexeme, {*node.left, *node.right});
+    }
+    case ExprType::GROUPING: {
+      Grouping node = std::get<Grouping>(expr.value);
+      assert(node.expression != NULL);
+      return parenthesize("group", {*node.expression});
+    }
+    case ExprType::LITERAL: {
+      Literal node = std::get<Literal>(expr.value);
       switch (node.value.type) {
         case ObjectType::NIL:
           return "nil";
@@ -28,22 +32,20 @@ std::string AstPrinter::print(const Expression &expr) {
           return std::to_string(value);
         }
       }
-    } else if constexpr(std::is_same_v<T, Unary>) {
-      return parenthesize(node.op.lexeme, {node.right});
+    }
+    case ExprType::UNARY: {
+      Unary node = std::get<Unary>(expr.value);
+      assert(node.right != NULL);
+      return parenthesize(node.op.lexeme, {*node.right});
+    }
+  }
 }
 
-    return "";
-  }, expr);
-}
-
-std::string AstPrinter::parenthesize(const std::string &name, std::vector<Expression> exprs) {
+std::string AstPrinter::parenthesize(const std::string &name, std::vector<Expr> exprs) {
   std::stringstream ss{};
   ss << '(' << name;
   for (auto &expr : exprs) {
-    std::string value = std::visit([this](auto &&node) -> std::string {
-      return print(node);
-    }, expr);
-    ss << ' ' << value;
+    ss << ' ' << print(expr);
   }
   ss << ')';
 

cclox_src/parser/expr.hh

@@ -2,45 +2,61 @@
 #define EXPR_HH
 
 #include "../tokenizer.hh"
+#include <memory>
 #include <string>
 #include <variant>
 
-struct Expr {};
+struct Expr;
 
-struct Binary : public Expr {
-  Binary(Expr left, Token op, Expr right) : left{left}, op{op}, right{right} {};
+using ExprPtr = std::shared_ptr<Expr>;
 
-  Expr left;
+struct Binary {
+  Binary(ExprPtr left, Token op, ExprPtr right) : left{left}, op{op}, right{right} {};
+
+  ExprPtr left;
   Token op;
-  Expr right;
+  ExprPtr right;
 };
 
-struct Grouping : public Expr {
-  Grouping(Expr expr) : expression{expr} {};
+struct Grouping {
+  Grouping(ExprPtr expr) : expression{expr} {};
 
-  Expr expression;
+  ExprPtr expression;
 };
 
-struct Literal : public Expr {
+struct Literal {
   Literal(Object value) : value{value} {};
 
   Object value;
 };
 
-struct Unary : public Expr {
-  Unary(Token op, Expr right) : op{op}, right{right} {};
+struct Unary {
+  Unary(Token op, ExprPtr right) : op{op}, right{right} {};
 
   Token op;
-  Expr right;
+  ExprPtr right;
 };
 
-using Expression = std::variant<Expr, Binary, Grouping, Literal, Unary>;
+enum class ExprType {
+  BINARY,
+  GROUPING,
+  LITERAL,
+  UNARY,
+};
+
+using Expression = std::variant<Binary, Grouping, Literal, Unary>;
+
+struct Expr {
+  Expr(ExprType type, Expression value) : type{type}, value{value} {}
+  ExprType type;
+  Expression value;
+};
 
 class AstPrinter {
 public:
-  std::string print(const Expression &expr);
+  std::string print(const Expr &expr);
 private:
-  std::string parenthesize(const std::string &name, std::vector<Expression> exprs);
+  std::string parenthesize(const std::string &name, std::vector<Expr> exprs);
 };
 
 #endif