expr.cpp

#include <iostream>
using namespace std;

class Expr {
public:
  virtual ~Expr() { }
  virtual int eval() = 0;
};

class Literal : public Expr {
  int x;
public:
  Literal(int val): x(val) { }
  virtual int eval() { return x; }
};

class PlusExpr : public Expr {
  Expr* a;
  Expr* b;
public:
  PlusExpr(Expr* e1, Expr* e2): a(e1), b(e2) { }
  virtual int eval() { return a->eval() + b->eval(); }
};

int main() {
  Literal* expr = new Literal(0);
  for (int i = 0; i < 2000000000; i++)
    expr->eval();
  return 0;
}

expr.nim

type
  TExpr = object ## abstract base class for an expression
  TLiteral = object of TExpr
    x: int
  TPlusExpr = object of TExpr
    a, b: ref TExpr
    
method eval(e: ref TExpr): int =
  # override this base method
  quit "to override!"

method eval(e: ref TLiteral): int = return e.x

method eval(e: ref TPlusExpr): int =
  # watch out: relies on dynamic binding
  return eval(e.a) + eval(e.b)

proc newLit(x: int): ref TLiteral =
  new(result)
  result.x = x
  
proc newPlus(a, b: ref TExpr): ref TPlusExpr =
  new(result)
  result.a = a
  result.b = b

var expr = newLit(0)
for i in 1..2000000000:
  discard eval(expr)