Editing Interpreter pattern (section)

== Example ==
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This C++11 implementation is based on the pre C++98 sample code in the book.

<syntaxhighlight lang="c++">
#include <iostream>
#include <map>
#include <cstring>

class Context;

class BooleanExp {
public:
  BooleanExp() = default;
  virtual ~BooleanExp() = default;
  virtual bool evaluate(Context&) = 0;
  virtual BooleanExp* replace(const char*, BooleanExp&) = 0;
  virtual BooleanExp* copy() const = 0;
};

class VariableExp;

class Context {
public:
  Context() :m() {}
  bool lookup(const VariableExp* key) { return m.at(key); }
  void assign(VariableExp* key, bool value) { m[key] = value; }
private:
  std::map<const VariableExp*, bool> m;
};

class VariableExp : public BooleanExp {
public:
  VariableExp(const char* name_) :name(nullptr) {
    name = strdup(name_);
  }
  virtual ~VariableExp() = default;
  virtual bool evaluate(Context& aContext) {
    return aContext.lookup(this);
  }
  virtual BooleanExp* replace( const char* name_, BooleanExp& exp ) {
    if (0 == strcmp(name_, name)) {
      return exp.copy();
    } else {
      return new VariableExp(name);
    }
  }
  virtual BooleanExp* copy() const {
    return new VariableExp(name);
  }
  VariableExp(const VariableExp&) = delete; // rule of three
  VariableExp& operator=(const VariableExp&) = delete;
private:
  char* name;
};

class AndExp : public BooleanExp {
public:
  AndExp(BooleanExp* op1, BooleanExp* op2)
    :operand1(nullptr), operand2(nullptr) {
    operand1 = op1;
    operand2 = op2;
  }
  virtual ~AndExp() = default;
  virtual bool evaluate(Context& aContext) {
    return operand1->evaluate(aContext) && operand2->evaluate(aContext);
  }
  virtual BooleanExp* replace(const char* name_, BooleanExp& exp) {
    return new AndExp(
        operand1->replace(name_, exp),
        operand2->replace(name_, exp)
      );
  }
  virtual BooleanExp* copy() const {
    return new AndExp(operand1->copy(), operand2->copy());
  }
  AndExp(const AndExp&) = delete; // rule of three
  AndExp& operator=(const AndExp&) = delete;
private:
  BooleanExp* operand1;
  BooleanExp* operand2;
};

int main() {
  BooleanExp* expression;
  Context context;
  VariableExp* x = new VariableExp("X");
  VariableExp* y = new VariableExp("Y");
  expression = new AndExp(x, y);

  context.assign(x, false);
  context.assign(y, true);
  bool result = expression->evaluate(context);
  std::cout << result << '\n';

  context.assign(x, true);
  context.assign(y, true);
  result = expression->evaluate(context);
  std::cout << result << '\n';
}
</syntaxhighlight>

The program output is:

<syntaxhighlight lang="c++">
0
1
</syntaxhighlight>