Interpreter pattern (original) (raw)

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Approach in computer programming

In computer programming, the interpreter pattern is a design pattern that specifies how to evaluate sentences in a language. The basic idea is to have a class for each symbol (terminal or nonterminal) in a specialized computer language. The syntax tree of a sentence in the language is an instance of the composite pattern and is used to evaluate (interpret) the sentence for a client.[1]: 243 See also Composite pattern.

The Interpreter[2]design pattern is one of the twenty-three well-known _GoF design patterns_that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

What problems can the Interpreter design pattern solve?

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Source:[3]

When a problem occurs very often, it could be considered to represent it as a sentence in a simple language (Domain Specific Languages) so that an interpreter can solve the problem by interpreting the sentence.

For example, when many different or complex search expressions must be specified. Implementing (hard-wiring) them directly into a class is inflexible because it commits the class to particular expressions and makes it impossible to specify new expressions or change existing ones independently from (without having to change) the class.

What solution does the Interpreter design pattern describe?

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The expression objects are composed recursively into a composite/tree structure that is called_abstract syntax tree_ (see Composite pattern).
The Interpreter pattern doesn't describe how to build an abstract syntax tree. This can be done either manually by a client or automatically by a parser.

See also the UML class and object diagram below.

UML class and object diagram

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A sample UML class and object diagram for the Interpreter design pattern.[4]

In the above UML class diagram, the Client class refers to the common AbstractExpression interface for interpreting an expressioninterpret(context).
The TerminalExpression class has no children and interprets an expression directly.
The NonTerminalExpression class maintains a container of child expressions (expressions) and forwards interpret requests to these expressions.

The object collaboration diagram shows the run-time interactions: The Client object sends an interpret request to the abstract syntax tree. The request is forwarded to (performed on) all objects downwards the tree structure.
The NonTerminalExpression objects (ntExpr1,ntExpr2) forward the request to their child expressions.
The TerminalExpression objects (tExpr1,tExpr2,…) perform the interpretation directly.

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

#include #include #include

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'; }

The program output is:

  1. ^ Gamma, Erich; Helm, Richard; Johnson, Ralph; Vlissides, John (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. ISBN 0-201-63361-2.
  2. ^ Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison Wesley. pp. 243ff. ISBN 0-201-63361-2.{{[cite book](/wiki/Template:Cite%5Fbook "Template:Cite book")}}: CS1 maint: multiple names: authors list (link)
  3. ^ "The Interpreter design pattern - Problem, Solution, and Applicability". w3sDesign.com. Retrieved 2017-08-12.
  4. ^ "The Interpreter design pattern - Structure and Collaboration". w3sDesign.com. Retrieved 2017-08-12.