解释器模式

#include <iostream>
#include <map>
#include <string>
using namespace std;

#define SAFE_DELETE(p) if ((p)) {delete (p); (p) = NULL;}

class Context
{
public:
    void addValue(string key, int value)
    {
        //_valueMap.insert(pair<string, int>(key, value));
        _valueMap[key] = value;
    
    }

    int getValue(string key) const
    {
        return _valueMap.at(key);
    }

private:
    map<string, int> _valueMap;
};

class AbstractExpression
{
public:
    virtual ~AbstractExpression() {}
    virtual int interpreter(const Context &context) = 0;
};

class AddNonterminalExpression : public AbstractExpression
{
public:
    AddNonterminalExpression(AbstractExpression *left, AbstractExpression *right) : _left(left), _right(right){}
    ~AddNonterminalExpression()
    {
        SAFE_DELETE(_left);
        SAFE_DELETE(_right);
    }

    virtual int interpreter(const Context &context)
    {
        return _left->interpreter(context) + _right->interpreter(context);
    }
private:
    AbstractExpression *_left;
    AbstractExpression *_right;
};


class SubstractNonterminalExpression : public AbstractExpression
{
public:
    SubstractNonterminalExpression(AbstractExpression *left, AbstractExpression *right) : _left(left), _right(right){}
    ~SubstractNonterminalExpression()
    {
        SAFE_DELETE(_left);
        SAFE_DELETE(_right);
    }

    virtual int interpreter(const Context &context)
    {
        return _left->interpreter(context) - _right->interpreter(context);
    }
private:
    AbstractExpression *_left;
    AbstractExpression *_right;
};


class TerminalExpression : public AbstractExpression
{
public:
    TerminalExpression(string key)
    {
        _key = key;
    }

    int interpreter(const Context &context)
    {
        return context.getValue(_key);
    }
private:
    string _key;
};

void test()
{
    Context cxt;
    cxt.addValue("a", 1);
    cxt.addValue("b", 3);
    cxt.addValue("c", 5);
    
    // 1 + 3
    AbstractExpression *addExpr = new AddNonterminalExpression(new TerminalExpression("a"), new TerminalExpression("b"));
    cout << addExpr->interpreter(cxt) << endl;
    
    // 4 - 5
    AbstractExpression *subExpr = new SubstractNonterminalExpression(addExpr, new TerminalExpression("c"));
    cout << subExpr->interpreter(cxt) << endl;
    delete subExpr;
}

int main()
{
    test();
    cin.get();
    return 0;
}