Java代码转C++代码，求懂两种语言的帮个忙

[elemusic]

改了半天改晕了.....好像Java的substring用法同C++不同，具体怎么变没搞清.....似乎Java的substring对空元素访问有些自动纠错处同C++不同吧。

要转的代码是这页前面的这一小段

http://www.oodesign.com/interpreter-pattern.html

这是我目前转的，但是不对的，哪位能帮着改一下使程序正常，达到罗马数字转阿拉伯数字的功能。或者说说Java那里怎么处理，C++中怎么等价变换过去也好。

#include <iostream>
#include <string>
#include <vector>

using namespace std;

class Context
{
public:
	string input;
	int output;
	Context(string str_){input=str_;}
	string GetInput()
	{
		return input;
	}
	void SetInput(string input_)
	{
		input = input_;
	}
	int GetOutput()
	{
		return output;
	}
	void SetOutput(int output_)
	{
		output = output_;
	}
};

class Expression
{
public:
	Expression(){}
	void interpret(Context* context)
	{
		if (context->GetInput().length() == 0) 
			return;


		if (context->GetInput().find(nine()))
		{
			context->SetOutput(context->GetOutput() + (9 * multiplier()));
			context->SetInput(context->GetInput().substr(2, len-2));//自己跟了一下，错误出在这里
//input长度为1时会有问题，其实我想可能还是不太对吧，主要不太清楚Java那个substring到底怎么处理的
		}
		else if (context->GetInput().find(five()))
		{
			context->SetOutput(context->GetOutput() + (5 * multiplier()));
			context->SetInput( context->GetInput().substr(1, len-1));
		}
		else if (context->GetInput().find(four()))
		{
			context->SetOutput(context->GetOutput() + (4 * multiplier()));
			context->SetInput(context->GetInput().substr(2, len-2));
		}

		while (context->GetInput().find(one()))
		{
			context->SetOutput(context->GetOutput() + (1 * multiplier()));
			string tempstr = context->GetInput().substr(1, len-1);
			if(tempstr != "")
			{
				context->SetInput(context->GetInput().substr(1, len-1));
			}
			else
			{
				break;
			}
			
		}
	}
	virtual string one() = 0;
	virtual string four() = 0;
	virtual string five() = 0;
	virtual string nine() = 0;
	virtual int multiplier() = 0;
};

class ThousandExpression : public Expression
{
public:
	ThousandExpression(){}
	string one(){return "M";}
	string four(){return " ";}
	string five(){return " ";}
	string nine(){return " ";}
	int multiplier(){return 1000;}
};

class HundredExpression : public Expression
{
public:
	HundredExpression(){}
	string one(){return "C";}
	string four(){return "CD";}
	string five(){return "D";}
	string nine(){return "CM";}
	int multiplier(){return 100;}
};

class TenExpression : public Expression
{
public:
	TenExpression(){}
	string one(){return "X";}
	string four(){return "XL";}
	string five(){return "L";}
	string nine(){return "XC";}
	int multiplier(){return 10;}
};

class OneExpression : public Expression
{
public:
	OneExpression(){}
	string one(){return "I";}
	string four(){return "IV";}
	string five(){return "V";}
	string nine(){return "IX";}
	int multiplier(){return 1;}
};

int main()
{
	// 罗马数字转阿拉伯数字
	string roman = "MCMXXVIII";

	Context* con = new Context(roman);

	vector<Expression*> vec;

	vec.push_back(new ThousandExpression());
	vec.push_back(new HundredExpression());
	vec.push_back(new TenExpression());
	vec.push_back(new OneExpression());

	vector<Expression*>::iterator pos;

	for(pos = vec.begin(); pos != vec.end(); ++pos)
	{
		(*pos)->interpret(con);
	}

	delete con;



	return 0;
}

[WorldCreater]

substring


public String substring(int beginIndex)

public String substring(int beginIndex,
int endIndex)

[elemusic]

我想问的是

context.setInput(context.getInput().substring(2));

对于input长度小于2的情况这句是怎么正常处理的，C++怎么转换，看起来好像Java对空元素直接访问有什么特别的处理？