评估C++字符串中的表达式:"来自$ {host}的Hi $ {user}"

Question

我正在寻找一种干净的C++方法来解析包含$ {}中包含的表达式的字符串,并从编程计算表达式构建结果字符串.

示例:如果我实现程序让"user"评估为"foo"等,"来自$ {host}的Hi $ {user}"将被评估为"Hi foo from bar".

我正在考虑的当前方法包括一个状态机,它一次从字符串中吃掉一个字符,并在达到'}'后评估表达式.任何提示或其他建议？

注意:boost ::非常欢迎!:-)

更新感谢前三个建议!不幸的是我让这个例子太简单了!我需要能够检查$ {}内的内容,所以这不是一个简单的搜索和替换.也许它会说$ {uppercase:foo}然后我必须使用"foo"作为hashmap中的一个键,然后将其转换为大写,但我在编写上面的原始问题时试图避免$ {}的内部细节...... :-)

Answer 1

#include <iostream>
#include <conio.h>
#include <string>
#include <map>

using namespace std;

struct Token
{
    enum E
    {
        Replace,
        Literal,
        Eos
    };
};

class ParseExp
{
private:
    enum State
    {
        State_Begin,
        State_Literal,
        State_StartRep,
        State_RepWord,
        State_EndRep
    };

    string          m_str;
    int             m_char;
    unsigned int    m_length;
    string          m_lexme;
    Token::E        m_token;
    State           m_state;

public:
    void Parse(const string& str)
    {
        m_char = 0;
        m_str = str;
        m_length = str.size();
    }

    Token::E NextToken()
    {
        if (m_char >= m_length)
            m_token = Token::Eos;

        m_lexme = "";
        m_state = State_Begin;
        bool stop = false;
        while (m_char <= m_length && !stop)
        {
            char ch = m_str[m_char++];
            switch (m_state)
            {
            case State_Begin:
                if (ch == '$')
                {
                    m_state = State_StartRep;
                    m_token = Token::Replace;
                    continue;
                }
                else
                {
                    m_state = State_Literal;
                    m_token = Token::Literal;
                }
                break;

            case State_StartRep:
                if (ch == '{')
                {
                    m_state = State_RepWord;
                    continue;
                }
                else
                    continue;
                break;

            case State_RepWord:
                if (ch == '}')
                {
                    stop = true;
                    continue;
                }
                break;

            case State_Literal:
                if (ch == '$')
                {
                    stop = true;
                    m_char--;
                    continue;
                }
            }

            m_lexme += ch;
        }

        return  m_token;
    }

    const string& Lexme() const
    {
        return m_lexme;
    }

    Token::E Token() const
    {
        return m_token;
    }
};

string DoReplace(const string& str, const map<string, string>& dict)
{
    ParseExp exp;
    exp.Parse(str);
    string ret = "";
    while (exp.NextToken() != Token::Eos)
    {
        if (exp.Token() == Token::Literal)
            ret += exp.Lexme();
        else
        {
            map<string, string>::const_iterator iter = dict.find(exp.Lexme());
            if (iter != dict.end())
                ret += (*iter).second;
            else
                ret += "undefined(" + exp.Lexme() + ")";
        }
    }
    return ret;
}

int main()
{
    map<string, string> words;
    words["hello"] = "hey";
    words["test"] = "bla";
    cout << DoReplace("${hello} world ${test} ${undef}", words);
    _getch();
}

我很乐意解释有关此代码的任何信息:)