自制计算器（一）：Scanner

    class Token 
    {
        // 略

    private:
        TokenType type_;
        int intValue_;
    };

    enum class TokenType 
    {
        INT,
        FLOAT,
        ADD,        // +
        SUB,        // -
        MUL,        // * 
        DIV,        // /
        LEFT_PAR,    // (
        RIGHT_PAR,    // )

        INVALID,    // 无效类型
    };

    Token Scanner::GetNextToken(stringstream &expression)
    {
        // first char
        auto currectChar = GetNextChar(expression);

        // state judge
        while (!expression.eof())
        {
            // 第一部分
            switch (state_)
            {
            case State::START:
                break;

            case State::NUMBER:
                return HandleNumberState(expression, currectChar);
                break;

            case State::OPERATOR:
                return HandleOperatorState(expression, currectChar);
                break;

            case State::ERROR:
                ErrorToken("error input");
                return Token(TokenType::INVALID);
            }

            string currectStr;
            currectStr.push_back(currectChar);

            //第二部分
            if (iswdigit(currectChar))
            {
                state_ = State::NUMBER;
            }
            else if (dict_.HasToken(currectStr))
            {
                state_ = State::OPERATOR;
            }
            else
            {
                state_ = State::ERROR;
            }
        }
        return Token(TokenType::INVALID);
    }        

    Token Scanner::HandleNumberState(stringstream &expression, char currectChar)
    {
        // first char
        string buffer;
        buffer.push_back(currectChar);

        while (!expression.eof() && isdigit(expression.peek()))
        { 
            buffer += GetNextChar(expression);
        }

        // reset state
        state_ = State::START;

        // string to int
        int value;
        std::stringstream stream(buffer);
        stream >> value;

        return Token(TokenType::INT, value);
    }

    Token Scanner::HandleOperatorState(stringstream &expression, char currectChar)
    {
        // first char
        string buffer;
        buffer.push_back(currectChar);
        auto token = dict_.FindToken(buffer);

        //reset state
        state_ = State::START;

        return token;
    }