c++判断一个字符串是否是数字

bool isNum(const string& str)
{
    bool bRet = false;
    bool point = false;
    if(str.length() == 0)
    {
        return bRet;
    }
    
    if (!isdigit(str[0]) && str[0] != ‘+‘ && str[0] != ‘-‘ )
    {
        return bRet;
    }

    if (str[1] == ‘.‘ && (str[0] == ‘+‘ || str[0] == ‘-‘))
    {
        return bRet;
    }

    for ( int i = 1; i < str.length(); i++ )
    {
        if (!isdigit(str[i]))
        {
            if (str[i] == ‘.‘ && !point)
            {
                point = true;
            } else
            {
                return bRet;
            }

        }
    }

    return true;
}