Linux C++：脚本中发生异常：basic_string::_S_construct NULL 无效
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C++ : Exception occurred in script: basic_string::_S_construct NULL not valid
提问by krisdigitx
I am returning a string or NULL from the database function to the main program, sometimes i get this error from the exception:
我从数据库函数向主程序返回一个字符串或NULL，有时我从异常中得到这个错误：
basic_string::_S_construct NULL not valid
i think its because of the return NULL value from the database function? any ideas???
我认为这是因为从数据库函数返回 NULL 值？有任何想法吗？？？
string database(string& ip, string& agent){
//this is just for explanation
.....
....
return NULL or return string
}
int main(){
string ip,host,proto,method,agent,request,newdec;
httplog.open("/var/log/redirect/httplog.log", ios::app);
try{
ip = getenv("IP");
host = getenv("CLIENT[host]");
proto = getenv("HTTP_PROTO");
method = getenv("HTTP_METHOD");
agent = getenv("CLIENT[user-agent]");
if (std::string::npos != host.find(string("dmnfmsdn.com")))
return 0;
if (std::string::npos != host.find(string("sdsdsds.com")))
return 0;
if (method=="POST")
return 0;
newdec = database(ip,agent);
if (newdec.empty())
return 0;
else {
httplog << "Redirecting to splash page for user IP: " << ip << endl;
cout << newdec;
cout.flush();
}
httplog.close();
return 0;
}
catch (exception& e){
httplog << "Exception occurred in script: " << e.what() << endl;
return 0;
}
return 0;
}
采纳答案by Armen Tsirunyan
You cannot return NULL(or 0) from a function that is declared to return stringbecause there is no appropriate implicit conversion. You might want to return an empty string though
您不能从声明为返回的函数返回NULL（或0），string因为没有适当的隐式转换。你可能想返回一个空字符串
return string();
or
或者
return "";
If you want to be able to distinguish between a NULLvalue and an empty string, then you will have to use either pointers (smart ones, preferrably), or, alternatively, you could use boost::optional
如果您希望能够区NULL分值和空字符串，则必须使用指针（最好是智能指针），或者，您可以使用boost::optional
回答by FaddishWorm
I would try changing it to return an empty string instead of null and check the string length.
我会尝试将其更改为返回一个空字符串而不是 null 并检查字符串长度。
回答by CB Bailey
It's a violation of std::string's contract to construct it from a null charpointer. Just return an empty string if the pointer that you want to construct it from is null.
std::string从空char指针构造它违反了的契约。如果要从中构造它的指针为空，则只返回一个空字符串。
E.g.
例如
return p == NULL ? std::string() : std::string(p);
