C++ : Exception occurred in script: basic_string::_S_construct NULL not valid

Question

I am returning a string or NULL from the database function to the main program, sometimes i get this error from the exception: ``` basic_string::_S_construct NULL not valid ``` i think its because of the return NULL value from the database function? any ideas??? ``` 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(); } httpl…

Accepted Answer

You cannot return `NULL` (or `0`) from a function that is declared to return `string` because there is no appropriate implicit conversion. You might want to return an empty string though

```
return string();
```

or

```
return "";
```

If you want to be able to distinguish between a `NULL` value and an empty string, then you will have to use either pointers (smart ones, preferrably), or, alternatively, you could use `std::optional` (or `boost::optional` before C++17).

C++ : Exception occurred in script: basic_string::_S_construct NULL not valid

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