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I'm trying to pass a query to MySQL via a C++ library. However, the query requires a const char* data type, so I tried...

//Initialization code ommitted.. includes declaration of conn, among others...

	const char* updateStr = ("update users set " + dbProperty + "='" + newValue + "' where id=" + ID + ";").c_str();
	mysql_query(conn, updateStr);

	mysql_close(conn);


dbProperty, newValue, and ID are all parameters of the function. However, my code still not working properly. I know that the API is set up correctly because I can use mysql_query to pull data from the database without an issue. It's writing that's the problem. The above query syntax is correct though because I tried it in the server console window, and it worked fine. I wanted to make sure that the char* was appending correctly so I tried..

std::cout << updateStr << std::endl;


And all I got printed was a dash. What's going on? Why can't I build a string, convert it to const char*, and use it? Any ideas?
Posted

1 solution

You are building a temporary std::string object and ask it for its starting address (that's what c_str() does). As the string is temporary it will go out of scope as soon as control moves to the next statement. So, by the time mysql_query is executed, the string contents is already undefined.

Instead, assign the concattenation to a std::string object:
std::string updateCommand = ....
mysql_query (conn, updateCommand.c_str());
   
Comments
Sergey Alexandrovich Kryukov 19-Mar-13 15:07pm
   
5ed!
—SA
nv3 19-Mar-13 15:18pm
   
Thanks, Sergey!
H.Brydon 19-Mar-13 23:15pm
   
me too...
nv3 20-Mar-13 2:52am
   
Thanks, H.Brydon!
RadXPictures 19-Mar-13 15:17pm
   
Thanks! I knew it had to be a simple solution for a pretty stupid question.. So thank you!
nv3 19-Mar-13 15:18pm
   
You are welcome.

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