Why isn't my const char* cast working correctly?

Question

0.00/5 (No votes)

See more:

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...

C++

//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..

C++

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 19-Mar-13 8:46am

RadXPictures

Add a Solution

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

nv3 · Accepted Answer · 2013-03-19T08:58:00

Solution 1

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:

C++

std::string updateCommand = ....
mysql_query (conn, updateCommand.c_str());

Posted 19-Mar-13 8:58am

nv3

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.