Programming Forums > Application Development > C++

Jimbo · Jul 16th, 2006, 12:17 PM

Quote:

Originally Posted by Jessehk

I personally think that DaWei, Narue, and Grumpy are some of, if not the most helpful members on this forum. Thanks to all of you!

++
And in the case of DaWei, possibly the most entertaining as well

kyoryu · Jul 17th, 2006, 12:14 PM

Quote:

Originally Posted by Sane

By the way, Narue, I'm not sure if you were calling my approach to it incorrect. Because all I had to do was add the word static before the declaration of "new_text". Which is still returning a pointer, but it's not as if I had to do much.

Question: What is the output of the following program? Why?

(Toggle Plain Text)

// insert proper .h files, omitted for brevity
using namespace std;
const char * doSomethingWithAString( const char * inString )
{
	// please ignore the lack of length checking.
	static char internal_string[50];
	memcpy( internal_string, inString, strlen( inString ) );
	return internal_string;
}
int main()
{
	const char * first_string = doSomethingWithAString( "Hello, " );
	const char * second_string = doSomethingWithAString( "world!\n" );
	cout << first_string << " " << second_string;
	return 0;

}

grumpy · Aug 22nd, 2006, 3:44 AM

Hmmm ..... I don't normally resurrect old threads, but someone sent me a PM suggesting I look at the last post in this thread.

The code given by kyoryu yields undefined behaviour, as doSomethingWithAString() does not append a zero byte on the end of internal_string.

In practice, what will usually be seen will be typically be the word "world!" on a line of it's own, possibly followed by a lot of random junk, then a space, then the text "world!" on a line of it's own again, again possibly following by a lot of random junk. On some systems, the program will crash, hang, or otherwise die miserably somewhere in the process of doing all this.

If kyoru was to change the memset() line to

(Toggle Plain Text)

memcpy( internal_string, inString, strlen( inString ) +1);

then the output would be two lines of text saying "world!"

Either way, this example illustrates quite nicely that simply making a local variable static does not ensure the program gives the behaviour the programmer might intend - a lot of people would expect this code to output the line "Hello world!".

Aug 22nd, 2006, 3:44 AM	#23
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,223 Rep Power: 5	Hmmm ..... I don't normally resurrect old threads, but someone sent me a PM suggesting I look at the last post in this thread. The code given by kyoryu yields undefined behaviour, as doSomethingWithAString() does not append a zero byte on the end of internal_string. In practice, what will usually be seen will be typically be the word "world!" on a line of it's own, possibly followed by a lot of random junk, then a space, then the text "world!" on a line of it's own again, again possibly following by a lot of random junk. On some systems, the program will crash, hang, or otherwise die miserably somewhere in the process of doing all this. If kyoru was to change the memset() line to (Toggle Plain Text) memcpy( internal_string, inString, strlen( inString ) +1); memcpy( internal_string, inString, strlen( inString ) +1); then the output would be two lines of text saying "world!" Either way, this example illustrates quite nicely that simply making a local variable static does not ensure the program gives the behaviour the programmer might intend - a lot of people would expect this code to output the line "Hello world!".

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