const_cast problem??

Bob Stanneveld

Hello, I've encountered a strange problem when I use the const_cast() operator. See the following code that behaves strange:

std::stringstream sstrMsg;

// bla bla bla

char* pszMsg = const_cast(sstrMsg.str().c_str());
SPrintBuffer(&pszMsg);

I've checked the return values of both stringstream::str() and std::string::c_str(). They both return the expected data. Strangely enough, after the cast, pszMsg points to some undefined memory address. (The address pointed to holds the value -18). I'm currently using the following workaround:

char* pszMsg = new char[sstrMsg.str().length()];
strncpy(pszMsg, sstrMsg.str().c_str(), sstrMsg.str().length());
SPrintBuffer(&pszMsg);
delete[] pszMsg;

Does anybody have a clue why const_cast is producing undesired results? Behind every great black man... ... is the police. - Conspiracy brother Blog[^]

BadKarma

Hi, If you mean the const_cast casting operator, I believe you have used it wrong. It should be used like this: char* pszMsg = const_cast<char*>(sstrMsg.str().c_str()); This shouldn't give any problems. codito ergo sum

John R Shaw

Your code looks very dangerous. Q1) Why would you need to cast away the constancy of the pointer returned by c_str()? How about using: const char* pszMsg = sstrMsg.str().c_str(); Q2) Why would a function named SPrintBuffer() need a pointer to a pointer of type char*? The name implies that it is just going to print out the contents of the buffer, not modify it. Note: Modifing the data return via c_str(), is a very bad idea. INTP Every thing is relative...

Bob Stanneveld

I know that the code looks very dangerous. I double checked the code for SPrintBuffer() and it doesn't modify the string. The reason that the reason the function takes just a char* and not a const char* is unknown to me. I need to add a link layer of a protocol to a program that my teacher wrote. Please don't tell me, the entire program is badly designed and all the code looks like spaghetti... If you want to see some obfuscated C code, this program is right for you! Behind every great black man... ... is the police. - Conspiracy brother Blog[^]

Bob Stanneveld

Hello, I used it just like you said it should be used. It's just that the '<' and the '>' 'disappeared' because I didn't use '<' and '>' respectively. Sometimes the parser for the message removes those in pre tags and sometimes it doesn't. Behind every great black man... ... is the police. - Conspiracy brother Blog[^]

John R Shaw

Sorry I have been gone for a few days.:( 1) It sounds like your teachers code would not pass a review by me, let alone a quality assurance team. If the function does not modify the data pointed to by the argument, then you should say so by making it a pointer to constant data. 2) You still should not need to use const_cast, instead you can just call the function like so: SPrintBuffer((char*)str.c_str());. Note: The const_cast should not be needed in this particular case. Note: In your original code you are trying to pass a char** and not a char*

Bob Stanneveld

John R. Shaw wrote:

Sorry I have been gone for a few days.

So have I... :-D

John R. Shaw wrote:

It sounds like your teachers code would not pass a review by me, let alone a quality assurance team.

That was my first thought and I offered to rewrite the entire program instead of doing all the other assignments. This was a no-go since the course is not going to be given anymore. Anyway, that's the way we learn things in college these days.. :sigh:

John R. Shaw wrote:

You still should not need to use const_cast, instead you can just call the function like so: SPrintBuffer((char*)str.c_str());.

I force myself to use straight C++ only. The C-Style casts are there fore backwards compatibility. They shouldn't make a difference. I'll try it and let you know if there it will work. Behind every great black man... ... is the police. - Conspiracy brother Blog[^]