Debug failure error

This is a discussion on Debug failure error within the C++ Programming forums, part of the General Programming Boards category; I'm trying to create my own string class, which I named CStr. First, I defined it as such (I left ...

  1. #1
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    Join Date
    Oct 2005
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    271

    Debug failure error

    I'm trying to create my own string class, which I named CStr.

    First, I defined it as such (I left out a lot of irrelevant code at the moment):

    Code:
    class CStr
    {
    	char* pData;
            int nLength;
    	int stlen(char* c){for(int i = 0; *c != '\0'; c++, i++); return i;}
    	char* stcpy(char*, const char*);
    	char* stcat(char*, const char*);
    public:
    	CStr(char*);
    	void set(char* c){stcpy(pData, c);}
    };
    Then the member function definitions:

    Code:
    char* CStr::stcpy(char* dest, const char* src)
    {
    	char* p;
    	p = dest;
    	if(nLength > 0)
    	{
    		delete [] pData;
    		pData = new char[stlen(src)];
    		p = dest = pData;
    	}
    	while(*(p++)=*(src++));
    	return dest;
    }
    
    CStr::CStr(char* c)
    {
    	pData = new char[stlen(c)];
    	stcpy(pData, c);
    	nLength = stlen(c);
    }
    Now here's the code from the main() function.

    Code:
    	char* p = "First input";
    	char q[256];
    	CStr c(p);
    	while(gets(q))
    		c.set(q);

    My problem is that I get a "Debug Error" message (in VC++ 2003)) which states as follows:
    DAMAGE: After Normal Block(#42) at XXXXX.

    This occurs at the line "delete [] pData" in the definition of stcpy() when it is called by "c.set(q)".

    Please tell me, what am I doing wrong here?

  2. #2
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    Join Date
    Aug 2005
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    1,267
    you don't allow room for the null-terminator

    Code:
    pData = new char[stlen(src)+1];
    why does strcpy() method have two parameters? It only needs one -- the source string. Destination string should always be pData, which does not have to be passed to the function.

  3. #3
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    Oct 2005
    Posts
    271
    Thanks, your correction worked perfectly.

    As for my stcpy function, I was curious how the actual strcpy function in string.h worked so I thought I might as well try my hand at mimicing it. I could have written one specifically for this class, but just wanted to make something generic.

  4. #4
    Registered User
    Join Date
    Apr 2003
    Posts
    2,662
    Hi,

    The defining aspect of a C-style string is that it ends in a '\0' character. However, that '\0' is not included in the strlen() count. That will constantly trip you up when dealing with C-style strings. The '\0' is required at the end of C-style strings because it indicates to all the <cstring> functions where the characters in the array end. There's no bounds checking in C++, so a while loop will go right past the end of an array and into uncharted memory if you let it. cout<< also uses that '\0' to determine when to stop outputing characters to the screen. So, don't forget about the '\0'!

    Your strcat() function will provide a little twist on that concept because you don't want to end up with:

    "string1\0string2\0"

    The <cstring> functions and cout<< use a while loop to advance along the array one character at a time, and the loop will terminate when a '\0' character is encountered. So those functions won't be able to see anything past the first '\0'.
    Last edited by 7stud; 11-13-2005 at 12:55 AM.

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