Debug Assertion Failed! String manipulation using pointer arithmetic - c++

Question

EDIT: Pastebin links to the entirety of the code at the bottom
for my CS215 course, I was given a class called String215 which is a basic string class to help in the understanding of dynamic memory allocation and pointer arithmetic with char arrays.
The class was given to me in a very basic skeleton form with prototypes but no implementations, along with a test function to test my implementations. I CAN NOT use any C String functions in this assignment.
The part of the program which is troubling is the append function, which just appends a parameter string215 object to the end of the current string215 object.
// Add a suffix to the end of this string. Allocates and frees memory.
void string215::append(const string215 &suffix)
{
char *output = new char[str_len(data)+suffix.length()+1];
for(int x = 0; x < str_len(data); x++) {
*output = *data;
output++;
data++;
}
for(int x = 0; x < suffix.length(); x++) {
*output = suffix.getchar(x);
output++;
}
*output = '\0';
output -= (str_len(data)+suffix.length()+1);
delete[] data;
data = output;
}
This portion of the code is tested in the 13th test of the test function as shown here:
string215 str("testing");
...
// Test 13: test that append works in a simple case.
curr_test++;
string215 suffix("123");
str.append(suffix);
if (strcmp(str.c_str(), "testing123") != 0) {
cerr << "Test " << curr_test << " failed." << endl;
failed++;
}
Here is the description of the append class:
Add the suffix to the end of this string. Allocates a new, larger, array; copies the old contents, followed by the suffix, to the new array; then frees the old array and updates the pointer to the new one.
My program aborts at the very end of the append function execution with the error message:
Debug Assertion Failed!
Program: [Source path]\dbgdel.cpp
Line: 52
Expression: _BLOCK_TYPE_IS_VALID(pHead->nBlockUse)
...
Abort || Retry || Ignore
I'm fairly certain it has something to do with my very poor memory management. I know it's not a lot to go on, but I've been struggling with this for hours on end and can't seem to figure it out.
Here's a pastebin of the .cpp and .h file for this program
string215.cpp: http://pastebin.com/Xh2SvDKJ
string215.h: http://pastebin.com/JfAJDEVN
Any help at all is greatly appreciated!
Thanks,
RAW-BERRY

Answer 1

You are changing data pointer before delete[]. You need to delete[] exactly the same value you got from new[].
Also, you are incrementing output pointer str_len(data)+suffix.length() times, and you take it back by str_len(data) + suffix.length() + 1.
I would use separate variables for iteration to solve these problems.

Answer 2

You increment output exactly str_len(data) + suffix.length() times. Note that you don't increment output after *output = '\0';.
So to go back to the start, you should use:
output -= (str_len(data) + suffix.length());
By the way, some of the code is not very efficient. For example, getchar uses a loop instead of simply returning data[index]. You use getchar in append, which means that the performance isn't great.
EDIT: As zch says, you use delete[] data after modifying data, but note that even before that you use str_len(data) after modifying data (when deciding how many bytes to go skip back), so the calculation is wrong (and my suggestion above is also wrong, because str_len(data) is now zero).

Answer 3

So I think your problem is with the line
for(int x = 0; x < str_len(data); x++) {
Notice that the size of 'data' is changing at each iteration of the loop. As you increment 'x', you are decreasing the length of 'data'. Suppose 'data' is a string holding "hello": in the first iteration of the loop x=0 and str_len(data)=5; in the second iteration x=1 and str_len(data)=4. Thus the for loop executes half as many times as you need it to and 'data' does not end up pointing to the end of the data string