glibc detected - double free or corruption - c++

Question

I get the following error messages when I submit the code (pasted below) to an online gcc compiler.
* glibc detected /run-1326102706-2046832693/solution: double free or corruption (!prev): 0x091901a8 ** =======
The code is as follows:
# include <iostream>
# include <string>
# include <list>
# include <cstring>
using namespace std;
int main()
{
int test_cases, i, score, str_len;
string str;
char first_char, current_char;
list <int> strlist;
list <int> :: iterator it;
cin>>test_cases;
char *cstr[test_cases]; //Creating an array of cstr pointers (test_cases number of pointers)
while(test_cases > 0)
{
cin>>str;
first_char = str.at(0);
str_len = str.length();
score = str_len;
strlist.clear();
cstr[test_cases-1] = new char[str_len];
strcpy(cstr[test_cases-1],str.c_str()); //copying the input str into cstr. This is done to minimize the complexity of std::string's at function.
for(i=1;i<str_len; i++)
{
current_char = *(cstr[test_cases-1]+i);
if (current_char == first_char)
{
score++; strlist.push_front(1);
it = strlist.begin();
if (it != strlist.end())
it++;
}
while (!strlist.empty() && it != strlist.end())
{
if (current_char == *(cstr[test_cases-1] + *(it)))
{
(*it)++;it++;score++;
}
else
it = strlist.erase(it);
}
if (!strlist.empty())
it = strlist.begin();
}
cout<<score<<endl;
delete(cstr[test_cases-1]);
test_cases--;
}
return 0;
}
As mentioned in the code itself, I initially used std::string, but found that the std::string.at function was quite slow (esepcially since this problem has really large input strings). So I decided to store the string input in a character array, so that direct indexing to a particular position would be possible.
Appreciate any help.

Answer 1

There are two problems that I can see:
cstr[test_cases-1] = new char[str_len]; // Not allocating space for terminating NULL.
delete(cstr[test_cases-1]); // Incorrect delete, should be delete[]
// As already pointed out by mooware
Change these two lines to:
cstr[test_cases-1] = new char[str_len + 1];
delete[] cstr[test_cases-1];

Answer 2

You are using array-new ("new char[str_len]") to allocate the strings, but scalar-delete ("delete(cstr[test_cases-1])") to delete them. You should always match the new- and delete-operators, so when you use array-new, also use array-delete ("delete[] cstr[test_cases-1]").

Answer 3

You have two bugs. One is here:
cstr[test_cases-1] = new char[str_len];
strcpy(cstr[test_cases-1],str.c_str());
You allocate one byte too few. That should be new char[str_len+1] since strcpy copies the terminator.
The other is here:
delete(cstr[test_cases-1]);
You cannot allocate with new[] and deallocate with delete. If you allocate with new[], you must deallocate with delete[].