c++ deleting char pointer allocated with new - c++

Question

In this code I am getting numbers from a file, when the first number is the size of the 2D array.
In my code I'm defining
char *filename=new char;
(I have to use char *filename, this is the exercise..)
Everything works fine, until the moment I try to delete. both delete and delete[] gives me error and crashing my program.
This is my full code:
#include <iostream>
#include <fstream>
using namespace std;
double **readmat(char *filename, int *size)/////question 2
{
ifstream read(filename);
cout << filename << endl;
if (!read)
{
cout << "Can't open file!" << endl;
exit(1);
}
read >> *size;
double **mat = new double*[*size];
for (int i = 0; i < *size; i++)
{
mat[i] = new double[*size];
for (int j = 0; j < *size; j++)
{
read >> mat[i][j];
}
}
read.close();
return mat;
}
int main()
{
int size;
char *filename = new char;
filename = "text.txt";
double **arr = readmat(filename, &size);
for (int i = 0; i < size; i++)
{
for (int j = 0; j < size; j++)
{
cout << arr[i][j]<<" , ";
}
cout << endl;
}
cout << endl;
delete filename; //<-------- this crashed my code
for (int i = 0; i < size; i++)
{
delete[] arr[i];
}
delete[] arr;
return 0;
}
This is how my file looks:
This is what the console app looks like after running the code:
Which is what I am expecting to get, but I get this error:
Does anyone have any idea what could this happen, and what I can do to fix it?

Answer 1

You are trying to delete a char* that is not pointing at memory allocated with new.
On this line:
char *filename = new char;
You do new some memory (a single char, not a string of chars). But then on this line:
filename = "text.txt";
You change the char* pointer to point at completely different memory, thus leaking the memory you new'ed.
Then on this line:
delete filename;
You try to delete the "text.txt" literal, not the char you new'ed. That is why you crash.
For what you are attempting to do, you need to do this instead:
char *filename = new char[strlen("text.txt")+1];
strcpy(filename, "text.txt");
...
delete[] filename;
However, you really should not be using new/new[] for filename at all. Use std::string instead:
#include <fstream>
#include <string>
double **readmat(const std::string &filename, int *size)
{
std::ifstream read(filename.c_str());
...
}
int main()
{
int size;
double **arr = readmat("text.txt", &size);
...
}
Alternatively:
#include <fstream>
#include <string>
double **readmat(const char *filename, int *size)
{
ifstream read(filename);
...
}
int main()
{
int size;
std::string filename = "text.txt";
double **arr = readmat(filename.c_str(), &size);
// or simply:
// double **arr = readmat("text.txt", &size);
...
}
And then, while you are at it, you should not be using new[] for your matrix, either. Use std::vector instead:
#include <vector>
std::vector< std::vector<double> > readmat(char *filename)
{
...
int size;
read >> size;
std::vector< std::vector<double> > mat(size);
for (int i = 0; i < size; i++)
{
mat[i].resize(size);
for (int j = 0; j < size; j++)
{
read >> mat[i][j];
}
}
return mat;
}
int main()
{
...
std::vector< std::vector<double> > arr = readmat("text.txt");
size_t size = arr.size();
for (size_t i = 0; i < size; i++)
{
for (size_t j = 0; j < size; j++)
{
std::cout << arr[i][j] << " , ";
}
std::cout << endl;
}
std::cout << endl;
return 0;
}

Answer 2

char *filename = new char;
filename = "text.txt";
This creates a new char, then leaks it because the pointer filename is reassigned to something that is statically declared.
Therefore, later on you delete something else than the original char.
Multiple issues here (using new instead of new[], etc). Suggestion, forget everything and use std::string and STL.

Answer 3

This is the source of your problem:
char *filename = new char;
filename = "text.txt";
filename no longer points to dynamically allocated memory, thus you can't delete it (and you're also leaking 1 byte of memory). Change your declaration to const char *filename = "test.txt"; and remove the delete filename;.

Answer 4

new char allocates a single character on the heap. Most functions that take a const char* as parameter expect a pointer to the first element of an array with the null character (\0) as delimiter (a C-style string).
You shouldn't even be able to assign a string literal to a variable of type char *, at least not in standard C++. You also don't need to dynamically allocate memory for string literals, simply use
const char *filename = "text.txt";
Then you also don't delete pointers to string literals. (That's what causes the error most likely, you deleted a pointer that pointed to a string literal)

Answer 5

Just replace
char* filename = new char;
with
const char* filename = "text.txt";
and remove
delete filename;
This is how your final code will look
int main()
{
int size;
const char *filename = "text.txt";
double **arr = readmat(filename, &size);
for (int i = 0; i < size; i++)
{
for (int j = 0; j < size; j++)
{
cout << arr[i][j]<<" , ";
}
cout << endl;
}
cout << endl;
for (int i = 0; i < size; i++)
{
delete[] arr[i];
}
delete[] arr;
return 0;
}