Matrix allocation using double pointer

Question

Hi

Please refer below code :

I have tried to allocate memory for matrix. Is double pointer only or best suitable approach ?

Also I got confused that why 32 + 48 bytes are allocated ? 48 (12 bytes of 4 rows is ok for me to digest) but what about 32 bytes at first line of output?

Also is there any memory leak ?

I will obviously go for smart pointer and will update code once I am more clear on my doubt of raw pointer implementation.

Thanks in advance.
https://code.sololearn.com/cjHxkNuLBRXX/?ref=app

Answer 1

A safe and easy way to implement a matrix in C++, is to use a single `std::vector` as storage, and just provide 2D indexing of that.

Answer 2

Ketan,
Isn't the 32 byte allocated for the int*? assuming a 64bit system, a pointer size would be 8 bytes (64 bits), and there you have 4 rows. So we just multiply number of rows (4) by sizeof( int* ) (8) and that's where the 32 bytes was for.

Each row has 3 columns, each carries an int (assumed 4 bytes each). And that makes 12 (3 columns * sizeof( int ) (4)).

Not sure about memory leaks, but as I remember, each row will need to be freed first, before <ptr> itself was to be freed.

Hope I got that right (cmiiw) ...

Answer 3

You're allocating 'r' pointers for the rows of 'c' ints

Matrix
|
V
int* R1 -> [M11, M12,..., M1c]
int* R2 -> [M21, M22,..., M2c]
.
.
.
int* Rr -> [Mr1, Mr2,..., Mrc]

Than yes, there is a memory leak as you are only freeing the array of pointers [R1,... Rr] and forgetting about all of the columns [Mxx]

Answer 4

Thanks Ipang , ♨️A.S. Raghuvanshi♨️ and Angelo

It is helpful..

I updated code for three different approach.

Can anyone help me to solve issue in current code ? Basically I am struggling to have array of pointer with unique pointer.

Answer 5

Ketan Lalcheta The problem is in the declaration of the matrix object.
What you want is a unique_ptr that holds an array of unique_ptrs that in turn hold arrays of ints. Not a unique_ptr that holds an array of int pointers.

In other words, just replace your declaration of matrix with this

unique_ptr<unique_ptr<int[]>[]> matrix(new unique_ptr<int[]>[r]);

and it should work.