Convert malloc to new - c++

Question

How to write the following code using new operator?
Please explain in detail.
Thanks in advance.
#include<alloc>
#define MAXROW 3
#define MAXCOL 5
using namespace std;
int main()
{
int (*p)[MAXCOL];
p = (int(*)[MAXCOL])malloc(MAXROW*sizeof(*p));
}

Answer 1

Quite simply, to answer the question literally:
p = new int[MAXROW][MAXCOL];
This allocates a 2D array (MAXROW by MAXCOL) on the free store and, as usual with new, returns a int(*)[MAXCOL] - the same type as decaying the 2D array. Don't forget to delete[] p;.
The last part brings up the importance of std::vector. Presumably, you know the size of the second dimension at compile-time. Therefore, a std::vector<std::array<int, MAXCOL>> would work with the added bonus of not requiring a delete[] statement, plus it knows its size (MAXROW). Please use this if at all possible.
In fact, in your example, both dimensions are known at compile-time, meaning a std::array<std::array<int, MAXCOL>, MAXROW> would also work here. That's typically preferable to dynamic allocation.
If neither dimension is known at compile-time, your best bet is usually a vector of vectors or a dedicated matrix class to increase performance when you know every inner vector is the same size.

Answer 2

Since this is C++ I will recomend using std::array and std::unique_ptr
Also when using malloc you should use free un-alloc or free the memory, if you use new you need to use delete; if you new[] you need to use delete[]
#include <cstdlib>
#include <memory>
#include <array>
#define MAXROW 3
#define MAXCOL 5
using namespace std;
int main()
{
int (*p)[MAXCOL];
p = (int(*)[MAXCOL])malloc(MAXROW*sizeof(*p));
free(p); //free memory
array<int,MAXCOL> *p1 = new array<int,MAXCOL>[MAXROW];
delete []p1; //use this to delete the variable
array<array<int,MAXCOL>,MAXROW> *p2 = new array<array<int,MAXCOL>,MAXROW>;
delete p2; // normal delete for this one
auto p3 = make_unique<array<array<int,MAXCOL>,MAXROW>>();
//no delete needed for p3, it is a smart pointer.
}

Answer 3

The literal question
” How to write the following code using new operator?
… means something else than you think it means.
The new operator is a simple allocation function roughly directly analogous to C's malloc, except the C++ new operator is replacable by a user defined one.
You probably mean a new expression. Such an expression invokes the new operator for allocation, and then invokes a constructor for initialization, if the allocated thing is of class type. And it's type safe.
Still, for your array you'd not want that either, but simply std::vector from the standard library.
Here's an example using a std::vector of vectors to create a matrix:
#include <vector>
using namespace std;
auto main()
-> int
{
int const n_rows = 3;
int const n_cols = 5;
using Row = vector<int>;
vector<Row> v( n_rows, Row( n_cols ) );
// E.g. v[1] is a row, and v[1][2] is an int item in that row.
}
Even if you don't so often use matrices, it can be a good idea to wrap the general notion of a matrix, in a class. A simple way is to use a single std::vector for storage, and provide e.g. an at function or an operator() for indexing from client code. If you don't yet feel like doing this yourself, then e.g. the Boost library provides a matrix class.