vj1207 - 1 year ago 78
C++ Question

# Allocating memory for array of pointers of int dynamically

If I want to dynamically allocate memory for array of pointers of int, how can I achieve this requirement?

Suppose I declare an array of pointers of int like this:

``````int (* mat)[];
``````

Is there a way I can allocate memory for K number of pointers dynamically and assign it to mat? If I do

``````mat = new int * [K];
``````

It gives error :
`cannot convert 'int**' to 'int (*)[]' in assignment`
. I understand this memory allocation is implicitly got converted to int **. Is there any way to allocate memory for above scenario?

Even when I try to assignment of statically allocated array of pointers of int to array of pointers of int, like this:

``````int (*mat)[] = NULL;
int (* array_pointers)[26];
mat = array_pointers;
``````

Compilation gives this error:
`cannot convert 'int (*)[26]' to 'int (*)[]' in assignment`
.

Can someone please explain to me why this is an error or why it should be an error?

In C, `int (* mat)[];` is a pointer to array of `int` with unspecified size (not an array of pointers). In C++ it is an error, the dimension cannot be omitted in C++.

Your question says `new int *[K]` so I assume this is really a C++ question. The expression `new T[n]` evaluates to a `T *` already, there is no implicit conversion.

The code to allocate an array of null pointers using `new` is:

``````int **mat = new int *[10]();
``````

Then `mat` points to the first one of those. Another option (less commonly used) is:

``````int * (*mat)[10] = new int *[1][10]();
``````

where `*mat` designates the entire array of 10 pointers.

NB. This sort of code is not useful for anything except demonstration purposes perhaps, whatever you are trying to do has a better solution.

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