CODEWITHSUNDEEP
c++memory-managementmatrix
Alon Shmiel
Alon Shmiel

Reputation: 7121

allocate a new memory for matrix

I have two classes:

class A

class B: class A

I want to create a matrix that each pointer point to B (mat of 1X2).

so I defined:

A **mat;

mat = new A*[1];
*mat = new B[2]; // call the constructor.

now I have 2 elements: mat[0][0], mat[0][1].

mat[0][0] is initialized but mat[0][1] is NULL.

please help.

Upvotes: 0

Views: 110

Answers (2)

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170055

If sizeof(A) < sizeof(B) than an array of A's is not equivalent to an array of B's. Since in the second allocation you allocate concreete objects which are larger than what *mat is supposed to point to, when you access the second element it doesn't access the second B element, but rather slices into the first.

That's my guess anyway, since you didn't tell us much about A and B.

EDIT

To answer your comment, I'd start by limiting the amount of raw pointers you use. Use a vector of vectors instead. Each element of the vector can be a raw pointer to an A. Which can have a run-time type of B or C.

#include <vector>

std::vector<std::vector<A*> > mat(1, std::vector<A*>(2, (A*)0));

Upvotes: 1

Brad
Brad

Reputation: 10660

If you need pointers to B, you don't need to specify anything about A. Since B derives A, you only have to allocate space for the larger one (B in this case).

For a matrix of pointers, you will need the following.

B *** mat;
mat = new B**[1]; //Dimension 1 
mat[0] = new B*[2]; //Dimension 2 (array of pointers)
mat[0][0] = new B(); //Allocate B in [0][0]
mat[0][1] = new B(); //Allocate B in [0][1]

But if you know it will always be 1x2, just use the following:

B * mat[1][2];

Upvotes: 1

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