CODEWITHSUNDEEP
c++cmallocnew-operatordynamic-memory-allocation
Jonathan Mee
Jonathan Mee

Reputation: 38949

Can I Allocate a Block of Memory with new?

So given this structure:

typedef struct {
    int* arr1;
    int* arr2;
} myStruct;

This answer described using a single malloc to allocate a myStruct and it's arrays at the same time:

myStruct* p = malloc(sizeof(*p) + 10 * sizeof(*p->arr1) + 10 * num * sizeof(*p->arr2);

if(p != NULL) {
    p->arr1 = (int*)(p + 1);
    p->arr2 = p->arr1 + 10;
}

What I'd like to know is there a similar way to do this with new?
Obviously I want to be able to allocate to a size that I receive at runtime as is done with the C example.

Upvotes: 1

Views: 1472

Answers (3)

James Reed
James Reed

Reputation: 491

Is there any reason you want to do it like in the link you provided? A little more context would help. Otherwise I would personally just use a constructor to do that:

    struct myStruct {
        int* arr1;
        int* arr2;
        myStruct(int num)
        {
                arr1 = new int[10];
                arr2 = new int[10*num];
        }
        ~myStruct()
        {
                delete[] arr1;
                delete[] arr2;
        }
};

int main()
{
        int num = 3;
        myStruct* a;
        a = new myStruct(3);
        delete a;
}

Upvotes: 1

Mark Ransom
Mark Ransom

Reputation: 308520

You can allocate a block of memory using new with an array of char, then use placement new to call the constructor on that block of memory.

Upvotes: 7

Ivaylo Strandjev
Ivaylo Strandjev

Reputation: 71009

In c++ we use new because it calls the constructors of the objects being allocated. So the proper way to achieve what you want is to have the constructor of the structure do the necessary allocations.

Upvotes: 1

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