How to initialize a dynamic array of customized struct in C++

Question

I have the following struct which I wanted to initialize

struct Box{
    int *dimval;
    int no;
    int dim;

    Box(dim){
        this->dim = dim;
        dimval = new int[dim]
    }

}

now in my main function. I wanted to initialize an array of Box struct, but this is problematic with my implementation.

int main(){
    Box *boxes;
    int num_box, dim;

    cin>>num_box>>dim;

    boxes = new Box[num_box](dim);// I know this is devil here. 
}

I want to have a dynamic array containing num_box Box items, each being initialized with a dynamic array of dim long. How can I do that?

but with default constructor, I cannot pass dim parameter in it — Daniel
– Daniel, Commented May 6, 2014 at 0:02
I am practicing writing c style code with c++, so stl data type is not preferred. how do you achieve this with a c code? — Daniel
– Daniel, Commented May 6, 2014 at 0:14
@Daniel I am practicing writing c style code with c++ Why? Why not just write straight C code instead of wasting time with C++, and in the process, change the tag to C instead of C++. — PaulMcKenzie
– PaulMcKenzie, Commented May 6, 2014 at 0:18

zneak · Accepted Answer · 2014-05-06 00:12:44Z

4

You cannot create an array of a type unless it has a default constructor, and then you can't initialize each of them. However, you can initialize a vector with a default object, which is pretty much what you're asking for here.

#include <vector>

int num_box, dim;
cin >> num_box >> dim;
vector<Box> boxes(num_box, Box(dim));

Note that you need a copy constructor to handle the copying of dimval...

#include <algorithm>

struct Box
{
    // ...

    Box(const Box& that)
    {
        this->dim = that.dim;
        this->no = that.no;
        this->dimval = new int[dim];
        copy(that.dimval, that.dimval + that.dim, this->dimval);
    }
};

... but you can use the default copy constructor if you simply replace your dimval pointer with a vector, too, since vectors handle copy (that would fix a leak, too).

struct Box
{
    vector<int> dimval;
    int no;
    int dim;

    Box(int dim)
    : dimval(dim)
    {
        this->dim = dim;
    }
}

edited May 6, 2014 at 0:12

answered May 6, 2014 at 0:06

zneak

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13 Comments

Daniel Over a year ago

any way to do it under the context of dynamic allocated pointer？ I am maintaining my version of dynamic array instead of using stl vector

zneak Over a year ago

You cannot invoke a constructor other than the default constructor when you create an array with new, and there needs to be a default constructor to be able to create an array with new. Your only other solution is to use a placement new, but this really opens a can of worms.

zneak Over a year ago

Anyways, here's how you'd do it, but I personally believe that you would shoot yourself in the foot if this is for anything more than learning purposes: stackoverflow.com/questions/222557/…

Chnossos Over a year ago

@Daniel Why you use your own version of std::vector if you can freely choose between the two ?

PaulMcKenzie Over a year ago

@Daniel I am maintaining my version of dynamic array instead of using stl vector and while you're doing that, some other C++ programmer is using std::vector and has made more progress in finishing their program.

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