A way of allocating multidimensional arrays dynamically

salute..

I am learning dynamic allocations for multidimensional arrays in a book and I found some ways for that, And now haven't problem in it. But the author of the book shows us a way, but it doesn't work correctly. It is this:

pbeans = new double [3][4];         // Allocate memory for a 3x4 array

And this is t开发者_StackOverflowhe error:

error C2440: '=' : cannot convert from 'int (*)[4]' to 'int *'

how should i define pbeans ( if this type of coding is legal)?

and what is the problem exactly?

Regards.

This is covered in my FAQ on arrays:

double (*pbeans)[4];
pbeans = new double[3][4];

// ...

delete[] pbeans;

For the "C declarator impaired", you could make that more readable with a typedef:

typedef double row[4];

row *pbeans;
pbeans = new row[3];

// ...

delete[] pbeans;

But in C++, we prefer RAII containers over raw pointers:

#include <vector>
#include <array>

std::vector<std::array<double, 4> > beans(3);

Note the complete absence of delete[] which makes this solution exception-safe.

You need to allocate each dimension of the array separately:

double **pbeans = new double*[3];
for (int i = 0; i < 3; ++i) {
    pbeans[i] = new double[4];
}

Here is a way to do it that allocates the memory contiguously on the heap:

typedef double MyGrid[3][4];

int main(int argc, char* argv[])
{
    MyGrid& x = *(reinterpret_cast<Grid*>(new double[12]));
    ...
    x[1][2] = 0.3333;
    ...
    delete[] &x;
    return 0;
}

Which you could easily turn into a more generic solution:

template<typename T, int x, int y>
struct Array2D
{
    typedef T CArrayType[x][y];
    typedef CArrayType& RefType;
    static CArrayType& New()
    {
        return *(reinterpret_cast<CArrayType*>(new T[x * y]));
    }
    static void Delete(RefType x)
    {
        delete[] &x;
    }
};

typedef Array2D<double, 3, 4> MyGrid;// define your 2d array with 3 rows / 4 columns.

int main(int argc, char* argv[])
{
    MyGrid::RefType j = MyGrid::New();
    ...
    j[1][2] = 0.3333;
    ...
    MyGrid::Delete(j);
    return 0;
}

The idea is to just generate the elements in 1D (x*y), and cast it to a 2D array. But since array types are value types, you need to deal in pointers-to-arrays. Using a reference makes it almost transparent.

Boost probably has something like this but I don't know boost well enough to say...