int* myArray = new int[10][20];
myArray[2][3] = 100;
int* myArray = new int[10*20];
int** myArray = new int*[10];
for(int x = 0; x < 10; x++)
{
myArray[x] = new int[20];
}
Accessing elements in 2D dynamically allocated array? (C++)
Author
Hi guys, is there anyway I can allocate a 2D array dynamically with 'new' and then access elements using the normal [] operator? Some code which doesn't work:
One approach is to declare like this:
But then it is only a 1D array. Or I could say something like:
Which then allows the correct syntax for access but means the array could be spread out in memory (bad for cache, can't clear with one call to memset(), etc).
Am I missing something obvious?
Thanks!
I think the easiest solution is to create an Array2D class that stores a 1D array internally and overload the [] operator (and maybe the () operator for 2D access).
Unfortunately, C++ does not support the direct dynamic allocation of true 2D arrays. Arrays in C++ are not first class types. In addition to the approach you've tried, you may want to use:
Here is another thread on the topic.
class MyArray { public: int data[10][20];};MyArray * ptr = new MyArray;Here is another thread on the topic.
Quote:Am I missing something obvious?
Yes:
std::vector< std::vector< int > > myArray;Quote:Which then allows the correct syntax for access but means the array could be spread out in memory (bad for cache, can't clear with one call to memset(), etc).
If you need fully continuous block of memory:
template < class T >class Array { Array( size_t w, size_t h ) : m_array( w * h ) , m_width(w) , m_height(h) { } inline T get( size_t x, size_t y ) { return m_array[ y * m_width + x ]; } void clear( T defaultValue ) { std::fill( m_array.begin(), m_array.end(), defaultValue ); }private: std::vector< T > m_array; size_t m_width; size_t m_height;};
Of the choices you presented, the 2nd approach is probably best...even a 2d array like, say, video memory is represented in 1 dimension.
The 3rd approach will work fine if you need a 2d array...but an array with any number of dimensions is usually best mapped in 1 dimension and accessed with multiplies or bit shifts.
Alternatively you could allocate 1 block of memory and assign it to a multi-dimensional array like
That would give you a 2d array in 1 contiguous block.
On a side note...honestly you should probably create some sort of class to do your memory allocations or use STL ... it will save you many moons of debugging.
Cheers
-Scott
The 3rd approach will work fine if you need a 2d array...but an array with any number of dimensions is usually best mapped in 1 dimension and accessed with multiplies or bit shifts.
Alternatively you could allocate 1 block of memory and assign it to a multi-dimensional array like
int *Tmp = new int[10*20];int **myArray = new int*[10]; // maybe (int*) ??for (int Index = 0; Index < 10; Index ++) {myArray[Index] = &Tmp[Index*20];}That would give you a 2d array in 1 contiguous block.
On a side note...honestly you should probably create some sort of class to do your memory allocations or use STL ... it will save you many moons of debugging.
Cheers
-Scott
Author
Wow, so many replies - thanks guys. So it wasn't trivial which is a relief cos I thought I was missing something. I'm not sure which approach I'll take yet - I'll probably do it the 'proper' way and use vectors or multi_array.
thanks for the help!
thanks for the help!
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