In c++ how do you cast a dynamic array to static?
Say
I have
int ** da;
da = new int*[9];
for (int i=0; i<9; i++) da[i] = new int[9];
and my function argument is of type
int[9][9], how do I cast da so my function can use it?
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What is your function specification?vickirk– vickirk2011-04-07 06:45:34 +00:00Commented Apr 7, 2011 at 6:45
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3 Answers
An int[9][9] is 81 integers consecutive in memory, taking 81*sizeof(int) bytes. An int *[9] is a sequence of 9 pointers to integer, taking 9*sizeof(int *) bytes. Each of these are setup to point to 9 distinct sequences of 9 ints.
You cannot use one in place of the other - no casting will change that these two are laid out completely different in memory.
Comments
you don't - you must move as necessary if function keeps its signature. example:
void function(int a[9][9]);
int tmp[9][9];
// move to temp
for (size_t i(0); i < 9; ++i) {
for (size_t j(0); j < 9; ++j) {
tmp[i][j] = da[i][j];
}
}
function(tmp);
// move to da
for (size_t i(0); i < 9; ++i) {
for (size_t j(0); j < 9; ++j) {
da[i][j] = tmp[i][j];
}
}
the reason: the layout and alignment of 2D arrays is explicitly defined, and implementation defined. the function's implementation expects the exact layout and alignment. any other layout would obviously introduce a bug.
even int tmp[9*9] is not guaranteed to be the same as int tmp[9][9].
fortunately, creating this on the stack and copying to/from is cheap.
Comments
It may not relate to this question. But it reminds me a trick in matrix using C. The good part is we only need to call malloc and free once. The bad part is.....
// create the buffer and assign the pointer array
int i, j;
int* buffer = (int*) malloc(sizeof(int) * 81);
int* matrix[9]; // 9 * 9
for (i = 0; i < 9; ++i)
matrix[i] = buffer + i * 9;
// assign some value using matrix[i][j]
for (i = 0; i < 9; ++i)
for (j = 0; j < 9; ++j)
matrix[i][j] = (i + 1) * (j + 1);
// retrieve the value in matrix
for (i = 0; i < 9; ++i)
for (j = 0; j < 9; ++j)
std::cout << matrix[i][j] << " ";
std::cout << std::endl;
// free the buffer
free(buffer);
