Re: Passing a 2 dimensional array from fortran to c++

Rich Townsend wrote:

glen herrmannsfeldt wrote: 

(snip)

First (float**) is not a two dimensional array.  It is a pointer to
pointer to float.  It is often used in place of 2D arrays, but it still
isn't one.  Some will even say that a C array declared like:

float x[10][10];

isn't a 2D array, either, but I will disagree.

How so? Surey, x[10] has type (float *)? I thought that in C,

x[i]

is purely syntactic sugar for *(x+i). Am I wrong? 

It is *(x+i) but the compiler knows what it is applied to.

float x[10][10];

will allocate 100 contiguous float values, just like a Fortran array.

When you say x[i][j] it computes 10*i+j, scaled by sizeof(float) to
find the address of the array element.  It will also do that for

*(*(x+i)+j) for an array with that declaration.

I believe the wording is that x[10] decays to (float*),
but x is not, and does not decay to, (float**).  You can't,
for example, pass x to a subroutine with a float** dummy argument.

As a function dummy argument declaration,

float *y;  and float y[]; are equivalent.

float **x; and float x[][10]; are not.

-- glen

.

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