• Damian (22/22) Jul 01 2013 void testref(ref int[] arr)
• =?UTF-8?B?QWxpIMOHZWhyZWxp?= (15/36) Jul 01 2013 When that call is made, a slice would have to be created to represent
• Adam D. Ruppe (9/10) Jul 01 2013 No. A static array is just a block of memory (like in C), whereas
• Damian (1/1) Jul 01 2013 Thanks Ali and Adam for the good explanations I understand now.

"Damian" <damianday hotmail.co.uk> writes:

void testref(ref int[] arr)
{
     arr[0] = 1;
}

void test(int[] arr)
{
     arr[0] = 1;
}

void main()
{
     //int[] buffer1 = new int[4]; // This works
     int[4] buffer1; // This doesn't
     int[4] buffer2;

     testref(buffer1);
     test(buffer2);
	
     assert(buffer1[0] == 1);
     assert(buffer2[0] == 1);
}

I'm not sure why my code doesn't work?? Isn't the buffer just an 
array with a fixed length? DMD is telling me 'buffer1 is not an 
lvalue'. The non ref version works fine?!

=?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:

On 07/01/2013 10:34 AM, Damian wrote:

 void testref(ref int[] arr)
 {
      arr[0] = 1;
 }

 void test(int[] arr)
 {
      arr[0] = 1;
 }

 void main()
 {
      //int[] buffer1 = new int[4]; // This works
      int[4] buffer1; // This doesn't
      int[4] buffer2;

      testref(buffer1);

When that call is made, a slice would have to be created to represent 
all of the elements of the fixed-length array buffer1. A slice would be 
needed because buffer1 is not a slice but testref() takes a slice.

By the simplest definition, that slice is an rvalue because it is not 
defined as a variable in the program.

And rvalues cannot be bound to non-const references. (If I am not 
mistaken not even to const references yet, if ever.)

      test(buffer2);

Similarly, when that call is made, a slice is created. The difference 
is, because the parameter is by-value, the slice gets copied to test(). 
Now there is no problem because 'arr' is just a local variable of test().

(Note that when I say a slice is created or a slice is copied, they are 
very cheap operations. A slice is nothing but the number of elements and 
a pointer to the first one of those elements. Just a size_t and a pointer.)

      assert(buffer1[0] == 1);
      assert(buffer2[0] == 1);
 }

 I'm not sure why my code doesn't work?? Isn't the buffer just an array
 with a fixed length? DMD is telling me 'buffer1 is not an lvalue'. The
 non ref version works fine?!

Ali

"Adam D. Ruppe" <destructionator gmail.com> writes:

On Monday, 1 July 2013 at 17:34:39 UTC, Damian wrote:
 Isn't the buffer just an array with a fixed length?

No. A static array is just a block of memory (like in C), whereas 
a slice is a struct with a length and pointer to a memory block.

A ref slice would write to the length/pointer, which doesn't 
exist with a static array.

You can get a slice from a static array by putting [] on the end, 
but to be ref, you'll need a variable with the type "int[]" 
because then the length and pointers can be changed by the 
function that accepts it.

"Damian" <damianday hotmail.co.uk> writes:

Thanks Ali and Adam for the good explanations I understand now.