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digitalmars.D.learn - Creating 1000 instances

reply =?UTF-8?B?0JLQuNGC0LDQu9C40Lkg0KTQsNC0?= =?UTF-8?B?0LXQtdCy?= writes:
We have:
     class File
     {
         // WIN32_FIND_DATAW data;
     }

     void fastReadDir()
     {
         File[] files;

         // reserve space, allocating instances
         files = new File[]( 1000 );  // <--- trouble here ?

         // filling instances
         auto file = files.ptr;

         writeln( file.data );    // <--- or trouble here ?

         // ...
     }

Got:
     SegFault

Goal:
     Allocate memory for 1000 instances at once.

Source:
     https://run.dlang.io/is/xfaXcv

Question:
     What is the true, fastest, beauty way to create 1000 
instances of the class File ?
Feb 19 2021
parent reply Ferhat =?UTF-8?B?S3VydHVsbXXFnw==?= <aferust gmail.com> writes:
On Friday, 19 February 2021 at 08:04:19 UTC, Виталий Фадеев wrote:

 We have:
     class File
     {
         // WIN32_FIND_DATAW data;
     }

     void fastReadDir()
     {
         File[] files;

         // reserve space, allocating instances
         files = new File[]( 1000 );  // <--- trouble here ?

         // filling instances
         auto file = files.ptr;

         writeln( file.data );    // <--- or trouble here ?

         // ...
     }

 Got:
     SegFault

 Goal:
     Allocate memory for 1000 instances at once.

 Source:
     https://run.dlang.io/is/xfaXcv

 Question:
     What is the true, fastest, beauty way to create 1000 
 instances of the class File ?


files = new File[]( 1000 );
files[] = new File(); // add this

Since classes are reference types all instances of files will be 
the same reference of "new File()", which you probably don't want.
Feb 19 2021
next sibling parent reply Ferhat =?UTF-8?B?S3VydHVsbXXFnw==?= <aferust gmail.com> writes:
On Friday, 19 February 2021 at 08:29:36 UTC, Ferhat Kurtulmuş 
wrote:

 On Friday, 19 February 2021 at 08:04:19 UTC, Виталий Фадеев 
 wrote:

 [...]


 files = new File[]( 1000 );
 files[] = new File(); // add this

 Since classes are reference types all instances of files will 
 be the same reference of "new File()", which you probably don't 
 want.


You can do

files[].each!((ref a) => a = new File);
Feb 19 2021
parent Ferhat =?UTF-8?B?S3VydHVsbXXFnw==?= <aferust gmail.com> writes:
On Friday, 19 February 2021 at 08:41:06 UTC, Ferhat Kurtulmuş 
wrote:

 On Friday, 19 February 2021 at 08:29:36 UTC, Ferhat Kurtulmuş 
 wrote:

 On Friday, 19 February 2021 at 08:04:19 UTC, Виталий Фадеев 
 wrote:

 [...]


 files = new File[]( 1000 );
 files[] = new File(); // add this

 Since classes are reference types all instances of files will 
 be the same reference of "new File()", which you probably 
 don't want.


 You can do

 files[].each!((ref a) => a = new File);


oh, now we can remove brackets

files.each!((ref a) => a = new File);
Feb 19 2021
prev sibling parent reply Siemargl <inqnone gmail.com> writes:
On Friday, 19 February 2021 at 08:29:36 UTC, Ferhat Kurtulmuş 
wrote:


 Since classes are reference types all instances of files will 
 be the same reference of "new File()", which you probably don't 
 want.


Is any differences between x and y definitions?

MyClass [] x, y;
x = new MyClass[7];

y= new MyClass[](8);
Feb 19 2021
next sibling parent Ferhat =?UTF-8?B?S3VydHVsbXXFnw==?= <aferust gmail.com> writes:
On Friday, 19 February 2021 at 10:02:05 UTC, Siemargl wrote:

 On Friday, 19 February 2021 at 08:29:36 UTC, Ferhat Kurtulmuş 
 wrote:


 Since classes are reference types all instances of files will 
 be the same reference of "new File()", which you probably 
 don't want.


 Is any differences between x and y definitions?

 MyClass [] x, y;
 x = new MyClass[7];

 y= new MyClass[](8);


Although I don't usually use the latter, I can say online d 
editor yields the same ASM output for both:

File[] files = new File[10];
File[] files = new File[](10);
Feb 19 2021
prev sibling parent Simen =?UTF-8?B?S2rDpnLDpXM=?= <simen.kjaras gmail.com> writes:
On Friday, 19 February 2021 at 10:02:05 UTC, Siemargl wrote:

 On Friday, 19 February 2021 at 08:29:36 UTC, Ferhat Kurtulmuş 
 wrote:


 Since classes are reference types all instances of files will 
 be the same reference of "new File()", which you probably 
 don't want.


 Is any differences between x and y definitions?

 MyClass [] x, y;
 x = new MyClass[7];

 y= new MyClass[](8);


The only part of the documentation I've found that talks about 
this is here:
https://dlang.org/spec/expression.html#new_expressions

The main difference I know of comes with multidimensional arrays:

     auto a = new int[4][4];
     pragma(msg, typeof(a)); // Prints int[4][]
     auto b = new int[][](4,4);
     pragma(msg, typeof(b)); // Prints int[][]

Since the former is a dynamic array of static arrays, the first 
size parameter cannot be passed at runtime:

     auto n = 4;
     // Error: variable n cannot be read at compile time
     auto c = new int[n][n];

But must be a compiletime constant:

     enum N = 4;
     auto d = new int[N][n];
     pragma(msg, typeof(d)); // Prints int[4][]

The other syntax however, can take runtime values, but does not 
encode the size in the type:

     auto e = new int[][](n,n);
     pragma(msg, typeof(e)); // Prints int[][]

The other big thing about the []() syntax is it actually 
initializes the arrays of arrays for you:

     assert(e[0].length == n);

If you were to use new int[][n], you would need to initialize 
each array of arrays manually:

     auto f = new int[][n];
     assert(f[0].length == 0); // it's empty
     foreach (i; 0..n) {
         f[i] = new int[n]; // Have to do this yourself.
     }

--
   Simen
Feb 19 2021