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digitalmars.D.learn - is pointer

reply bearophile <bearophileHUGS lycos.com> writes:
(I am looking for rough corners in D, or in my knowledge of D.)

In this page:
http://www.digitalmars.com/d/2.0/templates-revisited.html

In the section "Template Parameters" there is written:

P:P*,         // P must be a pointer type

-----------------

So I have written this D2 program:


template IsPointer1(T) {
    enum bool IsPointer1 = is(T : T*);
}
void main() {
    int* ptr;
    static assert(IsPointer1!(typeof(ptr))); // Err
}


But it asserts, do you know why?

-----------------

Then I have written a different program:


template IsPointer2(T) {
    enum bool IsPointer2 = is(typeof(*T)) || is(T == void*);
}
void main() {
    int* ptr;
    static assert(IsPointer2!(typeof(ptr))); // OK
    int[] arr;
    static assert(!IsPointer2!(typeof(arr))); // Err
}


Do you know why IsPointer2 is true for the dynamic array type too?

(I know in std.traits of Phobos 2 there is an isPointer).

Thank you,
bye,
bearophile
Mar 19 2010
next sibling parent reply Moritz Warning <moritzwarning web.de> writes:
On Fri, 19 Mar 2010 19:29:24 -0400, bearophile wrote:


 (I am looking for rough corners in D, or in my knowledge of D.)


[..]

 
 template IsPointer1(T) {
     enum bool IsPointer1 = is(T : T*);
 }
 void main() {
     int* ptr;
     static assert(IsPointer1!(typeof(ptr))); // Err
 }
 
 
 But it asserts, do you know why?


I think the problem is that is(T : T*) becomes is(int* : int**)
and that's false.




 Then I have written a different program:


[..]

     enum bool IsPointer2 = is(typeof(*T)) || is(T == void*);


[..]

 
 
 Do you know why IsPointer2 is true for the dynamic array type too?


.. no idea.
Mar 19 2010
parent reply Ellery Newcomer <ellery-newcomer utulsa.edu> writes:
On 03/19/2010 07:53 PM, Moritz Warning wrote:

 On Fri, 19 Mar 2010 19:29:24 -0400, bearophile wrote:


 (I am looking for rough corners in D, or in my knowledge of D.)


 [..]

 template IsPointer1(T) {
      enum bool IsPointer1 = is(T : T*);
 }
 void main() {
      int* ptr;
      static assert(IsPointer1!(typeof(ptr))); // Err
 }


 But it asserts, do you know why?


 I think the problem is that is(T : T*) becomes is(int* : int**)
 and that's false.




 Then I have written a different program:


 [..]

      enum bool IsPointer2 = is(typeof(*T)) || is(T == void*);


 [..]

 Do you know why IsPointer2 is true for the dynamic array type too?


 .. no idea.


Curious behavior DArrays exhibit:

void main(){
   int[] i = [5,4,3,2,1];
   writeln(*i); // prints 5
}

Ick.
Mar 19 2010
parent bearophile <bearophileHUGS lycos.com> writes:
Moritz Warning:

I think the problem is that is(T : T*) becomes is(int* : int**) and that's
false.<


Thank you, you can be right.
So D docs are wrong, or that syntax has a semantic bug, or I have not
understood the situation yet.
If someone else has ideas I'm all ears.

-------------

Ellery Newcomer:

 Curious behavior DArrays exhibit:


Thankfully this doesn't compile:

import std.stdio: writeln;
import std.c.stdlib: calloc;

struct Array {
    int* ptr;
    size_t length;
}

void main() {
    enum size_t N = 5;
    int* p = cast(int*)calloc(N, int.sizeof);
    Array a = Array(p, N);
    writeln(*a);
}

This solves the second question, I'll file it as a bug or specs bug. Thank you.

Bye,
bearophile
Mar 19 2010
prev sibling next sibling parent reply Daniel Keep <daniel.keep.lists gmail.com> writes:
bearophile wrote:

 (I am looking for rough corners in D, or in my knowledge of D.)
 
 In this page:
 http://www.digitalmars.com/d/2.0/templates-revisited.html
 
 In the section "Template Parameters" there is written:
 
 P:P*,         // P must be a pointer type
 
 -----------------
 
 So I have written this D2 program:
 
 
 template IsPointer1(T) {
     enum bool IsPointer1 = is(T : T*);
 }
 void main() {
     int* ptr;
     static assert(IsPointer1!(typeof(ptr))); // Err
 }
 
 
 But it asserts, do you know why?


You do realise that "Template Parameters" are a completely different
thing to "is expressions", right?
Mar 19 2010
parent bearophile <bearophileHUGS lycos.com> writes:
Daniel Keep:

 You do realise that "Template Parameters" are a completely different
 thing to "is expressions", right?


Nope. And from the other answers by Moritz and Ellery it seems I am not alone
:-)
You are right, this works:


template IsPointer1(T:T*) {
    enum bool IsPointer1 = true;
}
template IsPointer1(T) {
    enum bool IsPointer1 = false;
}
void main() {
    static assert(IsPointer1!(int*));
    static assert(!IsPointer1!(int));
    static assert(!IsPointer1!(int[]));
}


This syntax looks too much similar to the syntax with is():
template IsPointer1(T:T*) {
It's a syntax collision. You realize that I will not be the only one to fall in
this little trap.

Bye and thank you,
bearophile
Mar 20 2010
prev sibling parent Jacob Carlborg <doob me.com> writes:
On 2010-03-20 00.29, bearophile wrote:

 (I am looking for rough corners in D, or in my knowledge of D.)

 In this page:
 http://www.digitalmars.com/d/2.0/templates-revisited.html

 In the section "Template Parameters" there is written:

 P:P*,         // P must be a pointer type

 -----------------

 So I have written this D2 program:


 template IsPointer1(T) {
      enum bool IsPointer1 = is(T : T*);
 }
 void main() {
      int* ptr;
      static assert(IsPointer1!(typeof(ptr))); // Err
 }


 But it asserts, do you know why?


You can do like this:

template IsPointer (T)
{
	enum IsPointer = false;
}

template IsPointer (T : T*)
{
	enum IsPointer = true;
}


 -----------------

 Then I have written a different program:


 template IsPointer2(T) {
      enum bool IsPointer2 = is(typeof(*T)) || is(T == void*);
 }
 void main() {
      int* ptr;
      static assert(IsPointer2!(typeof(ptr))); // OK
      int[] arr;
      static assert(!IsPointer2!(typeof(arr))); // Err
 }


 Do you know why IsPointer2 is true for the dynamic array type too?

 (I know in std.traits of Phobos 2 there is an isPointer).

 Thank you,
 bye,
 bearophile
Mar 20 2010