• lili (4/4) Aug 23 2019 Hi:
• Jonathan M Davis (12/16) Aug 23 2019 D uses parens to restrict how much of the type is const.
• Max Haughton (5/21) Aug 23 2019 As to const pointers to mutable types, it can be done in a
• Jonathan M Davis (10/40) Aug 24 2019 You can create a wrapper struct that acts as a pointer and make it so th...

lili <akozhao tencent.com> writes:

Hi:
   In C we can definition const int *ncp_to_cv;
    or int * const cp_to_ncv;
   How to do this in D.

Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:

On Friday, August 23, 2019 10:14:56 PM MDT lili via Digitalmars-d-learn 
wrote:
 Hi:
    In C we can definition const int *ncp_to_cv;
     or int * const cp_to_ncv;
    How to do this in D.

D uses parens to restrict how much of the type is const.

const int* - const pointer to const int
const(int*) - const pointer to const int
const(int)* - mutable pointer to const int

Similarly,

const(int*)* - mutable pointer to const pointer to const int
const(int)** - mutable pointer to mutable pointer to const int

D's const is transitive, so it's not possible to have a const pointer to a
mutable type.

- Jonathan M Davis

Max Haughton <maxhaton gmail.com> writes:

On Saturday, 24 August 2019 at 05:03:43 UTC, Jonathan M Davis 
wrote:
 On Friday, August 23, 2019 10:14:56 PM MDT lili via 
 Digitalmars-d-learn wrote:
 Hi:
    In C we can definition const int *ncp_to_cv;
     or int * const cp_to_ncv;
    How to do this in D.

 D uses parens to restrict how much of the type is const.

 const int* - const pointer to const int
 const(int*) - const pointer to const int
 const(int)* - mutable pointer to const int

 Similarly,

 const(int*)* - mutable pointer to const pointer to const int
 const(int)** - mutable pointer to mutable pointer to const int

 D's const is transitive, so it's not possible to have a const 
 pointer to a mutable type.

 - Jonathan M Davis

As to const pointers to mutable types, it can be done in a 
library (Final in std.typecons). I don't know what the overhead 
is but I imagine it wraps it in a struct

Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:

On Saturday, August 24, 2019 12:48:33 AM MDT Max Haughton via Digitalmars-d-
learn wrote:
 On Saturday, 24 August 2019 at 05:03:43 UTC, Jonathan M Davis

 wrote:
 On Friday, August 23, 2019 10:14:56 PM MDT lili via

 Digitalmars-d-learn wrote:
 Hi:
    In C we can definition const int *ncp_to_cv;

     or int * const cp_to_ncv;

    How to do this in D.

 D uses parens to restrict how much of the type is const.

 const int* - const pointer to const int
 const(int*) - const pointer to const int
 const(int)* - mutable pointer to const int

 Similarly,

 const(int*)* - mutable pointer to const pointer to const int
 const(int)** - mutable pointer to mutable pointer to const int

 D's const is transitive, so it's not possible to have a const
 pointer to a mutable type.

 - Jonathan M Davis

 As to const pointers to mutable types, it can be done in a
 library (Final in std.typecons). I don't know what the overhead
 is but I imagine it wraps it in a struct

You can create a wrapper struct that acts as a pointer and make it so that
the struct can't be assigned to, making it emulate const, but you can't
actually make it const, since that would make the wrapped pointer const as
well. Personally, I think that const pointers to mutable data are useless
anyway, but IIRC, there's a library somewhere on code.dlang.org that has a
struct that tries to emulate such a pointer, since it gets brought up from
time to time.

- Jonathan M Davis