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digitalmars.D - How to extract the AA type?

reply Steven Schveighoffer <schveiguy yahoo.com> writes:
A nice puzzle for those template gurus out there.

I have a function like this:

auto foo(T: V[K], V, K)(T t)
{
    RealAAType!(T, V, K) aa = t;
    return aa;
}

So I need to know what to write RealAAType. What I'm looking for is a 
mechanism to write the exact AA type that is passed in. RealAAType 
should be an AA, and nothing else.

Here are 2 things that *don't* work:

alias RealAAType!(T, V, K) = T;

This doesn't work because T could be an alias this'd struct. I don't 
want T, I want the actual AA type.

alias RealAAType!(T, V, K) = V[K];

This doesn't work when the T has a type modifier. For instance if T is 
const(int[int]), then V[K] is really const(int)[int], and you can't 
assign it to t.

Here is are the tests I want to compile:

const(int[int]) x;
auto aa1 = foo(x);
static assert(typeof(aa1) == const(int[int]));

static struct S
{
    int[int] aa;
    alias aa this;
}

const(S) s;
auto aa2 = foo(s);
static assert(typeof(aa2) == const(int[int]));

-Steve
Dec 13 2017
next sibling parent reply jmh530 <john.michael.hall gmail.com> writes:
On Wednesday, 13 December 2017 at 16:00:32 UTC, Steven 
Schveighoffer wrote:

 A nice puzzle for those template gurus out there.

 I have a function like this:

 auto foo(T: V[K], V, K)(T t)
 {
    RealAAType!(T, V, K) aa = t;
    return aa;
 }

 So I need to know what to write RealAAType. What I'm looking 
 for is a mechanism to write the exact AA type that is passed 
 in. RealAAType should be an AA, and nothing else.


[snip]

How about:

template RealAAType!(T, V, K)
{
     import std.traits : CopyConstness;

     alias RealAAType = CopyConstness!(T, V[K]);
}
Dec 13 2017
parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 12/13/17 11:30 AM, jmh530 wrote:

 On Wednesday, 13 December 2017 at 16:00:32 UTC, Steven Schveighoffer wrote:

 A nice puzzle for those template gurus out there.

 I have a function like this:

 auto foo(T: V[K], V, K)(T t)
 {
    RealAAType!(T, V, K) aa = t;
    return aa;
 }

 So I need to know what to write RealAAType. What I'm looking for is a 
 mechanism to write the exact AA type that is passed in. RealAAType 
 should be an AA, and nothing else.


 [snip]
 
 How about:
 
 template RealAAType!(T, V, K)
 {
      import std.traits : CopyConstness;
 
      alias RealAAType = CopyConstness!(T, V[K]);
 }


Yes, that seems to work, I was worried that something like this would 
give me multiple type modifiers (.e.g const(const(int)[int])

Now, I need to figure out how to do this in object.d :)

BTW, the PR that spurred this for reference: 
https://github.com/dlang/druntime/pull/1992

-Steve
Dec 13 2017
parent Steven Schveighoffer <schveiguy yahoo.com> writes:
On 12/13/17 3:23 PM, Steven Schveighoffer wrote:

 On 12/13/17 11:30 AM, jmh530 wrote:

 How about:

 template RealAAType!(T, V, K)
 {
      import std.traits : CopyConstness;

      alias RealAAType = CopyConstness!(T, V[K]);
 }


 
 Yes, that seems to work, I was worried that something like this would 
 give me multiple type modifiers (.e.g const(const(int)[int])
 
 Now, I need to figure out how to do this in object.d :)
 
 BTW, the PR that spurred this for reference: 
 https://github.com/dlang/druntime/pull/1992


After thinking about it, I've decided to simply use the workaround I 
implemented, just apply const to the AA in all cases, since I have to 
cast to void * anyway.

Thanks for the tips, I learned a little bit here :)

-Steve
Dec 14 2017
prev sibling parent Jack Applegame <japplegame gmail.com> writes:
auto foo(T: V[K], V, K)(T t)
{
     CopyConstness!(T, V[K]) aa = t;
     return aa;
}

https://run.dlang.io/is/LSMa5C
Dec 13 2017