Re: Express "Class argument may not be null" ?
On 8/9/17 1:05 PM, Johan wrote: On Wednesday, 9 August 2017 at 12:47:49 UTC, Steven Schveighoffer wrote: On 8/8/17 3:59 PM, Johan Engelen wrote: In C++, it is clear that the _caller_ is doing the dereferencing, and the dereference is also explicit. In fact it's not doing any dereferencing. It's just under the hood passing a pointer. Come on Steven, I thought you knew I am a compiler dev. Of course I do! I'm not a compiler dev though :) The caller _is_ doing dereferencing. That's what the '*' means semantically and it matters. [1] As a layman user, I consider dereferencing to be when you actually use the data pointed at by the pointer (i.e. you've followed the reference). What you are doing when passing a dereferenced pointer through a reference is the same as just passing a pointer, it doesn't seem to me like there should be a difference. However, the in contract does actually enforce the requirement. And adds null pointer checks even when clearly not needed. Clearly not needed? I thought the point was to ensure the reference is not null? I meant code like this: `k->something(); foo(*k);` or `auto k = new Klass(); foo(*k);`. Anyway, the point was to express the intent nicely. I wasn't expecting having to explain the merits of passing by reference. Sorry, not a C++ developer. I just think in terms of how the code translates to the instructions to the computer. To me, pointers and references seem very similar. [1] Going off-topic. The alternative is dereferencing inside the callee (which is apparently the only option D semantically provides for classes). I showed the impact of this semantic difference in my DConf 2017 talk; here are links to examples: https://godbolt.org/g/rgvHTC (C++) https://godbolt.org/g/NCTeXo (D) It's a pity we are not able to pass classes "by reference" (per C++ parlance), for reasoning and readability, and performance. I still have to come up with a rationale for not having an explicit "*" for classes for when I am going to teach D. I didn't think this comes from the dereferencing, I thought it came from the ability to rebind. That is, inside the loop, the pointer potentially could have been set to something different, so it has to reload the data. What you really want perhaps is a class parameter that cannot be rebound? That would more accurately duplicate a ref parameter. -Steve
Re: Express "Class argument may not be null" ?
On Wednesday, 9 August 2017 at 12:47:49 UTC, Steven Schveighoffer wrote: On 8/8/17 3:59 PM, Johan Engelen wrote: In C++, it is clear that the _caller_ is doing the dereferencing, and the dereference is also explicit. In fact it's not doing any dereferencing. It's just under the hood passing a pointer. Come on Steven, I thought you knew I am a compiler dev. The caller _is_ doing dereferencing. That's what the '*' means semantically and it matters. [1] However, the in contract does actually enforce the requirement. And adds null pointer checks even when clearly not needed. Clearly not needed? I thought the point was to ensure the reference is not null? I meant code like this: `k->something(); foo(*k);` or `auto k = new Klass(); foo(*k);`. Anyway, the point was to express the intent nicely. I wasn't expecting having to explain the merits of passing by reference. - Johan [1] Going off-topic. The alternative is dereferencing inside the callee (which is apparently the only option D semantically provides for classes). I showed the impact of this semantic difference in my DConf 2017 talk; here are links to examples: https://godbolt.org/g/rgvHTC (C++) https://godbolt.org/g/NCTeXo (D) It's a pity we are not able to pass classes "by reference" (per C++ parlance), for reasoning and readability, and performance. I still have to come up with a rationale for not having an explicit "*" for classes for when I am going to teach D.
Re: Express "Class argument may not be null" ?
On 8/8/17 4:00 PM, Andre Kostur wrote:
On 2017-08-08 12:38 PM, Steven Schveighoffer wrote:
On 8/8/17 2:56 PM, ag0aep6g wrote:
On 08/08/2017 08:34 PM, Johan Engelen wrote:
How would you express the function interface intent that a
reference to a class may not be null?
For a function "void foo(Klass)", calling "foo(null)" is valid. How
do I express that that is invalid? (let's leave erroring with a
compile error aside for now)
Something equivalent to C++'s pass by reference: "void foo(Klass&)".
[snip]
But you can pass null in a ref parameter:
void f(ref int x) @safe {}
void main() @safe
{
int* p = null;
f(*p);
}
Note that C++ also can do this, so I'm not sure the & is accomplishing
the correct goal:
void foo(Klass&);
int main()
{
Klass *k = NULL;
foo(*k);
}
However, the in contract does actually enforce the requirement.
To be fair: it cannot be done in C++ without first invoking Undefined
Behaviour (such as dereferencing a nullptr).
If your "insurance" that null pointers aren't passed is the threat of
undefined behavior, then it leaves a lot to be desired.
It's possible, and does happen. The "just don't write bugs" approach
doesn't scale.
-Steve
Re: Express "Class argument may not be null" ?
On 8/8/17 3:59 PM, Johan Engelen wrote:
On Tuesday, 8 August 2017 at 19:38:19 UTC, Steven Schveighoffer wrote:
Note that C++ also can do this, so I'm not sure the & is accomplishing
the correct goal:
void foo(Klass&);
int main()
{
Klass *k = NULL;
foo(*k);
}
In C++, it is clear that the _caller_ is doing the dereferencing, and
the dereference is also explicit.
In fact it's not doing any dereferencing. It's just under the hood
passing a pointer.
However, the in contract does actually enforce the requirement.
And adds null pointer checks even when clearly not needed.
Clearly not needed? I thought the point was to ensure the reference is
not null?
-Steve
Re: Express "Class argument may not be null" ?
I was about to say "use NotNull" but there still isn't one in std.typecons. ugh. But it is just a wrapper type that checks null in the contracts too, so you can do it at the function itself.
Re: Express "Class argument may not be null" ?
On 2017-08-08 12:38 PM, Steven Schveighoffer wrote:
On 8/8/17 2:56 PM, ag0aep6g wrote:
On 08/08/2017 08:34 PM, Johan Engelen wrote:
How would you express the function interface intent that a
reference to a class may not be null?
For a function "void foo(Klass)", calling "foo(null)" is valid. How
do I express that that is invalid? (let's leave erroring with a
compile error aside for now)
Something equivalent to C++'s pass by reference: "void foo(Klass&)".
[snip]
But you can pass null in a ref parameter:
void f(ref int x) @safe {}
void main() @safe
{
int* p = null;
f(*p);
}
Note that C++ also can do this, so I'm not sure the & is accomplishing
the correct goal:
void foo(Klass&);
int main()
{
Klass *k = NULL;
foo(*k);
}
However, the in contract does actually enforce the requirement.
To be fair: it cannot be done in C++ without first invoking Undefined
Behaviour (such as dereferencing a nullptr).
Re: Express "Class argument may not be null" ?
On Tuesday, 8 August 2017 at 19:38:19 UTC, Steven Schveighoffer
wrote:
Note that C++ also can do this, so I'm not sure the & is
accomplishing the correct goal:
void foo(Klass&);
int main()
{
Klass *k = NULL;
foo(*k);
}
In C++, it is clear that the _caller_ is doing the dereferencing,
and the dereference is also explicit.
However, the in contract does actually enforce the requirement.
And adds null pointer checks even when clearly not needed.
- Johan
Re: Express "Class argument may not be null" ?
On Tuesday, 8 August 2017 at 18:57:48 UTC, Steven Schveighoffer
wrote:
On 8/8/17 2:34 PM, Johan Engelen wrote:
Hi all,
How would you express the function interface intent that a
reference to a class may not be null?
For a function "void foo(Klass)", calling "foo(null)" is
valid. How do I express that that is invalid? (let's leave
erroring with a compile error aside for now)
There isn't a way to do this in the type itself.
One can always create a null class instance via:
MyObj obj;
There is no way to disallow this somehow in the definition of
MyObj. With structs, you can @disable this(), and it's still
possible but harder to do so.
Ok thanks, so this could be a reason for not being allowed to
express the non-null-ness.
(I still haven't found peace with the absence of an explicit *
for classes)
I would say, however, that if you wanted to express the
*intent*, even without a compile-time error, you could use a
contract:
void foo(Klass k) in {assert(k !is null);};
Thanks. I regret leaving compile-time errors out, because in that
case adding it to the function documentation would suffice.
(Btw: "Error: function foo in and out contracts require function
body". But who uses .di files anyway. ;-)
Cheers,
Johan
Re: Express "Class argument may not be null" ?
On 8/8/17 2:56 PM, ag0aep6g wrote:
On 08/08/2017 08:34 PM, Johan Engelen wrote:
How would you express the function interface intent that a
reference to a class may not be null?
For a function "void foo(Klass)", calling "foo(null)" is valid. How do
I express that that is invalid? (let's leave erroring with a compile
error aside for now)
Something equivalent to C++'s pass by reference: "void foo(Klass&)".
[snip]
But you can pass null in a ref parameter:
void f(ref int x) @safe {}
void main() @safe
{
int* p = null;
f(*p);
}
Note that C++ also can do this, so I'm not sure the & is accomplishing
the correct goal:
void foo(Klass&);
int main()
{
Klass *k = NULL;
foo(*k);
}
However, the in contract does actually enforce the requirement.
-Steve
Re: Express "Class argument may not be null" ?
On 08/08/2017 08:34 PM, Johan Engelen wrote:
How would you express the function interface intent that a reference
to a class may not be null?
For a function "void foo(Klass)", calling "foo(null)" is valid. How do I
express that that is invalid? (let's leave erroring with a compile error
aside for now)
Something equivalent to C++'s pass by reference: "void foo(Klass&)".
A contract might be the best you can do:
void foo(Klass k)
in { assert(k !is null); }
body {}
Or throw an exception.
(note: I mean D classes, for structs "ref" works)
But you can pass null in a ref parameter:
void f(ref int x) @safe {}
void main() @safe
{
int* p = null;
f(*p);
}
Re: Express "Class argument may not be null" ?
On 8/8/17 2:34 PM, Johan Engelen wrote:
Hi all,
How would you express the function interface intent that a reference
to a class may not be null?
For a function "void foo(Klass)", calling "foo(null)" is valid. How do I
express that that is invalid? (let's leave erroring with a compile error
aside for now)
There isn't a way to do this in the type itself.
One can always create a null class instance via:
MyObj obj;
There is no way to disallow this somehow in the definition of MyObj.
With structs, you can @disable this(), and it's still possible but
harder to do so.
I would say, however, that if you wanted to express the *intent*, even
without a compile-time error, you could use a contract:
void foo(Klass k) in {assert(k !is null);};
Since the contract is part of the signature, this should be symantically
what you want.
However, this has to be done on every function that would accept a
Klass, there's no way to bake it into the type itself.
-Steve
