Re: How to check if variable of some type can be of null value?

[JG via Digitalmars-d-learn]

On Saturday, 24 July 2021 at 19:39:02 UTC, Alexey wrote:

On Saturday, 24 July 2021 at 18:10:07 UTC, Alexey wrote:

I've tried to use ```typeof(t) is cast(t)null```, but compiler exits with error and so this can't be used for checking this issue.

The goal I with to achieve by this check - is to use template and to assign value to variable basing on it's ability to accept null as a value.

currently I ended up using ```__traits(compiles, cast(T1)null)``` for this check. but don't know is this really semantically correct.

There are probably better ways. However, this seems to work:
```d
import std;
enum canBeSetToNull(T) = __traits(compiles,(T.init is null));

interface I1
{
}

class C1 : I1
{
}

struct S1
{
}

    struct S2
    {
        int a=1;
    }

    void main()
    {
        auto c1 = new C1;

        I1 i1 = c1;

        auto s1 = S1();
        auto s2 = S2();

        static assert(canBeSetToNull!(typeof(c1)));
        static assert(canBeSetToNull!(typeof(i1)));
        static assert(!canBeSetToNull!(typeof(s1)));
        static assert(!canBeSetToNull!(typeof(s2)));
        static assert(!canBeSetToNull!(int));
        static assert(canBeSetToNull!(int*));

}```