What's the use of 'typeof','typeid','__traits','is' ?

[Paul Backus]

On Sunday, 24 August 2025 at 03:13:38 UTC, David T. Oxygen wrote:
> Hello every kind and helpful D programmers, I need your help.
>
> I've met two problems about getting types.
> In D, we can use many keywords to get the type of an object, 
> such as `typeof`,`typeid`,`is`.
> However, I don't know the differences between them well. I also 
> don't know how to use them in D...

`typeof` is evaluated at compile time and gives the type of an 
expression or symbol as seen by the compiler.

`typeid` is evaluated at runtime and gives the [`TypeInfo`][1] 
object containing information about an object's runtime type.

[1]: https://dlang.org/library/object/type_info.html

The difference between `typeof` and `typeid` can be seen in the 
following program:

     class Example {}

     void main()
     {
         auto object = new Example;
         check(object);
     }

     void check(Object o)
     {
         import std.stdio;

         writeln(typeof(o).stringof); // Object
         writeln(typeid(o).toString); // Example
     }