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digitalmars.D - what's the semantics of 'varX is varY', in particular `strX is strY`?

reply mw <mingwu gmail.com> writes:
I've thought it's compare by reference, as in e.g. `assert(obj 
!is null)`

but

```
   string strX = "2020-07-29";
   string strY = "2020-07-29";
   string strZ = "2020-07-30";
   assert(strX !is strY);  // assertion failed
   assert(strX !is strZ);  // assertion pass
```

so here `is` means compare strings by their contents? where is 
the exact definition of the  semantics of 'varX is varY'?


BTW, on page:

https://dlang.org/spec/lex.html#keywords

`is` directly link to:

https://dlang.org/spec/expression.html#IsExpression

which only talks about is(expr), not 'varX is varY'


Thanks.
Aug 05 2020
next sibling parent Adam D. Ruppe <destructionator gmail.com> writes:
On Wednesday, 5 August 2020 at 17:29:09 UTC, mw wrote:

 so here `is` means compare strings by their contents?


They both refer to the same string there... the literal gets 
reused by the compiler. Put a .dup on one of the strings and then 
is will not return true anymore.


 where is the exact definition of the  semantics of 'varX is 
 varY'?


It checks if the memory for the variable is the same.
Aug 05 2020
prev sibling parent Paul Backus <snarwin gmail.com> writes:
On Wednesday, 5 August 2020 at 17:29:09 UTC, mw wrote:

 I've thought it's compare by reference, as in e.g. `assert(obj 
 !is null)`

 but

 ```
   string strX = "2020-07-29";
   string strY = "2020-07-29";
   string strZ = "2020-07-30";
   assert(strX !is strY);  // assertion failed
   assert(strX !is strZ);  // assertion pass
 ```

 so here `is` means compare strings by their contents? where is 
 the exact definition of the  semantics of 'varX is varY'?


It's documented under the name "Identity Expression":

https://dlang.org/spec/expression.html#IdentityExpression

You're correct that it's comparing by reference. The reason strX 
and strY are equal is that the compiler noticed you wrote two 
identical string literals, so it combined them in the binary. You 
can tell this is true because `assert(strX.ptr == strY.ptr)` 
passes.
Aug 05 2020