Here's my code:

module grammar;

class Grammar(T : ulong) {
    this(const T[] str) {
        auto grammar = str in grammarCache;

        if (grammar) {
            this = grammar.dup;
        } else {
            this = approximateSmallestGrammar(str);
            grammarCache[str] = this.dup;
        }
    }

    static Grammar approximateSmallestGrammar(const T[] str) {
        return new Grammar();
    }

    @property Grammar dup() {

    }

private:
    this() {}
    static Grammar[T[]] grammarCache;
};


Compiler says 'this' is not an lvalue.  How would I accomplish what I want?

On Friday, July 24, 2015 01:30:55 Enjoys Math via Digitalmars-d-learn wrote:
> Here's my code:
>
> module grammar;
>
> class Grammar(T : ulong) {
>      this(const T[] str) {
>          auto grammar = str in grammarCache;
>
>          if (grammar) {
>              this = grammar.dup;
>          } else {
>              this = approximateSmallestGrammar(str);
>              grammarCache[str] = this.dup;
>          }
>      }
>
>      static Grammar approximateSmallestGrammar(const T[] str) {
>          return new Grammar();
>      }
>
>      @property Grammar dup() {
>
>      }
>
> private:
>      this() {}
>      static Grammar[T[]] grammarCache;
> };
>
>
> Compiler says 'this' is not an lvalue.  How would I accomplish what I want?

Assign to the members individually rather than the whole object at once.

- Jonathan M Davis

On 7/23/15 9:30 PM, Enjoys Math wrote:
> Here's my code:
>
> module grammar;
>
> class Grammar(T : ulong) {
>      this(const T[] str) {
>          auto grammar = str in grammarCache;
>
>          if (grammar) {
>              this = grammar.dup;
>          } else {
>              this = approximateSmallestGrammar(str);
>              grammarCache[str] = this.dup;
>          }
>      }
>
>      static Grammar approximateSmallestGrammar(const T[] str) {
>          return new Grammar();
>      }
>
>      @property Grammar dup() {
>
>      }
>
> private:
>      this() {}
>      static Grammar[T[]] grammarCache;
> };
>
>
> Compiler says 'this' is not an lvalue.  How would I accomplish what I want?
>

You're approaching this wrong. Do the lookup before deciding whether to instantiate a new object:

static Grammar getGrammar(const T[] str) {
     if(auto x = str in grammarCache)
         return *x;
     else
     {
         auto g = new Grammar;
         grammarCache[str] = g;
         return g;
     }
}

If you always want to dup, then do it outside the lookup. Don't do it in the constructor, you already have an object by then.

-Steve

On Friday, 24 July 2015 at 03:12:43 UTC, Steven Schveighoffer wrote:
> On 7/23/15 9:30 PM, Enjoys Math wrote:
>>[...]
>
> You're approaching this wrong. Do the lookup before deciding whether to instantiate a new object:
>
> static Grammar getGrammar(const T[] str) {
>      if(auto x = str in grammarCache)
>          return *x;
>      else
>      {
>          auto g = new Grammar;
>          grammarCache[str] = g;
>          return g;
>      }
> }
>
> If you always want to dup, then do it outside the lookup. Don't do it in the constructor, you already have an object by then.
>
> -Steve

Thanks.  That sounds like a good approach

Apart from what others have said, for a class `this` is the _reference_ to the current object, i.e. a "pointer". If the compiler allowed assigning to it, it would not modify the contents of your object.

If you want to assign all of the elements at once, you can use `tupleof` (untested):

    this.tupleof = other.tupleof;

On 7/24/15 4:47 AM, "Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm@gmx.net>" wrote:
> Apart from what others have said, for a class `this` is the _reference_
> to the current object, i.e. a "pointer". If the compiler allowed
> assigning to it, it would not modify the contents of your object.
>
> If you want to assign all of the elements at once, you can use `tupleof`
> (untested):
>
>      this.tupleof = other.tupleof;

I'm quite certain this wouldn't copy the derived data. So be careful when doing this, you can only do this on final classes.

-Steve

On Friday, 24 July 2015 at 14:12:54 UTC, Steven Schveighoffer wrote:
> On 7/24/15 4:47 AM, "Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm@gmx.net>" wrote:
>> Apart from what others have said, for a class `this` is the _reference_
>> to the current object, i.e. a "pointer". If the compiler allowed
>> assigning to it, it would not modify the contents of your object.
>>
>> If you want to assign all of the elements at once, you can use `tupleof`
>> (untested):
>>
>>      this.tupleof = other.tupleof;
>
> I'm quite certain this wouldn't copy the derived data. So be careful when doing this, you can only do this on final classes.

Right, this is dangerous for classes.