Error: xxx is not an lvalue

[Denis Koroskin]

On Sun, 03 May 2009 01:02:41 +0400, Tyro[a.c.edwards] <nospam at home.com> wrote:

> On 5/3/2009 3:44 AM, flourish wrote:
>> Hi,
>>
>> why does the following code not compile -- or how to declare a (usable)
>> property of an associative array field?
>>
>> //////////////////////
>> class Test
>> {
>>    private int[int] _testMap;
>>    public int[int] testMap() {return _testMap;}
>> }
>>
>
> testMap() is a getter and can only return the value it is designed to.
> You must implement a setter to accomplish what you are trying to di.  
> Unfortunately I'm not sure how you do that with AAs so I'll leave that  
> to someone that's a little smarter than I.
>
> I think that maybe implementing opIndexAssign might work for you though:
>
> public void opIndexAssign(int val, int ndx){ _testMap[ndx] = val; }
>
> Then you can simply "test[0] = 1;" to get the desired effect.
>
>> void main()
>> {
>>    Test test = new Test();
>>    test.testMap[0] = 1;
>> }
>> /////////////////
>>
>> *** Error: test.testMap() is not an lvalue
>>
>>
>> Regards,
>>      flourish
>>
>>
>>
>

No, you are wrong, testMap works like a property here. I believe, original code _should_ work. For example, it works if you replace int[int] with int[].
All the trouble is because of an awful properties implementation in D, and temporaries not being able to be passed by reference:

struct Foo
{
    private int[] _arr;	
    int[] arr() { return _arr; }
}

void doSomething(int[] a)
{
}

void doSomethingElse(ref int[] a)
{
}

void doSomethingElseConst(ref const(int)[] a)
{
}

void main()
{
    Foo foo;
    foo.arr[0] = 1; // works
    //foo.arr.length = 1; // doesn't

    foo.arr().doSomething(); // works
    //foo.arr.doSomething(); // doesn't

    //foo.arr().doSomethingElse(); 		// doesn't
    //foo.arr().doSomethingElseConst();	// doesn't
}