Calls to struct methods and immutable

[Ali Çehreli]

On 11/15/2012 10:26 AM, Joseph Rushton Wakeling wrote:
> On 11/15/2012 06:40 PM, Ali Çehreli wrote:
>> b) The user wants to play safe:
>>
>> auto makeFoo()
>> {
>> Foo foo;
>> foreach (i; 0..10)
>> foo.add( /* new data point */ );
>> return foo;
>> }
>>
>> void main()
>> {
>> immutable foo = makeFoo();
>> }
>>
>> Both of those compile with dmd 2.060.
>
> Really? I'm using from-GitHub dmd, and with the above example I get a
> "cannot implicitly convert expression to immutable" error, e.g.:
>
> Error: cannot implicitly convert expression (testSets(nfRaw,0.1L)) of
> type TestData!(ulong,ulong) to immutable(TestData!(ulong,ulong))
>

The following program compiles without any errors with dmd 2.060:

struct Foo(T0, T1)
{
     T0 t0;
     T1 t1;
}

auto testSets(T0, T1)(T0 t0, T1 t1)
{
     auto foo = Foo!(T0, T1)(t0, t1);
     return foo;
}

void main()
{
     ulong nfRaw;
     immutable foo = testSets(nfRaw,0.1L);
}

So far it makes sense to me: There shouldn't be any problem with making 
a copy of a value type and marking that copy as immutable.

Unless there exists a member that would make this unsafe. Let's add an 
int[] member to Foo:

struct Foo(T0, T1)
{
     T0 t0;
     T1 t1;
     int[] a;
}

auto testSets(T0, T1)(T0 t0, T1 t1, int[] a)
{
     auto foo = Foo!(T0, T1)(t0, t1, a);
     return foo;
}

void main()
{
     ulong nfRaw;
     int[] a = [ 42 ];
     immutable foo = testSets(nfRaw, 0.1L, a); // <-- compilation error
     assert(foo.a[0] == 42);
     a[0] = 43;
     assert(foo.a[1] == 43);  // <-- if compiled, this would be a bug
}

Do you have a reference type in your struct?

Ali