Joseph Wakeling wrote:
I also have some C++ experience, but it seems to be confusing as much as
complementary with respect to D ... :-)
Current source of confusion relates to declaring objects of a class whose
constructor takes input -- confusion because I can write,
class Foo
{
int x;
uint y;
this()
{
x = -1;
y = 2;
}
}
void main()
{
Foo f;
}
and have no problem, but if instead the constructor is,
this(int z)
{
x = z;
y = 2;
}
... it seems like I have to write instead,
auto f = new Foo(-1);
... and if I write as per C++,
Foo f(-1);
... I get back a compiler error: "found 'f' when expecting ';' following
'statement'". Am I right in thinking that the 'new' syntax is necessary when
the
class has a constructor which takes input?
You have stumbled upon a major difference between C++ and D. In D,
classes are reference types, and objects are allocated dynamically on
the heap. This means that if you simply type
Foo f;
then f is null -- it is a reference to absolutely nothing -- regardless
of whether Foo defines a constructor. If you try to use it for
anything, you will get a null dereference error.
To allocate a new object of type Foo, you use 'new':
Foo f = new Foo; // No constructor
Foo f = new Foo(-1);
In both cases, f now points to a valid object of type Foo. The C++
syntax you are referring to does not work in D.
If you want more C++-like behaviour, i.e. if you want to allocate on the
stack, use a struct:
struct Bar
{
uint x = -1;
uint y = 2;
this(uint z) { x = z; }
}
Bar b; // Use default x and y values
Bar b = Bar(0); // Set x = 0
Note that you can also allocate a struct on the heap by using 'new'. In
this case, it returns a pointer:
Bar* b = new Bar(123);
This creates confusion also because in C++ one associates 'new' with dynamic
allocation of memory, and it requires a consequent 'delete' statement. I know
that D has GC, and I know that it also has 'delete' statements, but ... this one
is 'ouch' for me :-P
Now you see, 'new' is for dynamic memory allocation in D as well, it's
just that for classes it is required. You normally don't need to worry
about 'delete', as the GC will take care of deallocation.
-Lars
