On Monday, 18 August 2014 at 10:01:59 UTC, maik klein wrote:
First of all I don't want to insult anyone on language design,
I just want to know the reason behind the "always on" GC.
I know that the GC as several advantages over reference
counting, especially when it comes to immutable data structures.
What I don't (correct me if i am wrong) understand is why every
heap allocation has to be garbage collected, like classes,
dynamic arrays etc.
Does a GC still have advantages over heap allocations that do
not need to be reference counted such as the unique_ptr in c++?
The dlang homepage stats:
Destructors are used to deallocate resources acquired by an
object. For most classes, this resource is allocated memory.
With garbage collection, most destructors then become empty and
can be discarded entirely.
If I understand it correctly it means that D has a GC, so most
classes don't need a destructor anymore because they don't need
to do any cleanup. I am not totally convinced that this would
be a good trade off in general. Maybe someone could shine some
light on this statement?
A good reason is the ability to write lock-free algorithms, which
are very hard to implement without GC support. This is the main
reason why C++11 has a GC API and Herb Sutter will be discussing
about GC in C++ at CppCon.
Reference counting is only a win over GC with compiler support
for reducing increment/decrement operations via dataflow analysis.
C++ programs with heavy use of unique_ptr/shared_ptr/weak_ptr are
slower than other languages with GC support, because those
classes are plain library types without compiler support. Of
course, compiler vendors can have blessed library types, but the
standard does not require it.
RC also has performance impact when deleting big data structures.
You can optimize this away big using asynchronous deletion, but
eventually you are just doing GC with another name.
As for feasibility of a GC in a systems programming language,
just think the Mesa/Cedar environment at Xerox PARC was a system
programming language using reference counting with a GC for
collecting cycles.
--
Paulo
