Re-posted due to formatting issues:
I think I should have used the term object based for non-OO languages.
Scope based finalization is a sweet spot of C++ with scoped based
destructors. I'm coding a lot of Java at the moment and having to code
finally blocks or use the try-with-resources statement is tedious. But
my point was mainly about polymorphism and how it can be implemented in
languages that have function pointers. Of course, in a language that has
no destructors or finalization you have to explicitly call a destructor.
Here is a nifty piece of C++ code that wraps the C stdio FILE structure
which implements RAII to call fclose() by using a implicit pointer
operator! It's one of the many C++ features many people are not aware
of. Props to Jerry Coffin who invented it.
class file {
typedef FILE *ptr;
ptr wrapped_file;
public:
file(std::string const &name, std::string const &mode =
std::string("r")) :
wrapped_file(fopen(name.c_str(), mode.c_str()))
{ }
operator ptr() const { return wrapped_file; }
~file() { if (wrapped_file) fclose(wrapped_file); }
};
-----------------------------------------------------------------------------
file f("myfile.txt", "w");
if (!f) {
fprintf(stderr, "Unable to open file\n");
return 0;
}
fprintf(f, "Hello world");
On 31/05/2017 9:06 PM, Charles Mills wrote:
There's no reason why you can't write OO code in assembler.
And there's no reason you could not write OO code by keying the bits in through
a debugger.
The point of the OO constructs is not that there is no other way to do things.
Obviously, any object program that can be produced by a compiler could have
been written in HLASM.
The point of OO constructs is never having to say "was there something I was
supposed to call when I was finished with this record (DSECT, struct, class, ...)?"
What was its name? Where is the pointer to it? What are the calling parameters? Or worse,
I just realized I am going to need a cleanup routine for this record. Now I have to go
back and make sure that every piece of code that uses it gets modified to call the
cleanup routine. You just delete the class and if it has a destructor, it gets called.
Voila!
Charles
-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On Behalf
Of David Crayford
Sent: Tuesday, May 30, 2017 7:46 PM
To: IBM-MAIN@LISTSERV.UA.EDU
Subject: Re: Question about C++
On 30/05/2017 9:52 PM, Charles Mills wrote:
You're trying to scare the poor man!
After I learned OO, I realized the problem in trying to communicate the
concepts.
- The easy-to-grasp explanations are stupid: Animal is a class. Dog and Cat are
child classes of Animal. Fido is an instance of Dog. You could have an
inherited public overridden method Speak() and say myAnimal.Speak() and if it
were a Dog it would bark and if it were a Cat it would Meow ...
Agreed. Inheritance should generally be avoided anyway. It has it's place but
composition should be preferred 9 times out of 10. Inheritance is tightly
coupled and can become incomprehensible once it gets a few layers deep.
- The real problems that are solved by the significant features are too hard to
explain in a simple tutorial. I solved a problem the other day with a very
sparsely-implemented virtual polymorphic method -- but it would take me an hour
to explain what the problem was.
Polymorphism is the key principle and you don't need an OO language to use it.
Any language with function pointers can implement polymorphism.
A case in point is the z/OS C stdio runtime which supports many different types
of data source. fopen() is the factory function which populates the read/write
(virtual) function pointers in the FILE structure. There's no reason why you
can't write OO code in assembler. I see lots of assembler code with
constitutional logic littered throughout which call different functions
depending on some type which would benefit from an OO design.
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