Operator overloading in C

[jacob navia]

C++ introduced an interesting feature (among others): operator overloading.

The idea is to build a mechanism for the user defining its own number types and the operations to be
done with them.

I decided to build it in the lcc-win32 distribution, and this feature allowed me to introduce 350
bit floats.

Yes, 350. We have a lot of fast machines now. Why not use them? The extra-precision is useful in
some algorithms.

This extension to C is transparent, since it doesn't change anything in the basics of the language.
The syntax used is:

TYPE operator+(TYPE a,TYPE b)
{
}

When in your program you use
TYPE a,b;
...
a+b;
Means that you call the operator function with those two numbers. This is similar to C++, but since
there are no classes (C is not object oriented) everything is quite simple.

Note that this is an extension compatible with C and it is important to know that the C standard
doesn't forbid extensions. They should not introduce new keywords though.

In my implementation it means that

int operator = 6;

still works as it should. No new keywords.

The operator symbol is recognized if (and only if):
1) The level is global, i.e. not within a function definition.
2) It is followed by one of the defined operator symbols (+, -, etc)
3) This must be followed by a normal function definition. The name of this function is the sequence
of types in the argument list, the "signature" in technical terms.

Is this useful?

Yes, for numbers it is ideal. It is less so, when the data types aren't quantities but other
relationships or data. Take, for instance
String a,b,c;
...
c = a+b;

This means that a+b is different from b+a. Not very clear.

Operator overloding *can* be useful, specially for letting the users build new kinds of numbers,
that fulfill special needs.

What do you think?

Voice your opinion. In any case Lcc-win32 is available at:

http://www.cs.virginia.edu/~lcc-win32

[bd]

On Fri, 08 Aug 2003 21:53:32 +0200, jacob navia wrote:

C++ introduced an interesting feature (among others): operator overloading.

The idea is to build a mechanism for the user defining its own number types and the operations to be
done with them.

I decided to build it in the lcc-win32 distribution, and this feature allowed me to introduce 350
bit floats.

[snip]

This is offtopic for comp.lang.c. comp.std.c is for discussion of changes
to the standard.

--
Freenet distribution not available
"Let's show this prehistoric bitch how we do things downtown!"
-- The Ghostbusters

[Martijn]

bd wrote:

I decided to build it in the lcc-win32 distribution, and this
feature allowed me to introduce 350 bit floats.

[snip]

This is offtopic for comp.lang.c. comp.std.c is for discussion of
changes
to the standard.

How do you gather that he is trying to change the standard? He is merely
suggesting an extension. I quote: "and it is important to know that the C
standard doesn't forbid extensions".

I guess is more of a moral one (which may or may not be off topic).

In regards to the original question: I wouldn't use it because I prefer to
stick to the standard, but I find the concept very neat and I do think it
would be a useful extension which could clarify and simplify code.

Good luck,

--
Martijn Haak
http://www.sereneconcepts.nl

[jacob navia]

"David Rubin" <bo***********@nomail.com> wrote in message news:3F***************@nomail.com...

jacob navia wrote:

[snip - I introduced operator overloading as an extension to my compiler
suite]

What do you think?

If you want to use C++ features, use C++.

/david

Obviously I do not agree with that. C++ is a very complex language, and is missing an essential
facility I found only in C: I like to build my structures/data as I want, without any object
oriented framework imposed by the language.

There are many things innovative in C++, but not all of it. This idea solves a problem that can't be
solved in C easily, without adding any C++ complexity.

jacob

[jacob navia]

"bd" <bd*****@bd-home-comp.no-ip.org> wrote in message
news:pa****************************@bd-home-comp.no-ip.org...

On Fri, 08 Aug 2003 21:53:32 +0200, jacob navia wrote:

C++ introduced an interesting feature (among others): operator overloading.

The idea is to build a mechanism for the user defining its own number types and the operations to be done with them.

I decided to build it in the lcc-win32 distribution, and this feature allowed me to introduce 350 bit floats.

[snip]

This is offtopic for comp.lang.c. comp.std.c is for discussion of changes
to the standard.

I wouldn't say the standard should be changed, at least not before a good discussion. This is what
comp.lang.c is all about isn't it?

Discussion about C.

Operator overloading allows the easy introduction of new numeric types without much problems. I am
not saying is "the solution" for all problems but is "a solution" for the introduction of
1) Higher precision numbers
2) Bignums (arbitrary precision numbers)
3) Rationals (Numbers expressed as integer ratios 5/9, for instance)
4) Quaternions

and many others. To do this in C requires
divide(add(a,b),sub(a,b))
instead of
(a+b)/(a-b)

jacob

[Malcolm]

"jacob navia" <ja*********@jacob.remcomp.fr> wrote in message

Operator overloading allows the easy introduction of new numeric
types without much problems

I wrote a big number package in C++. It had plenty of problems, for instance
I added a user defined cast to long to try to simplify the code, and
promptly broke everything I had written previously.

[jacob navia]

"Malcolm" <ma*****@55bank.freeserve.co.uk> wrote in message news:bh**********@news6.svr.pol.co.uk...

"jacob navia" <ja*********@jacob.remcomp.fr> wrote in message

Operator overloading allows the easy introduction of new numeric
types without much problems

I wrote a big number package in C++. It had plenty of problems, for instance
I added a user defined cast to long to try to simplify the code, and
promptly broke everything I had written previously.

Well, give lcc-win32 a try. I will be glad to help you if you find problems with it.
jacob

[jacob navia]

"Zeljko Vrba" <mo****@fly.srk.fer.hr> wrote in message news:sl*******************@fly.srk.fer.hr...

In article <bh**********@news-reader5.wanadoo.fr>, jacob navia wrote:

What do you think?
I wouldn't use it because of:
- it is supported only by your compiler; if I want to make my program available
to general public (i.e. UNIX systems, non x86 systems), the feature is useless
- I don't find prefix, function-call synax hard, and it's standard C
- unclear semantics: given

BIGNUM operator+(BIGNUM, double)

and an expression like a+2 (where a is BIGNUM), will 2 get converted to
double or an error will result?

If you compile this
#include <stdio.h>
typedef struct tagB {
int a;
int b;
} B;

B operator+(B a,double d)
{
printf("operator + called\n");
return a;
}

int main(void)
{
B a,b;

a = a+2;
return 0;
}
This produces
"operator + called"
Will this allow me to define additional
BIGNUM operator+(BIGNUM, int)
Yes. But in that case the int operator+ will be called. The algorithm first seeks a correct match
without any promotions. Then it will do "the usual conversions", then it will look if there are
defined casts that could match. I say C++ features are best left to C++.

I say operator overloading is useful.

[Stefano Ghirlanda]

"jacob navia" <ja*********@jacob.remcomp.fr> writes:

C++ introduced an interesting feature (among others): operator
overloading.

<explanation snipped>

Just my 2c: operator (and function) overloading are about the only C++
features I miss since I switched (back) to C.

I think a number of replies to your post reflect the fear that, as
soon as one starts to introduce new features, it will be impossible to
stop and C will soon become as complex as C++. And I don't deny that
there may be some truth in this claim.

Good luck,

--
Stefano

[Zeljko Vrba]

In article <bh**********@news-reader3.wanadoo.fr>, jacob navia wrote:

I say operator overloading is useful.

I didn't say it isn't. But then use C++. Nobody forces you to use classes,
inheritance, access levels and other C++ novelties. You can use it just as
an "enhanced C" using only features that you like/need/feel comfortable with.

Don't get me wrong: you did a tremendous amount of work coding a C compiler
and I admire that (having listened through 2 semesters of formal languages
and automata theory at faculty I know what is involved :)).

I just don't feel comfortable about spreading around another variant of C
with its own extensions that are incompatible with others in a fundamental
way (code written for your compiler would need a major revision if/when need
arises to port it to ANSI C).

Diversity of dialects killed LISP (the only other language I find beautiful
besides C) and Basic (at least until Gates "standardized" it). We shouldn't
be doing it to C too.

[jacob navia]

"Stefano Ghirlanda" <St***************@intercult.su.se> wrote in message
news:87************@ethology.intercult.su.se...

"jacob navia" <ja*********@jacob.remcomp.fr> writes:

C++ introduced an interesting feature (among others): operator
overloading.

<explanation snipped>

Just my 2c: operator (and function) overloading are about the only C++
features I miss since I switched (back) to C.

I think a number of replies to your post reflect the fear that, as
soon as one starts to introduce new features, it will be impossible to
stop and C will soon become as complex as C++. And I don't deny that
there may be some truth in this claim.

Yes, and the experience of C++ should be a warning to all that start the same way:

KNOW WHEN TO STOP.

jacob

[Serve La]

"Zeljko Vrba" <mo****@fly.srk.fer.hr> wrote in message
news:sl******************@fly.srk.fer.hr...

In article <bh**********@news-reader3.wanadoo.fr>, jacob navia wrote:

I say operator overloading is useful.

I didn't say it isn't. But then use C++. Nobody forces you to use classes,
inheritance, access levels and other C++ novelties. You can use it just as
an "enhanced C" using only features that you like/need/feel comfortable

with.

wrong. If you work with people that use C++, you can be sure that at some
point you have to use class, inheritance, access levels and other C++
novelties. This is only true if you work alone.

[Andre]

Serve La wrote:

"Zeljko Vrba" <mo****@fly.srk.fer.hr> wrote in message
news:sl******************@fly.srk.fer.hr...

In article <bh**********@news-reader3.wanadoo.fr>, jacob navia wrote:

I say operator overloading is useful.

I didn't say it isn't. But then use C++. Nobody forces you to use classes,
inheritance, access levels and other C++ novelties. You can use it just as
an "enhanced C" using only features that you like/need/feel comfortable

with.

wrong. If you work with people that use C++, you can be sure that at some
point you have to use class, inheritance, access levels and other C++
novelties. This is only true if you work alone.

Exactly. If you're into component development and enjoy abstraction and
reusability, there's always going to be class, inheritance etc involved.
Also, what Zeljko said is also only true when you deliberately force an
application, that could benifit from Object orientation, to comply in a
non-OO fashion. Like Serve said, if you're working alone, you're the boss.

-Andre

[Dan Pop]

In <bh**********@news-reader5.wanadoo.fr> "jacob navia" <ja*********@jacob.remcomp.fr> writes:

The syntax used is:

TYPE operator+(TYPE a,TYPE b)
{
}

When in your program you use
TYPE a,b;
...
a+b;
Means that you call the operator function with those two numbers. This is similar to C++, but since
there are no classes (C is not object oriented) everything is quite simple.

Note that this is an extension compatible with C and it is important to know that the C standard
doesn't forbid extensions. They should not introduce new keywords though.
The C standard requires a diagnostic for

TYPE operator+(TYPE a,TYPE b) {}

because it is syntactically invalid in C. The only extensions allowed
by the C standard are those based on undefined behaviour. So, to make
your extension compatible with standard C, replace "operator" by a an
identifier in the implementation name space, e.g. __operator__:

TYPE __operator__+(TYPE a,TYPE b) {}

invokes undefined behaviour (a reserved identifier not explicitly defined
by the C standard is used), therefore no diagnostic is needed, and your
compiler is free to silently overload the + operator as a result.

This technique of adding extensions is widely used by GNU C, so that
people can still use them if they invoke gcc in standard C mode. If your
compiler has also a non-standard C mode, you can use both "operator" and
"__operator__" in that mode. When invoked in standard C mode, "operator"
becomes an indentifier in the program namespace, though.
In my implementation it means that

int operator = 6;

still works as it should. No new keywords.
It doesn't matter, TYPE operator+(TYPE a,TYPE b) {} still requires a
diagnostic, according to the C syntax.
The operator symbol is recognized if (and only if):
1) The level is global, i.e. not within a function definition.
2) It is followed by one of the defined operator symbols (+, -, etc)
3) This must be followed by a normal function definition. The name of this function is the sequence
of types in the argument list, the "signature" in technical terms.
That's wrong, again. Such names are in the program name space, you can't
define them by magic. Use the __ prefix to fix the issue. But you have
an additional problem: if the function signature is the function name,
how do you distiguish between operators with identical signatures, e.g.

TYPE operator+(TYPE a,TYPE b) {...}
TYPE operator-(TYPE a,TYPE b) {...}
Is this useful?

The answer is mostly an issue of religion. For better acceptance, don't
allow TYPE to be any of the C scalar types.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[jacob navia]

"Dan Pop" <Da*****@cern.ch> wrote in message news:bh**********@sunnews.cern.ch...

In <bh**********@news-reader5.wanadoo.fr> "jacob navia" <ja*********@jacob.remcomp.fr> writes:
[snip] But you have
an additional problem: if the function signature is the function name,
how do you distiguish between operators with identical signatures, e.g.

TYPE operator+(TYPE a,TYPE b) {...}
TYPE operator-(TYPE a,TYPE b) {...}

The name of the first operator is:

_operator_plus_TYPE_TYPE
the second is
_operator_minus_TYPE_TYPE
The biggest problem I had is the pointer problem.
The operator + can't be defined with types like

TYPE operator+(TYPE a,int b)

since the operation a+7

is used in the language already (means a + 7 *sizeof(a)) and would
clash with standard C. This restriction is applied to all operators.

In any way, the compiler has several flags that make it reject any
non-standard grammar like this (-ansic) or even go back to ansi 89.

jacob

[Jun Woong]

"Dan Pop" <Da*****@cern.ch> wrote in message news:bh**********@sunnews.cern.ch...

In <bh**********@news-reader2.wanadoo.fr> "jacob navia" <ja*********@jacob.remcomp.fr> writes:

"Dan Pop" <Da*****@cern.ch> wrote in message news:bh**********@sunnews.cern.ch...

In <bh**********@news-reader5.wanadoo.fr> "jacob navia" <ja*********@jacob.remcomp.fr> writes:

[snip]

But you have
an additional problem: if the function signature is the function name,
how do you distiguish between operators with identical signatures, e.g.

TYPE operator+(TYPE a,TYPE b) {...}
TYPE operator-(TYPE a,TYPE b) {...}

The name of the first operator is:

_operator_plus_TYPE_TYPE
the second is
_operator_minus_TYPE_TYPE

That's not good enough, you really need *two* preceding underscores.

One can think they are good enough. Those function names doesn't
pollute the users' valid name space.
--
Jun, Woong (my******@hanmail.net)
Dept. of Physics, Univ. of Seoul

[Richard Bos]

"Jun Woong" <my******@hanmail.net> wrote:

"Dan Pop" <Da*****@cern.ch> wrote in message news:bh**********@sunnews.cern.ch...

In <bh**********@news-reader2.wanadoo.fr> "jacob navia" <ja*********@jacob.remcomp.fr> writes:

The name of the first operator is:

_operator_plus_TYPE_TYPE
the second is
_operator_minus_TYPE_TYPE

That's not good enough, you really need *two* preceding underscores.

One can think they are good enough. Those function names doesn't
pollute the users' valid name space.

Not on a file scope level, but they can be blocked within a block scope,
and then you're in trouble - you try debugging why just this operator
won't work for just these types in just this function, and no others.
You could, of course, reserve these names for yourself, but it's better
to make sure.

Richard

[Jun Woong]

"Richard Bos" <rl*@hoekstra-uitgeverij.nl> wrote in message news:3f***************@news.nl.net...

"Jun Woong" <my******@hanmail.net> wrote:

"Dan Pop" <Da*****@cern.ch> wrote in message news:bh**********@sunnews.cern.ch...

In <bh**********@news-reader2.wanadoo.fr> "jacob navia" <ja*********@jacob.remcomp.fr> writes:

>The name of the first operator is:
>
>_operator_plus_TYPE_TYPE
>the second is
>_operator_minus_TYPE_TYPE

That's not good enough, you really need *two* preceding underscores.

One can think they are good enough. Those function names doesn't
pollute the users' valid name space.

Not on a file scope level, but they can be blocked within a block scope,

Oh, I missed that those names can be inserted where the overloaded
operators are actually used in an expression.

Thanks.
--
Jun, Woong (my******@hanmail.net)
Dept. of Physics, Univ. of Seoul

[jacob navia]

"Dan Pop" <Da*****@cern.ch> wrote in message news:bh**********@sunnews.cern.ch...

In <bh**********@news-reader2.wanadoo.fr> "jacob navia" <ja*********@jacob.remcomp.fr> writes:

TYPE operator+(TYPE a,TYPE b) {...}
TYPE operator-(TYPE a,TYPE b) {...}

The name of the first operator is:

_operator_plus_TYPE_TYPE
the second is
_operator_minus_TYPE_TYPE

That's not good enough, you really need *two* preceding underscores.

Of course, the compiler adds another underscore, as with any other function. The
resulting name in the object file (that can be dumped) is:

__operator_minus_TYPE_TYPE
As I already said, operator overloading should not be implemented for
scalar types.

This would be an enormous restriction Dan.

I see the usage of this mainly for defining new types of NUMBERS.
Then, it is paramount to have conversion features, and interaction with
native types. For instance when using the qfloat data type (350 bits
precision) you can write:

qfloat b = 67;
qfloat c = b+1;

This is because there is an operator defined like this:

qfloat operator+(qfloat,int);

In standard C it is not possible to add an integer to a structure, so
I STAY within UB (undefined behavior) and my implementation
can't clash with the C standard.

Am I correct?
Can you explain your motivations why this is incorrect?

Thanks

jacob

[jacob navia]

I believe Dan is referring to int operator+(int, int); and similar - any
operation that is already defined by the C standard.
Of course. One of the arguments *must* be a user defined type.

There are two kinds operators:
monadic (! for instance) and dyadic (+, -, etc)
At least one argument must be a user defined type, and there is a table
in the documentation indicating where pointers are possible. For instance
pointer multiplication or division is undefined in C, so you can safely
define operators to those functions that take pointers.

Addition and above all subtraction can't take plain pointers.

A simplification is done with the cast operator

typea operator()(typeb a) { }

This means that when you write:
typeb b;
typea a;

a = (typea)b;

the above conversion function is called.

If you do not add this, the code becomes messy, with too many ad hoc
operators that define the same thing.

A big problem with my schema is type aliasing.

When you write:
typedef unsigned long DWORD;

and later:
type operator+(type,DWORD);

should the operator accept unsigned ints?
Or only DWORDS?

Questions and questions. One of the reasons of this discussion is
to hear to opinion of other people more clever than me.

What do you think?

jacob

P.S. concerning your other remark: It's not the name in the object file, it's the name in the C namespace
that matters. _operator_minus_TYPE_TYPE is a valid name for a user
program to use for a block-scope identifier.
I thought that identifiers beginning with _ are reserved and should not be used.
Should I add a new underscore? (Not very difficult to do).
"Kevin Easton" <[e-mail]> wrote in message
news:ne********************@tomato.pcug.org.au... jacob navia <ja*********@jacob.remcomp.fr> wrote:

"Dan Pop" <Da*****@cern.ch> wrote in message news:bh**********@sunnews.cern.ch...

In <bh**********@news-reader2.wanadoo.fr> "jacob navia" <ja*********@jacob.remcomp.fr> writes:

>> TYPE operator+(TYPE a,TYPE b) {...}
>> TYPE operator-(TYPE a,TYPE b) {...}
>>
>
>The name of the first operator is:
>
>_operator_plus_TYPE_TYPE
>the second is
>_operator_minus_TYPE_TYPE

That's not good enough, you really need *two* preceding underscores.

Of course, the compiler adds another underscore, as with any other function. The
resulting name in the object file (that can be dumped) is:

__operator_minus_TYPE_TYPE

It's not the name in the object file, it's the name in the C namespace
that matters. _operator_minus_TYPE_TYPE is a valid name for a user
program to use for a block-scope identifier.

As I already said, operator overloading should not be implemented for
scalar types.

This would be an enormous restriction Dan.

I see the usage of this mainly for defining new types of NUMBERS.
Then, it is paramount to have conversion features, and interaction with
native types. For instance when using the qfloat data type (350 bits
precision) you can write:

qfloat b = 67;
qfloat c = b+1;

This is because there is an operator defined like this:

qfloat operator+(qfloat,int);

In standard C it is not possible to add an integer to a structure, so
I STAY within UB (undefined behavior) and my implementation
can't clash with the C standard.

Am I correct?
Can you explain your motivations why this is incorrect?

I believe Dan is referring to int operator+(int, int); and similar - any
operation that is already defined by the C standard.

- Kevin.