## digitalmars.D - Why is int implicitly convertible to ulong?

• Hannes Steffenhagen (5/5) Feb 16 2014 isImplicitlyConvertible!(int,ulong) is true. Maybe this is just
• =?UTF-8?B?QWxpIMOHZWhyZWxp?= (12/17) Feb 16 2014 I don't know all of the reasons but it is at least about convenience. It...
• Xinok (6/11) Feb 16 2014 IIRC, the way they put it is that "information is not lost," so
• Jonathan M Davis (29/34) Feb 16 2014 signed to unsigned conversions (and vice versa) are implicit as are
• Xinok (17/76) Feb 17 2014 I've been bitten by signed / unsigned comparisons before, and I'm
"Hannes Steffenhagen" <cubicentertain gmail.com> writes:
```isImplicitlyConvertible!(int,ulong) is true. Maybe this is just
me, but I get the impression that this is quite nuts. Why is an
implicit conversion from a signed to an unsigned type possible?
The other way round would be at least somewhat understandable if
there's a static check that the values actually fit.
```
Feb 16 2014
=?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
```On 02/16/2014 01:35 PM, Hannes Steffenhagen wrote:
isImplicitlyConvertible!(int,ulong) is true. Maybe this is just me, but
I get the impression that this is quite nuts. Why is an implicit
conversion from a signed to an unsigned type possible? The other way
round would be at least somewhat understandable if there's a static
check that the values actually fit.

I don't know all of the reasons but it is at least about convenience. It
is possible to write expressions like 'u + diff' without explicit casts:

ulong u = 10;
int diff = -3;
auto a = u + diff;
static assert(is (typeof(a) == ulong));

Related, the type of 'a' above is implied as ulong due to arithmetic
conversions, which are sometimes very confusing as well. See "Usual
Arithmetic Conversions" here:

http://dlang.org/type.html

Ali
```
Feb 16 2014
"Xinok" <xinok live.com> writes:
```On Sunday, 16 February 2014 at 21:35:02 UTC, Hannes Steffenhagen
wrote:
isImplicitlyConvertible!(int,ulong) is true. Maybe this is just
me, but I get the impression that this is quite nuts. Why is an
implicit conversion from a signed to an unsigned type possible?
The other way round would be at least somewhat understandable
if there's a static check that the values actually fit.

IIRC, the way they put it is that "information is not lost," so
you could always cast back to an int and get the original value.
The same is not true for casting to a smaller type, e.g. int to
byte, the original value may be lost.
```
Feb 16 2014
Jonathan M Davis <jmdavisProg gmx.com> writes:
```On Sunday, February 16, 2014 21:35:01 Hannes Steffenhagen wrote:
isImplicitlyConvertible!(int,ulong) is true. Maybe this is just
me, but I get the impression that this is quite nuts. Why is an
implicit conversion from a signed to an unsigned type possible?
The other way round would be at least somewhat understandable if
there's a static check that the values actually fit.

signed to unsigned conversions (and vice versa) are implicit as are
conversions from smaller integral types to larger integral types. Converting
from smaller integral types to larger really doesn't cause any problems, but
the signed to unsigned (or vice versa) can cause issues - one of the biggest
of those being the comparison of signed and unsigned values, and IIRC, there
was some discussion on making that a warning or error. However, while there
are occasional problems from the conversion being implicit, if it weren't
implicit, you'd be forced to cast a lot more when the signed and unsigned
types interact, which would lead to messier code and could actually increase
the number of bugs, because if you got in the habit of casting everywhere to
get the signed to unsigned conversions to work, you'd risk accidentally doing
stuff like casting a ulong to int and losing data, since the compiler would
assume that you knew what you were doing with the cast.

So, it's a tradeoff, and neither making the signed to unsigned (or vice versa)
conversions explicit nor implicit would be without problems. Walter went with
it being implicit, which matches what C does. However, unlike C, conversions
that actually lose data (e.g. long -> int) do require casts so that it's
easier to catch those problems. But no data is actually lost with a sign
conversions, as casting it back to what it was will result in the same value
(unlike with converting to a smaller integral value).

Of slightly bigger concern IMHO is that bool and the character types are all
treated as integral types, which is at times useful but also risks some
entertaining bugs. But again, it's a matter of tradeoffs. If they required
casts when interacting with integral types, then a lot more casting would be
required, risking a different set of bugs. There really isn't a right answer
as to whether the conversions should be implicit or explicit. It just comes
down to the tradeoffs that you prefer.

- Jonathan M Davis
```
Feb 16 2014
"Xinok" <xinok live.com> writes:
```On Monday, 17 February 2014 at 04:40:52 UTC, Jonathan M Davis
wrote:
On Sunday, February 16, 2014 21:35:01 Hannes Steffenhagen wrote:
isImplicitlyConvertible!(int,ulong) is true. Maybe this is just
me, but I get the impression that this is quite nuts. Why is an
implicit conversion from a signed to an unsigned type possible?
The other way round would be at least somewhat understandable
if
there's a static check that the values actually fit.

signed to unsigned conversions (and vice versa) are implicit as
are
conversions from smaller integral types to larger integral
types. Converting
from smaller integral types to larger really doesn't cause any
problems, but
the signed to unsigned (or vice versa) can cause issues - one
of the biggest
of those being the comparison of signed and unsigned values,
and IIRC, there
was some discussion on making that a warning or error. However,
while there
are occasional problems from the conversion being implicit, if
it weren't
implicit, you'd be forced to cast a lot more when the signed
and unsigned
types interact, which would lead to messier code and could
actually increase
the number of bugs, because if you got in the habit of casting
everywhere to
get the signed to unsigned conversions to work, you'd risk
accidentally doing
stuff like casting a ulong to int and losing data, since the
compiler would
assume that you knew what you were doing with the cast.

So, it's a tradeoff, and neither making the signed to unsigned
(or vice versa)
conversions explicit nor implicit would be without problems.
Walter went with
it being implicit, which matches what C does. However, unlike
C, conversions
that actually lose data (e.g. long -> int) do require casts so
that it's
easier to catch those problems. But no data is actually lost
with a sign
conversions, as casting it back to what it was will result in
the same value
(unlike with converting to a smaller integral value).

Of slightly bigger concern IMHO is that bool and the character
types are all
treated as integral types, which is at times useful but also
risks some
entertaining bugs. But again, it's a matter of tradeoffs. If
they required
casts when interacting with integral types, then a lot more
casting would be
required, risking a different set of bugs. There really isn't a
as to whether the conversions should be implicit or explicit.
It just comes
down to the tradeoffs that you prefer.

- Jonathan M Davis

I've been bitten by signed / unsigned comparisons before, and I'm
sure others have been as well. On the other hand, I can't recall
any bugs that were due to an explicit cast. I can see how
explicit casts might cause unexpected bugs (if the original type
changes but is still a valid cast), but in my personal
experience, explicit casts are safer than implicit casts.

Walter decided to adopt C-style switches for D, to simplify
translating code. However, implicit fall-through is notorious for
causing bugs in C. So as a tradeoff, D still allows fall-through
but only by explicitly writing "goto case;".

We could speculate all day, but ultimately it comes down to
experience of what works and what doesn't. If something is
generally safe in practice, then perhaps we're better with
leaving it alone. But if something is a known nuisance for
causing bugs, then find a better solution.
```
Feb 17 2014
"Hannes Steffenhagen" <cubicentertain gmail.com> writes:
```The specific problem here was when working with std.json.

std.json distinguishes between UINTEGER and INTEGER, so I had
code like

static if(is(T : ulong)) {
// must be UINTEGER
} else static if(is(T : long)) {
// can be either INTEGER or UINTEGER
}

I've since found out about isSigned and isUnsigned, still it was
mighty confusing for me that the first case was selected for
signed types.
```
Feb 21 2014
Jonathan M Davis <jmdavisProg gmx.com> writes:
```On Friday, February 21, 2014 12:41:07 Hannes Steffenhagen wrote:
The specific problem here was when working with std.json.

std.json distinguishes between UINTEGER and INTEGER, so I had
code like

static if(is(T : ulong)) {
// must be UINTEGER
} else static if(is(T : long)) {
// can be either INTEGER or UINTEGER
}

I've since found out about isSigned and isUnsigned, still it was
mighty confusing for me that the first case was selected for
signed types.

Well, it is using : - which means implicit conversion, which is always
something that you should be careful with in generic code. Technically,
something like

struct S
{
ulong ul;
alias ul this;
}

would match it as well, and depending on what the code inside the static if is
like, it might work, but there's also a good chance that it wouldn't. Using
is(T == ulong) if you want to require that it be exactly ulong, or
isUnsigned!T if you want any unsigned integral type. Using implicit conversion
in generic code _can_ be useful, but you need to be _very_ careful with it.

- Jonathan M Davis
```
Feb 21 2014
=?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
```On 02/21/2014 04:41 AM, Hannes Steffenhagen wrote:
The specific problem here was when working with std.json.

std.json distinguishes between UINTEGER and INTEGER, so I had code like

static if(is(T : ulong)) {
// must be UINTEGER
} else static if(is(T : long)) {
// can be either INTEGER or UINTEGER
}

I've since found out about isSigned and isUnsigned, still it was mighty
confusing for me that the first case was selected for signed types.

I have something like the following in an experimental std.json code
that converts to JSONValue.

Since std.json uses long for the value, I attempt to detect data loss
when the value is a large ulong:

JSONValue to(Target : JSONValue, T)(T value)
{
/* ... */

} else static if (is (T : long)) {
static if (is (T == ulong)) {
enforce(value <= long.max,
format("Data loss: %s %s cannot fit a long.",
T.stringof, value));
}

json.type = JSON_TYPE.INTEGER;
json.integer = value;

} else static if (is (T : real)) {

Ali
```
Feb 21 2014