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This section provides definitions of terms used in the Numeric Conversion library.
As defined by the C++ Object Model (§1.7) the storage or memory on which a C++ program runs is a contiguous sequence of bytes where each byte is a contiguous sequence of bits.
An object is a region of storage (§1.8) and has a type (§3.9).
A type is a discrete set of values.
An object of type T
has an
object representation which is the
sequence of bytes stored in the object (§3.9/4)
An object of type T
has a
value representation which is the set
of bits that determine the value of an object of that
type (§3.9/4). For POD types (§3.9/10),
this bitset is given by the object representation, but not all the bits in
the storage need to participate in the value representation (except for character
types): for example, some bits might be used for padding or there may be
trap-bits.
The typed value that is held by an object is the value which is determined by its value representation.
An abstract value (untyped) is the conceptual information that is represented in a type (i.e. the number π).
The intrinsic value of an object is the binary value of the sequence of unsigned characters which form its object representation.
Abstract values can be represented in a given type.
To represent an abstract value V
in a type T
is to obtain a typed value v
which corresponds to the abstract value V
.
The operation is denoted using the rep()
operator, as in: v=rep(V)
. v
is the representation
of V
in the type T
.
For example, the abstract value π can be represented in the type double
as the double
value M_PI
and in the type int
as the
int value
3
Conversely, typed values can be abstracted.
To abstract a typed value v
of type T
is to obtain the abstract value V
whose representation in T
is v
.
The operation is denoted using the abt()
operator, as in: V=abt(v)
.
V
is the abstraction
of v
of type T
.
Abstraction is just an abstract operation (you can't do it); but it is defined nevertheless because it will be used to give the definitions in the rest of this document.
The C++ language defines fundamental types (§3.9.1). The following subsets of the fundamental types are intended to represent numbers:
{signed
char,
signed short
int,
signed int, signed long int}
Can be used to represent general integer
numbers (both negative and positive).
{unsigned
char,
unsigned short
int,
unsigned int, unsigned
long int}
Can be used to represent positive
integer numbers with modulo-arithmetic.
{float,double,long double}
Can be used to represent real numbers.
{{signed
integers},{unsigned integers}, bool, char and wchar_t}
{{integer
types},{floating types}}
The integer types are required to have a binary value representation.
Additionally, the signed/unsigned integer types of the same base type (short
, int
or long
) are required to have
the same value representation, that is:
int i = -3 ; // suppose value representation is: 10011 (sign bit + 4 magnitude bits) unsigned int u = i ; // u is required to have the same 10011 as its value representation.
In other words, the integer types signed/unsigned X use the same value representation but a different interpretation of it; that is, their typed values might differ.
Another consequence of this is that the range for signed X is always a smaller subset of the range of unsigned X, as required by §3.9.1/3.
Note | |
---|---|
Always remember that unsigned types, unlike signed types, have modulo-arithmetic; that is, they do not overflow. This means that: - Always be extra careful when mixing signed/unsigned types - Use unsigned types only when you need modulo arithmetic or very very large numbers. Don't use unsigned types just because you intend to deal with positive values only (you can do this with signed types as well). |
This section introduces the following definitions intended to integrate arithmetic types with user-defined types which behave like numbers. Some definitions are purposely broad in order to include a vast variety of user-defined number types.
Within this library, the term number refers to an abstract numeric value.
A type is numeric if:
A numeric type is signed if the abstract values it represent include negative numbers.
A numeric type is unsigned if the abstract values it represent exclude negative numbers.
A numeric type is modulo if it has modulo-arithmetic (does not overflow).
A numeric type is integer if the abstract values it represent are whole numbers.
A numeric type is floating if the abstract values it represent are real numbers.
An arithmetic value is the typed value of an arithmetic type
A numeric value is the typed value of a numeric type
These definitions simply generalize the standard notions of arithmetic types and values by introducing a superset called numeric. All arithmetic types and values are numeric types and values, but not vice versa, since user-defined numeric types are not arithmetic types.
The following examples clarify the differences between arithmetic and numeric types (and values):
// A numeric type which is not an arithmetic type (is user-defined) // and which is intended to represent integer numbers (i.e., an 'integer' numeric type) class MyInt { MyInt ( long long v ) ; long long to_builtin(); } ; namespace std { template<> numeric_limits<MyInt> { ... } ; } // A 'floating' numeric type (double) which is also an arithmetic type (built-in), // with a float numeric value. double pi = M_PI ; // A 'floating' numeric type with a whole numeric value. // NOTE: numeric values are typed valued, hence, they are, for instance, // integer or floating, despite the value itself being whole or including // a fractional part. double two = 2.0 ; // An integer numeric type with an integer numeric value. MyInt i(1234);
Given a number set N
, some
of its elements are representable in a numeric type T
.
The set of representable values of type T
,
or numeric set of T
, is a
set of numeric values whose elements are the representation of some subset
of N
.
For example, the interval of int
values [INT_MIN,INT_MAX]
is the set of representable values of type
int
, i.e. the int
numeric set, and corresponds to the representation
of the elements of the interval of abstract values [abt(INT_MIN),abt(INT_MAX)]
from the integer numbers.
Similarly, the interval of double
values [-DBL_MAX,DBL_MAX]
is the double
numeric set, which corresponds to the subset of the real numbers from abt(-DBL_MAX)
to abt(DBL_MAX)
.
Let next(x)
denote the lowest numeric value greater than x.
Let prev(x)
denote the highest numeric value lower then x.
Let v=prev(next(V))
and v=next(prev(V))
be identities that relate a numeric typed value v
with a number V
.
An ordered pair of numeric values x
,y
s.t. x<y
are
consecutive iff next(x)==y
.
The abstract distance between consecutive numeric values is usually referred
to as a Unit in the Last Place, or
ulp for short. A ulp is a quantity whose
abstract magnitude is relative to the numeric values it corresponds to: If
the numeric set is not evenly distributed, that is, if the abstract distance
between consecutive numeric values varies along the set -as is the case with
the floating-point types-, the magnitude of 1ulp after the numeric value
x
might be (usually is) different
from the magnitude of a 1ulp after the numeric value y for x!=y
.
Since numbers are inherently ordered, a numeric set
of type T
is an ordered sequence
of numeric values (of type T
)
of the form:
REP(T)={l,next(l),next(next(l)),...,prev(prev(h)),prev(h),h}
where l
and h
are respectively the lowest and highest
values of type T
, called
the boundary values of type T
.
A numeric set is discrete. It has a size
which is the number of numeric values in the set, a width
which is the abstract difference between the highest and lowest boundary
values: [abt(h)-abt(l)]
, and a density
which is the relation between its size and width: density=size/width
.
The integer types have density 1, which means that there are no unrepresentable
integer numbers between abt(l)
and abt(h)
(i.e.
there are no gaps). On the other hand, floating types have density much smaller
than 1, which means that there are real numbers unrepresented between consecutive
floating values (i.e. there are gaps).
The interval of abstract values [abt(l),abt(h)]
is the range of the type T
,
denoted R(T)
.
A range is a set of abstract values and not a set of numeric values. In other
documents, such as the C++ standard, the word range
is sometimes used as synonym for numeric
set
, that is, as the ordered sequence
of numeric values from l
to h
. In this document, however,
a range is an abstract interval which subtends the numeric set.
For example, the sequence [-DBL_MAX,DBL_MAX]
is the numeric set of the type double
,
and the real interval [abt(-DBL_MAX),abt(DBL_MAX)]
is its range.
Notice, for instance, that the range of a floating-point type is continuous unlike its numeric set.
This definition was chosen because:
This definition allows for a concise definition of subranged
as given in the last section.
The width of a numeric set, as defined, is exactly equivalent to the width of a range.
The precision of a type is given by the width or density of the numeric set.
For integer types, which have density 1, the precision is conceptually equivalent to the range and is determined by the number of bits used in the value representation: The higher the number of bits the bigger the size of the numeric set, the wider the range, and the higher the precision.
For floating types, which have density <<1, the precision is given not by the width of the range but by the density. In a typical implementation, the range is determined by the number of bits used in the exponent, and the precision by the number of bits used in the mantissa (giving the maximum number of significant digits that can be exactly represented). The higher the number of exponent bits the wider the range, while the higher the number of mantissa bits, the higher the precision.
Given an abstract value V
and a type T
with its corresponding
range [abt(l),abt(h)]
:
If V <
abt(l)
or
V >
abt(h)
, V
is not representable
(cannot be represented) in the type T
,
or, equivalently, it's representation in the type T
is out of range, or overflows.
V <
abt(l)
,
the overflow is negative.
V >
abt(h)
,
the overflow is positive.
If V >=
abt(l)
and
V <=
abt(h)
, V
is representable
(can be represented) in the type T
,
or, equivalently, its representation in the type T
is in range, or does
not overflow.
Notice that a numeric type, such as a C++ unsigned type, can define that
any V
does not overflow by
always representing not V
itself but the abstract value U
= [ V % (abt(h)+1)
]
, which is always in range.
Given an abstract value V
represented in the type T
as v
, the roundoff
error of the representation is the abstract difference: (abt(v)-V)
.
Notice that a representation is an operation, hence, the roundoff error corresponds to the representation operation and not to the numeric value itself (i.e. numeric values do not have any error themselves)
V
is exactly representable
in the type T
.
V
is inexactly representable
in the type T
.
If a representation v
in
a type T
-either exact or
inexact-, is any of the adjacents of V
in that type, that is, if v==prev
or v==next
, the representation is faithfully
rounded. If the choice between prev
and next
matches a given
rounding direction, it is correctly
rounded.
All exact representations are correctly rounded, but not all inexact representations
are. In particular, C++ requires numeric conversions (described below) and
the result of arithmetic operations (not covered by this document) to be
correctly rounded, but batch operations propagate roundoff, thus final results
are usually incorrectly rounded, that is, the numeric value r
which is the computed result is neither
of the adjacents of the abstract value R
which is the theoretical result.
Because a correctly rounded representation is always one of adjacents of the abstract value being represented, the roundoff is guaranteed to be at most 1ulp.
The following examples summarize the given definitions. Consider:
Int
representing
integer numbers with a numeric set: {-2,-1,0,1,2}
and
range: [-2,2]
Cardinal
representing integer numbers with a numeric set:
{0,1,2,3,4,5,6,7,8,9}
and range: [0,9]
(no
modulo-arithmetic here)
Real
representing
real numbers with a numeric set: {-2.0,-1.5,-1.0,-0.5,-0.0,+0.0,+0.5,+1.0,+1.5,+2.0}
and
range: [-2.0,+2.0]
Whole
representing real numbers with a numeric set: {-2.0,-1.0,0.0,+1.0,+2.0}
and
range: [-2.0,+2.0]
First, notice that the types Real
and Whole
both represent
real numbers, have the same range, but different precision.
1
(an
abstract value) can be exactly represented in any of these types.
-1
can be exactly represented in Int
,
Real
and Whole
, but cannot be represented in
Cardinal
, yielding negative
overflow.
1.5
can be
exactly represented in Real
,
and inexactly represented in the other types.
1.5
is represented as
either 1
or 2
in any of the types (except Real
), the representation is correctly
rounded.
0.5
is represented as
+1.5
in the type Real
, it
is incorrectly rounded.
(-2.0,-1.5)
are the Real
adjacents
of any real number in the interval [-2.0,-1.5]
, yet there are no Real
adjacents for x <
-2.0
,
nor for x >
+2.0
.
The C++ language defines Standard Conversions (§4) some of which are conversions between arithmetic types.
These are Integral promotions (§4.5), Integral conversions (§4.7), Floating point promotions (§4.6), Floating point conversions (§4.8) and Floating-integral conversions (§4.9).
In the sequel, integral and floating point promotions are called arithmetic promotions, and these plus integral, floating-point and floating-integral conversions are called arithmetic conversions (i.e, promotions are conversions).
Promotions, both Integral and Floating point, are value-preserving, which means that the typed value is not changed with the conversion.
In the sequel, consider a source typed value s
of type S
, the source abstract
value N=abt(s)
, a destination type T
;
and whenever possible, a result typed value t
of type T
.
Integer to integer conversions are always defined:
T
is unsigned, the
abstract value which is effectively represented is not N
but M=[ N % ( abt(h) + 1
) ]
,
where h
is the highest
unsigned typed value of type T
.
T
is signed and N
is not directly representable, the
result t
is implementation-defined, which means that
the C++ implementation is required to produce a value t
even if it is totally unrelated to s
.
Floating to Floating conversions are defined only if N
is representable; if it is not, the conversion has undefined
behavior.
N
is exactly representable,
t
is required to be the
exact representation.
N
is inexactly representable,
t
is required to be one
of the two adjacents, with an implementation-defined choice of rounding
direction; that is, the conversion is required to be correctly rounded.
Floating to Integer conversions represent not N
but M=trunc(N)
, were
trunc()
is to truncate: i.e. to remove the fractional part, if any.
M
is not representable
in T
, the conversion
has undefined behavior (unless
T
is bool
,
see §4.12).
Integer to Floating conversions are always defined.
N
is exactly representable,
t
is required to be the
exact representation.
N
is inexactly representable,
t
is required to be one
of the two adjacents, with an implementation-defined choice of rounding
direction; that is, the conversion is required to be correctly rounded.
Given a source type S
and
a destination type T
, there
is a conversion direction denoted: S->T
.
For any two ranges the following range relation can
be defined: A range X
can
be entirely contained in a range Y
,
in which case it is said that X
is enclosed by Y
.
Formally:
R(S)
is enclosed byR(T)
iif(R(S) intersection R(T)) == R(S)
.
If the source type range, R(S)
,
is not enclosed in the target type range, R(T)
;
that is, if (R(S)
& R(T))
!= R(S)
,
the conversion direction is said to be subranged,
which means that R(S)
is not
entirely contained in R(T)
and
therefore there is some portion of the source range which falls outside the
target range. In other words, if a conversion direction S->T
is subranged, there are values in S
which cannot be represented in T
because they are out of range. Notice that for S->T
,
the adjective subranged applies to T
.
Examples:
Given the following numeric types all representing real numbers:
X
with numeric set {-2.0,-1.0,0.0,+1.0,+2.0}
and
range [-2.0,+2.0]
Y
with numeric set {-2.0,-1.5,-1.0,-0.5,0.0,+0.5,+1.0,+1.5,+2.0}
and range [-2.0,+2.0]
Z
with numeric set {-1.0,0.0,+1.0}
and range [-1.0,+1.0]
For:
R(X) & R(Y) == R(X)
,
then X->Y
is not subranged. Thus, all values
of type X
are representable
in the type Y
.
R(Y) & R(X) == R(Y)
,
then Y->X
is not subranged. Thus, all values
of type Y
are representable
in the type X
, but
in this case, some values are inexactly representable
(all the halves). (note: it is to permit this case that a range is
an interval of abstract values and not an interval of typed values)
R(X) & R(Z) != R(X)
,
then X->Z
is subranged. Thus, some values
of type X
are not representable
in the type Z
, they
fall out of range (-2.0
and +2.0)
.
It is possible that R(S)
is not
enclosed by R(T)
, while
neither is R(T)
enclosed
by R(S)
; for
example, UNSIG=[0,255]
is not enclosed by SIG=[-128,127]
; neither
is SIG
enclosed by UNSIG
. This implies that is possible that
a conversion direction is subranged both ways. This occurs when a mixture
of signed/unsigned types are involved and indicates that in both directions
there are values which can fall out of range.
Given the range relation (subranged or not) of a conversion direction S->T
, it is possible to classify S
and T
as supertype and subtype:
If the conversion is subranged, which means that T
cannot represent all possible values of type S
,
S
is the supertype and T
the subtype; otherwise, T
is the supertype and S
the subtype.
For example:
R(float)=[-FLT_MAX,FLT_MAX]
andR(double)=[-DBL_MAX,DBL_MAX]
If FLT_MAX <
DBL_MAX
:
double->float
is subranged and supertype=double
,
subtype=float
.
float->double
is not subranged and supertype=double
, subtype=float
.
Notice that while double->float
is subranged, float->double
is not, which yields the same supertype,subtype for both directions.
Now consider:
R(int)=[INT_MIN,INT_MAX]
andR(unsigned int)=[0,UINT_MAX]
A C++ implementation is required to have UINT_MAX
> INT_MAX
(§3.9/3), so:
supertype=int
, subtype=unsigned
.
supertype=unsigned
,
subtype=int
.
In this case, the conversion is subranged in both directions and the supertype,subtype pairs are not invariant (under inversion of direction). This indicates that none of the types can represent all the values of the other.
When the supertype is the same for both S->T
and T->S
, it is effectively indicating a type
which can represent all the values of the subtype. Consequently, if a conversion
X->Y
is not subranged, but the opposite (Y->X)
is,
so that the supertype is always Y
,
it is said that the direction X->Y
is correctly rounded value preserving, meaning
that all such conversions are guaranteed to produce results in range and
correctly rounded (even if inexact). For example, all integer to floating
conversions are correctly rounded value preserving.