Constructors¶
Constructors are functions that create new objects — specifically, instances of Composite Types. In Julia, type objects also serve as constructor functions: they create new instances of themselves when applied to an argument tuple as a function. This much was already mentioned briefly when composite types were introduced. For example:
type Foo
bar
baz
end
julia> foo = Foo(1,2)
Foo(1,2)
julia> foo.bar
1
julia> foo.baz
2
For many types, forming new objects by binding their field values together is all that is ever needed to create instances. There are, however, cases where more functionality is required when creating composite objects. Sometimes invariants must be enforced, either by checking arguments or by transforming them. Recursive data structures, especially those that may be self-referential, often cannot be constructed cleanly without first being created in an incomplete state and then altered programmatically to be made whole, as a separate step from object creation. Sometimes, it’s just convenient to be able to construct objects with fewer or different types of parameters than they have fields. Julia’s system for object construction addresses all of these cases and more.
Outer Constructor Methods¶
A constructor is just like any other function in Julia in that its
overall behavior is defined by the combined behavior of its methods.
Accordingly, you can add functionality to a constructor by simply
defining new methods. For example, let’s say you want to add a
constructor method for Foo
objects that takes only one argument and
uses the given value for both the bar
and baz
fields. This is
simple:
Foo(x) = Foo(x,x)
julia> Foo(1)
Foo(1,1)
You could also add a zero-argument Foo
constructor method that
supplies default values for both of the bar
and baz
fields:
Foo() = Foo(0)
julia> Foo()
Foo(0,0)
Here the zero-argument constructor method calls the single-argument constructor method, which in turn calls the automatically provided two-argument constructor method. For reasons that will become clear very shortly, additional constructor methods declared as normal methods like this are called outer constructor methods. Outer constructor methods can only ever create a new instance by calling another constructor method, such as the automatically provided default one.
Inner Constructor Methods¶
While outer constructor methods succeed in addressing the problem of providing additional convenience methods for constructing objects, they fail to address the other two use cases mentioned in the introduction of this chapter: enforcing invariants, and allowing construction of self-referential objects. For these problems, one needs inner constructor methods. An inner constructor method is much like an outer constructor method, with two differences:
- It is declared inside the block of a type declaration, rather than outside of it like normal methods.
- It has access to a special locally existent function called
new
that creates objects of the block’s type.
For example, suppose one wants to declare a type that holds a pair of real numbers, subject to the constraint that the first number is not greater than the second one. One could declare it like this:
type OrderedPair
x::Real
y::Real
OrderedPair(x,y) = x > y ? error("out of order") : new(x,y)
end
Now OrderedPair
objects can only be constructed such that
x <= y
:
julia> OrderedPair(1,2)
OrderedPair(1,2)
julia> OrderedPair(2,1)
out of order
in OrderedPair at none:5
You can still reach in and directly change the field values to violate this invariant (support for immutable composites is planned but not yet implemented), but messing around with an object’s internals uninvited is considered poor form. You (or someone else) can also provide additional outer constructor methods at any later point, but once a type is declared, there is no way to add more inner constructor methods. Since outer constructor methods can only create objects by calling other constructor methods, ultimately, some inner constructor must be called to create an object. This guarantees that all objects of the declared type must come into existence by a call to one of the inner constructor methods provided with the type, thereby giving some degree of real enforcement of a type’s invariants, at least for object creation.
If any inner constructor method is defined, no default constructor
method is provided: it is presumed that you have supplied yourself with
all the inner constructors you need. The default constructor is
equivalent to writing your own inner constructor method that takes all
of the object’s fields as parameters (constrained to be of the correct
type, if the corresponding field has a type), and passes them to
new
, returning the resulting object:
type Foo
bar
baz
Foo(bar,baz) = new(bar,baz)
end
This declaration has the same effect as the earlier definition of the
Foo
type without an explicit inner constructor method. The following
two types are equivalent — one with a default constructor, the other
with an explicit constructor:
type T1
x::Int64
end
type T2
x::Int64
T2(x::Int64) = new(x)
end
julia> T1(1)
T1(1)
julia> T2(1)
T2(1)
julia> T1(1.0)
no method T1(Float64,)
in method_missing at /Users/stefan/projects/julia/base/base.jl:58
julia> T2(1.0)
no method T2(Float64,)
in method_missing at /Users/stefan/projects/julia/base/base.jl:58
It is considered good form to provide as few inner constructor methods as possible: only those taking all arguments explicitly and enforcing essential error checking and transformation. Additional convenience constructor methods, supplying default values or auxiliary transformations, should be provided as outer constructors that call the inner constructors to do the heavy lifting. This separation is typically quite natural.
Incomplete Initialization¶
The final problem which has still not been addressed is construction of self-referential objects, or more generally, recursive data structures. Since the fundamental difficulty may not be immediately obvious, let us briefly explain it. Consider the following recursive type declaration:
type SelfReferential
obj::SelfReferential
end
This type may appear innocuous enough, until one considers how to
construct an instance of it. If a
is an instance of
SelfReferential
, then a second instance can be created by the call:
b = SelfReferential(a)
But how does one construct the first instance when no instance exists to
provide as a valid value for its obj
field? The only solution is to
allow creating an incompletely initialized instance of
SelfReferential
with an unassigned obj
field, and using that
incomplete instance as a valid value for the obj
field of another
instance, such as, for example, itself.
To allow for the creation of incompletely initialized objects, Julia
allows the new
function to be called with fewer than the number of
fields that the type has, returning an object with the unspecified
fields uninitialized. The inner constructor method can then use the
incomplete object, finishing its initialization before returning it.
Here, for example, we take another crack at defining the
SelfReferential
type, with a zero-argument inner constructor
returning instances having obj
fields pointing to themselves:
type SelfReferential
obj::SelfReferential
SelfReferential() = (x = new(); x.obj = x)
end
We can verify that this constructor works and constructs objects that are, in fact, self-referential:
x = SelfReferential();
julia> is(x, x)
true
julia> is(x, x.obj)
true
julia> is(x, x.obj.obj)
true
Although it is generally a good idea to return a fully initialized object from an inner constructor, incompletely initialized objects can be returned:
type Incomplete
xx
Incomplete() = new()
end
julia> z = Incomplete();
While you are allowed to create objects with uninitialized fields, any access to an uninitialized field is an immediate error:
julia> z.xx
access to undefined reference
This prevents uninitialized fields from propagating throughout a program
or forcing programmers to continually check for uninitialized fields,
the way they have to check for null
values everywhere in Java: if a
field is uninitialized and it is used in any way, an error is thrown
immediately so no error checking is required. You can also pass
incomplete objects to other functions from inner constructors to
delegate their completion:
type Lazy
xx
Lazy(v) = complete_me(new(), v)
end
As with incomplete objects returned from constructors, if
complete_me
or any of its callees try to access the xx
field of
the Lazy
object before it has been initialized, an error will be
thrown immediately.
Parametric Constructors¶
Parametric types add a few wrinkles to the constructor story. Recall from Parametric Types that, by default, instances of parametric composite types can be constructed either with explicitly given type parameters or with type parameters implied by the types of the arguments given to the constructor. Here are some examples:
type Point{T<:Real}
x::T
y::T
end
## implicit T ##
julia> Point(1,2)
Point(1,2)
julia> Point(1.0,2.5)
Point(1.0,2.5)
julia> Point(1,2.5)
no method Point(Int64,Float64)
in method_missing at /Users/stefan/projects/julia/base/base.jl:58
## explicit T ##
julia> Point{Int64}(1,2)
Point(1,2)
julia> Point{Int64}(1.0,2.5)
no method Point(Float64,Float64)
in method_missing at /Users/stefan/projects/julia/base/base.jl:58
julia> Point{Float64}(1.0,2.5)
Point(1.0,2.5)
julia> Point{Float64}(1,2)
no method Point(Int64,Int64)
in method_missing at /Users/stefan/projects/julia/base/base.jl:58
As you can see, for constructor calls with explicit type parameters, the
arguments must match that specific type: Point{Int64}(1,2)
works,
but Point{Int64}(1.0,2.5)
does not. When the type is implied by the
arguments to the constructor call, as in Point(1,2)
, then the types
of the arguments must agree — otherwise the T
cannot be determined —
but any pair of real arguments with matching type may be given to the
generic Point
constructor.
What’s really going on here is that Point
, Point{Float64}
and
Point{Int64}
are all different constructor functions. In fact,
Point{T}
is a distinct constructor function for each type T
.
Without any explicitly provided inner constructors, the declaration of
the composite type Point{T<:Real}
automatically provides an inner
constructor, Point{T}
, for each possible type T<:Real
, that
behaves just like non-parametric default inner constructors do. It also
provides a single general outer Point
constructor that takes pairs
of real arguments, which must be of the same type. This automatic
provision of constructors is equivalent to the following explicit
declaration:
type Point{T<:Real}
x::T
y::T
Point(x::T, y::T) = new(x,y)
end
Point{T<:Real}(x::T, y::T) = Point{T}(x,y)
Some features of parametric constructor definitions at work here deserve
comment. First, inner constructor declarations always define methods of
Point{T}
rather than methods of the general Point
constructor
function. Since Point
is not a concrete type, it makes no sense for
it to even have inner constructor methods at all. Thus, the inner method
declaration Point(x::T, y::T) = new(x,y)
provides an inner
constructor method for each value of T
. It is thus this method
declaration that defines the behavior of constructor calls with explicit
type parameters like Point{Int64}(1,2)
and
Point{Float64}(1.0,2.0)
. The outer constructor declaration, on the
other hand, defines a method for the general Point
constructor which
only applies to pairs of values of the same real type. This declaration
makes constructor calls without explicit type parameters, like
Point(1,2)
and Point(1.0,2.5)
, work. Since the method
declaration restricts the arguments to being of the same type, calls
like Point(1,2.5)
, with arguments of different types, result in “no
method” errors.
Suppose we wanted to make the constructor call Point(1,2.5)
work by
“promoting” the integer value 1
to the floating-point value 1.0
.
The simplest way to achieve this is to define the following additional
outer constructor method:
Point(x::Int64, y::Float64) = Point(convert(Float64,x),y)
This method uses the convert
function to explicitly convert x
to
Float64
and then delegates construction to the general constructor
for the case where both arguments are Float64
. With this method
definition what was previously a “no method” error now successfully
creates a point of type Point{Float64}
:
julia> Point(1,2.5)
Point(1.0,2.5)
julia> typeof(ans)
Point{Float64}
However, other similar calls still don’t work:
julia> Point(1.5,2)
no method Point(Float64,Int64)
For a much more general way of making all such calls work sensibly, see
Conversion and Promotion. At the risk
of spoiling the suspense, we can reveal here that the all it takes is
the following outer method definition to make all calls to the general
Point
constructor work as one would expect:
Point(x::Real, y::Real) = Point(promote(x,y)...)
The promote
function converts all its arguments to a common type
— in this case Float64
. With this method definition, the Point
constructor promotes its arguments the same way that numeric operators
like +
do, and works for all kinds of real numbers:
julia> Point(1.5,2)
Point(1.5,2.0)
julia> Point(1,1//2)
Point(1//1,1//2)
julia> Point(1.0,1//2)
Point(1.0,0.5)
Thus, while the implicit type parameter constructors provided by default in Julia are fairly strict, it is possible to make them behave in a more relaxed but sensible manner quite easily. Moreover, since constructors can leverage all of the power of the type system, methods, and multiple dispatch, defining sophisticated behavior is typically quite simple.
Case Study: Rational¶
Perhaps the best way to tie all these pieces together is to present a real world example of a parametric composite type and its constructor methods. To that end, here is beginning of rational.jl, which implements Julia’s Rational Numbers:
type Rational{T<:Integer} <: Real
num::T
den::T
function Rational(num::T, den::T)
if num == 0 && den == 0
error("invalid rational: 0//0")
end
g = gcd(den, num)
num = div(num, g)
den = div(den, g)
new(num, den)
end
end
Rational{T<:Integer}(n::T, d::T) = Rational{T}(n,d)
Rational(n::Integer, d::Integer) = Rational(promote(n,d)...)
Rational(n::Integer) = Rational(n,one(n))
//(n::Integer, d::Integer) = Rational(n,d)
//(x::Rational, y::Integer) = x.num // (x.den*y)
//(x::Integer, y::Rational) = (x*y.den) // y.num
//(x::Complex, y::Real) = complex(real(x)//y, imag(x)//y)
//(x::Real, y::Complex) = x*y'//real(y*y')
function //(x::Complex, y::Complex)
xy = x*y'
yy = real(y*y')
complex(real(xy)//yy, imag(xy)//yy)
end
The first line — type Rational{T<:Int} <: Real
— declares that
Rational
takes one type parameter of an integer type, and is itself
a real type. The field declarations num::T
and den::T
indicate
that the data held in a Rational{T}
object are a pair of integers of
type T
, one representing the rational value’s numerator and the
other representing its denominator.
Now things get interesting. Rational
has a single inner constructor
method which checks that both of num
and den
aren’t zero and
ensures that every rational is constructed in “lowest terms” with a
non-negative denominator. This is accomplished by dividing the given
numerator and denominator values by their greatest common divisor,
computed using the gcd
function. Since gcd
returns the greatest
common divisor of its arguments with sign matching the first argument
(den
here), after this division the new value of den
is
guaranteed to be non-negative. Because this is the only inner
constructor for Rational
, we can be certain that Rational
objects are always constructed in this normalized form.
Rational
also provides several outer constructor methods for
convenience. The first is the “standard” general constructor that infers
the type parameter T
from the type of the numerator and denominator
when they have the same type. The second applies when the given
numerator and denominator values have different types: it promotes them
to a common type and then delegates construction to the outer
constructor for arguments of matching type. The third outer constructor
turns integer values into rationals by supplying a value of 1
as the
denominator.
Following the outer constructor definitions, we have a number of methods
for the //
operator, which provides a syntax for writing rationals.
Before these definitions, //
is a completely undefined operator with
only syntax and no meaning. Afterwards, it behaves just as described in
Rational Numbers
— its entire behavior is defined in these few lines. The first and most
basic definition just makes a//b
construct a Rational
by
applying the Rational
constructor to a
and b
when they are
integers. When one of the operands of //
is already a rational
number, we construct a new rational for the resulting ratio slightly
differently; this behavior is actually identical to division of a
rational with an integer. Finally, applying //
to complex integral
values creates an instance of Complex{Rational}
— a complex number
whose real and imaginary parts are rationals:
julia> (1 + 2im)//(1 - 2im)
-3//5 + 4//5im
julia> typeof(ans)
ComplexPair{Rational{Int64}}
julia> ans <: Complex{Rational}
true
Thus, although the //
operator usually returns an instance of
Rational
, if either of its arguments are complex integers, it will
return an instance of Complex{Rational}
instead. The interested
reader should consider perusing the rest of
rational.jl:
it is short, self-contained, and implements an entire basic Julia type
in just a little over a hundred lines of code.