Omar Lakkis wrote:
How can I define a static member of a class? not a static method,
rather a static field that would be accessed by all instances of the
class.
To define a static field in an Object class (the R.oo package), I
recommend you to use a private field <.field> and then create a virtual
field <field> by defining methods get<Field>() and set<Field(). These
methods should get and set the field <.field> of the _static_ instance
of the class. Below is an example, which also modifies the static field
already in the constructor something which otherwise may be tricky. I
will add this example to the help pages of R.oo in the next release.
Hope it helps
Henrik Bengtsson (author of R.oo)
######################################################################
# Example illustrating how to "emulate" static fields using virtual
# fields, i.e. get- and set-methods. Here we use a private static
# field '.count' of the static class instance 'MyClass', i.e.
# MyClass$.count. Then we define a virtual field 'count' via method
# getCount() to access this static field. This will make all queries
# for 'count' of any object to use the static field instead. In the
# same way is assignment controlled via the setCount() method. A
# side effect of this way of coding is that all MyClass instances will
# also have the private field '.count' (set to zero except for the
# static field that is).
######################################################################
library(R.oo)
setConstructorS3("MyClass", function(...) {
# Create an instance (the static class instance included)
this <- extend(Object(), "MyClass",
.count = 0
)
# In order for a static field to be updated in the
# constructor it has to be done after extend().
this$count <- this$count + 1;
# Return the object
this;
})
setMethodS3("as.character", "MyClass", function(this, ...) {
paste(class(this)[1], ": Number of instances: ", this$count, sep="");
})
# Get virtual field 'count', e.g. obj$count.
setMethodS3("getCount", "MyClass", function(this, ...) {
MyClass$.count;
})
# Set virtual field 'count', e.g. obj$count <- value.
setMethodS3("setCount", "MyClass", function(this, value, ...) {
MyClass$.count <- value;
})
# Create four instances of class 'MyClass'
obj <- lapply(1:4, MyClass)
print(obj)
print(MyClass$count)
print(obj[[1]]$count)
######################################################################
# Example illustrating how to "emulate" static fields using virtual
# fields, i.e. get- and set-methods. Here we use a private static
# field '.count' of the static class instance 'MyClass', i.e.
# MyClass$.count. Then we define a virtual field 'count' via method
# getCount() to access this static field. This will make all queries
# for 'count' of any object to use the static field instead. In the
# same way is assignment controlled via the setCount() method. A
# side effect of this way of coding is that all MyClass instances will
# also have the private field '.count' (set to zero except for the
# static field that is).
######################################################################
library(R.oo)
setConstructorS3("MyClass", function(...) {
# Create an instance (the static class instance included)
this <- extend(Object(), "MyClass",
.count = 0
)
# In order for a static field to be updated in the
# constructor it has to be done after extend().
this$count <- this$count + 1;
# Return the object
this;
})
setMethodS3("as.character", "MyClass", function(this, ...) {
paste(class(this)[1], ": Number of instances: ", this$count, sep="");
})
# Get virtual field 'count', e.g. obj$count.
setMethodS3("getCount", "MyClass", function(this, ...) {
MyClass$.count;
})
# Set virtual field 'count', e.g. obj$count <- value.
setMethodS3("setCount", "MyClass", function(this, value, ...) {
MyClass$.count <- value;
})
# Create four instances of class 'MyClass'
obj <- lapply(1:4, MyClass)
print(obj)
print(MyClass$count)
print(obj[[1]]$count)
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