CREATE ¶Defining words are used to create new entries in the dictionary. The
simplest defining word is CREATE. CREATE is used like
this:
CREATE new-word1
CREATE is a parsing word, i.e., it takes an argument from the
input stream (new-word1 in our example). It generates a
dictionary entry for new-word1. When new-word1 is
executed, all that it does is leave an address on the stack. The address
represents the value of the data space pointer (HERE) at the time
that new-word1 was defined. Therefore, CREATE is a way of
associating a name with the address of a region of memory.
Create ( "name" – ) core “Create”
Note that Standard Forth guarantees only for create that its body
is in dictionary data space (i.e., where here, allot
etc. work, see Dictionary allocation). Also, in Standard Forth only
created words can be modified with does>
(see User-defined Defining Words). And in Standard Forth >body
can only be applied to created words.
By extending this example to reserve some memory in data space, we end up with something like a variable. Here are two different ways to do it:
CREATE new-word2 1 cells allot \ reserve 1 cell - initial value undefined CREATE new-word3 4 , \ reserve 1 cell and initialise it (to 4)
The variable can be examined and modified using @ (“fetch”) and
! (“store”) like this:
new-word2 @ . \ get address, fetch from it and display 1234 new-word2 ! \ new value, get address, store to it
A similar mechanism can be used to create arrays. For example, an 80-character text input buffer:
CREATE text-buf 80 chars allot text-buf 0 chars + c@ \ the 1st character (offset 0) text-buf 3 chars + c@ \ the 4th character (offset 3)
You can build arbitrarily complex data structures by allocating appropriate areas of memory. For further discussions of this, and to learn about some Gforth tools that make it easier, See Structures.