Understanding The Stack

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Summary

Understanding the stack

One of the most important concepts to understand in order to embed squirrel,is 'the virtual machine stack'.

The majority of Squirrel's API functions exchanges values with C/C++ through a stack of squirrel objects.

Squirrel objects are strings, integers, floats, tables, arrays etc...

To better understand take a look at the following code C

        sq_pushfloat(v,1.5f); //pushes a float
        sq_pushroottable(v); //pushes the root table
        sq_pushstring(v,"I'm a string",-1); //pushed a string

and immagine that the result in Squirrel's stack would be something like this table

positive index	negative index	object
3(top)	-1	"I'm a string"
2	-2	root table
1	-3	1.5

Using the stack

After a virtual machine is created(calling *sq_open()) the stack is empty.

positive index

negative index

object

let's assume we want to create a global variable called test and set its value to 123(integer). This is done through the API function sq_createslot.

from the reference manual sq_createslot

pops a key and a value from the stack and performs a set operation on the table or class that is at position idx in the stack, if the slot does not exits it will be created.

        sq_pushroottable(v);
        sq_pushstring(v,"test",-1);
        sq_pushinteger(v,123);
        sq_createslot(v,-3);

this is the equivalent of the following squirrel code

        test <- 123;

step by step

given an empty stack

positive index

negative index

object

        sq_pushroottable(v);

pushes the root table that is the target object where we want to create the new slot

positive index	negative index	object
1	-1	root table

        sq_pushstring(v,"test",-1);

pushes the name of the slot we want to create

positive index	negative index	object
2	-1	"test"
1	-2	root table

        sq_pushinteger(v,123);

pushes the value of the slot we want to create

positive index	negative index	object
3	-1	123
2	-2	"test"
1	-3	root table

        sq_createslot(v,-3);

pops the value and the key and sets the object at position -3 (from the top of the stack) that is the root table we pushed.

positive index	negative index	object
1	-1	root table

as you can notice in the tabular representation of the stack, there are 2 sets of indexes;positive and negative. All API function that expect a stack index as parameter can be used by passing an absolute positive index or a negative index from the top of the stack. 0 is an invalid index.

It can sound wierd, but the negative index is the most useful because allows to write context independent code.

TO BE CONTINUED