Dynamic Languages and Variable Allocation

Question

How does a dynamic language decide how much memory to allocate for a variable? eg. How does the compiler change variable= 5 to variable ="xxx" without too much memory overhead? When does it use the hardware stack and when does it use the memory heap?

Answer 1

The compiler allocates enough memory for each variable to hold a pointer plus whatever metadata the language runtime requires. But I think you mean to be asking how much memory is allocated for each object. In that case the answer is that it depends on the type of object. When a variable gets assigned to a different object, the pointer associated with that variable changes what it points to.

Answer 2

The answer, of course, varies by language - both the hosted dynamic language and the lower-level implementation language. That which applies to Perl does not necessarily apply to Python, nor does what applies in Tcl apply in Java or LISP or ... well, do they count as dynamic languages.

In Perl, there's a C-level structure that goes by the name SV (scalar variable) that contains different storage for different versions of the variable's value. These often heap-based; the storage for strings always ends up being heap based, though a pure numeric value that has never been converted to string might be in an SV that is strictly on the stack. In Perl, these things are reference counted (and mortalized, or immortalized, and all sorts of other interesting terms). More complicated types (AV, HV, RV, etc) are based on SV.