Prof. Harley Flanders wrote:
Suppose I have defined
type Vec = array[1..1000] of Extended; // 10,000 bytes
Not always 10,000 bytes, it depends on how many bytes an Extended has, e.g. on some platforms 8 bytes, like a double, or 16 bytes like a longdouble.
var A, B, C, D: Vec;
function Sum(const A, B: Vec): Vec;
In the assignment
D := Sum(Sum(A, B), C);
there is an intermediate result Sum(A, B), going to an unnamed temporary vector.
The intermediate vector will be placed on the stack.
Does garbage collection pick up that space?
The space for the temporary vector is automatically freed, but the term "garbage collection" http://en.wikipedia.org/wiki/Garbage_collection_(computer_science) usually refers to automatic heap management methods, not to call stack unwinding http://en.wikipedia.org/wiki/Call_stack.
Similar question with
operator + (const A, B: Vec) C: Vec;
D := (A+B)+C;
More complicated: Delphi mode, so I have "Result" available
var Zero: Vec; // Zero[J] = 0 for all J
function Foo(A: Vec; N: Word): Vec;
var B: Vec;
begin if N = 0 then Result := Zero else if Odd(N) then Result := A + Foo(A, N - 1) else begin B := (Foo(A, N div 2) Result := B + B; end; end; // Foo
Garbage collection handles this correctly?
See the above answer.
Whatever the answers, how do I PROVE them by measuring stack, heap, or whatever?
You don't need to worry about deallocation of temporary space for function results. However, ask yourself if there is enough stack space to put "big" function results on the stack, especially if you are also using recursion. That depends. The alternative is to use a VAR parameter for the result of the calculation (but be aware that a local variable will also be allocated on the call stack).
Regards,
Adriaan van Os