Re: Automatic and Dynamic Storage
V wrote:
Hi,
I now that C++ has no concept of stack or heap, but use the more
abstract "automatic storage" and "dynamic storage" definitions
(section 3.7 of the standard). Normally, I think that automatic
storage objects are stored in the stack, and dynamic storage objects
are stored in the heap. Also Herb Sutter, in his "Exceptional C++",
item #35, said that: "the stack stores automatic variables".
Said so, suppose we have those simple classes:
class A {
public:
A() {
std::cout << "A()\n";
}
~A() {
std::cout << "~A()\n";
}
};
and:
class B {
public:
B() {
std::cout << "B()\n";
}
~B() {
std::cout << "~B()\n";
}
private:
A a;
};
(note that B has a data member "a" of type A).
Now, if I write:
B *ptr = new B();
the object of type B pointed by "ptr" has "dynamic storage duration",
or, in other words, is stored into the heap (Herb Sutter's talk of
"free store", but this is not the point of this question).
So far, I agree.
Now, inside
B there is the data member "a", that has "automatic storage duration".
Here I disagree.
"a" is a member of the B object. All members have the same storage
duration as the (larger) object they are a part of. Therefor, the
member "a" also has dynamic storage duration.
But if "a" has automatic storage duration, it's stored into the stack?
In my opinion no: rather, I think that is stored "somewhere else".
"a" is an integral part of the B object, so it is located in the same
memory area.
So, quoting Herb Sutter, we can say that "the stack stores automatic
variables", but "not all automatic variables are stored into the
stack".
Question #1: is my deduction correct?
No. Because your premiss was incorrect, your deduction is moot.
Question #2: if data member "a" is not stored into the stack, where
else can be stored? (Ok, this is an implementation-dependent question,
but I'm interested into the "ideas" behind this.)
This is not entirely implementation dependent. The data member "a" (of
the B object referred to by ptr) is stored somewhere in the memory
block that contains the B object.
At which offset from the start of the memory block this A sub-object is
located is implementation defined, but it must be located somewhere
within the block.
Best regards,
V.
Bart v Ingen Schenau
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