Re: Questions about default constructed STL container elements
On Jun 18, 6:43 pm, "Niels Dekker - no return address"
<nore...@this.is.invalid> wrote:
The Working Draft of the C++ Standard
(www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2606.pdf) specifies for a
few container constructors and resize member functions that they will provide
"default constructed elements". For example, about one of the constructors of
std::vector [vector.cons]:
explicit vector(size_type n);
Effects: Constructs a vector with n default constructed elements.
A default constructed element is default-initialized, right? (According to
the definition of default-initialization, [dcl.init].) So a default
constructed int is zero, right?
No, it's the other way around: a default-initialized non-POD class
object is default-constructed - but a default-constructed object is
not one that was necessarily default-initialized. In particular,
objects created inside a Standard Library container are default-
constructed - but they are but not default- (or value- or zero-)
initialized.
While a default constructed aggregate struct may not be entirely initialized:
struct Foo { int i; std::string s; };
For a default constructed Foo, its int data member will have an indeterminate
value, right?
Yes, the "i" data member will have an indeterminate initial valae for
a default-constructed Foo object (such as one created inside a
std::vector). The "i" data in a default-initialized Foo object (such
as one in a constructor-initializer list) - on the other hand - will
have an initial value of zero. It's important to remember here that a
default-initialized POD class object - is not default-constructed.
Somehow I thought that the Standard would state that those STL container
elements would be value-initialized, but apparently I was wrong...
No, only declared objects can be value-, default- or zero-initialized.
In other words, these kinds of object initialization require a
specific "syntactical context." In this case, the user program does
not declare the objects that a std::vector's constructor creates
inside the vector (nor does the std::vector implementation itself have
any obligation to declare them itself), In fact, a std::vector
constructor simply default-constructs the elements that need to be
placed in the container - and does so without initializing those
elements in any of the ways described in.?8.5 (Initializers) of the C+
+ Standard.
Greg
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