Re: couldn't understand the below section from TC++PL
On Feb 10, 8:23 pm, "Bo Persson" <b...@gmb.dk> wrote:
[...]
Before using cout or cin, you must include <iostream> which
should take care of the initialization. Perhaps it uses a
special pragma, or perhaps it uses the Init class presented in
the book. Or some linker magic. We don't have to know!
Or perhaps it doesn't do anything. The standard doesn't require
anything here.
The next version of the standard will require that <iostream>
define an instance of std::ios::Init, ensuring that if a file
includes <iostream>, the standard stream objects will have been
constructed before any objects with namespace scope which follow
the include in the same file. But this doesn't really buy you
much:
User.cc
#include "SomeClass.hh"
SomeClass hasStaticLifetime ;
// ...
SomeClass.cc
#include <iostream>
SomeClass::SomeClass()
{
std::cout << ... ;
}
The include of <iostream> in SomeClass.cc will guarantee that
std::cout is constructed before any objects with namespace scope
in SomeClass.cc, but it makes no guarantees with respect to
objects in User.cc
Note too that the guarantee only applies to objects at namespace
scope. If you're using the singleton idiom, for example, the
actual "global" object will be at block scope or allocated with
new, and once again, you have no guarantee.
In sum, anytime you use one of the standard streams in a
constructor, you should also define an instance of
std::ios_base::Init, e.g.:
SomeClass::SomeClass()
{
static std::ios_base::Init dummyToControlInitialization ;
std::cout << ...
}
Similarly, the standard guarantees that the standard streams are
*never* destructed; the final decrementation in
std::ios_base::Init::~Init() only flushes them. But since the
same considerations that affect order of initialization affect
order of destruction, you should take care to flush the stream
anytime you output in a destructor. (This is usually taken care
of implicitly, by std::endl. And you should be using std::endl
systematically, unless you're sure that more output will
immediately follow. Which can't be the case for the final
output in a destructor.)
(This *should* be standard practice, and it *should* be
explained in any beginner's text. A lot of things that should
be aren't, however.)
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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