utility/doc/value_init.qbk

[/
 / Copyright (c) 2012 Marshall Clow
 / Copyright (c) 2021, Alan Freitas
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[/===============]
[#sec:value_init]
[section Value Init]

[/===============]

[section Introduction]

Constructing and initializing objects in a generic way is difficult in
C++. The problem is that there are several different rules that apply
for initialization. Depending on the type, the value of a newly constructed
object can be zero-initialized (logically 0), default-constructed (using
the default constructor), or indeterminate. When writing generic code,
this problem must be addressed. The template __value_initialized__ provides
a solution with consistent syntax for value initialization of scalar,
union and class types. Moreover, __value_initialized__ offers a workaround to various
compiler issues regarding value-initialization.

Furthermore, a `const` object __initialized_value__ is provided,
to avoid repeating the type name when retrieving the value from a
`__value_initialized__<T>` object.

There are various ways to initialize a variable, in C++. The following
declarations all ['may] have a local variable initialized to its default
value:

```
T1 var1;
T2 var2 = 0;
T3 var3 = {};
T4 var4 = T4();
```

Unfortunately, whether or not any of those declarations correctly
initialize the variable very much depends on its type. The first
declaration is valid for any __DefaultConstructible__ type by definition.

However, it does not always do an initialization. It correctly initializes
the variable when it's an instance of a class, and the author of the class
has provided a proper default constructor. On the other hand, the value of
`var1` is ['indeterminate] when its type is an arithmetic type, like `int`,
`float`, or `char`.

An arithmetic variable is of course initialized properly by the second declaration,
`T2 var2 = 0`. But this initialization form will not usually work for a
class type, unless the class was especially written to support being
initialized that way.

The third form, `T3 var3 = {}`, initializes an aggregate, typically a "C-style"
`struct` or a "C-style" array. However, at the time this library was developed,
the syntax did not allow for a class that has an explicitly declared constructor.

The fourth form is the most generic form of them, as it can be used to initialize
arithmetic types, class types, aggregates, pointers, and other types.
The declaration,  `T4 var4 = T4()`, should be read as follows: First a temporary
object is created, by `T4()`. This object is [link sec:valueinit value-initialized].
Next the temporary object is copied to the named variable, `var4`. Afterwards,
the temporary is destroyed. While the copying and the destruction are likely to
be optimized away, C++ still requires the type `T4` to be __CopyConstructible__.
So `T4` needs to be ['both] __DefaultConstructible__ ['and] __CopyConstructible__.

A class may not be CopyConstructible, for example because it may have a
private and undefined copy constructor, or because it may be derived from
`boost::noncopyable`. Scott Meyers \[[link sec:references 2]\] explains why a
class would be defined like that.

There is another, less obvious disadvantage to the fourth form, `T4 var4 = T4()`:
It suffers from various [link sec:compiler_issues compiler issues], causing
a variable to be left uninitialized in some compiler specific cases.

The template __value_initialized__ offers a generic way to initialize
an object, like `T4 var4 = T4()`, but without requiring its type
to be __CopyConstructible__. And it offers a workaround to those compiler issues
regarding value-initialization as well. It allows getting an initialized
variable of any type; it ['only] requires the type to be __DefaultConstructible__.
A properly ['value-initialized] object of type `T` is constructed by the following
declaration:

```
value_initialized<T> var;
```

The template __initialized__ offers both value-initialization and direct-initialization.
It is especially useful as a data member type, allowing the very same object
to be either direct-initialized or value-initialized.

The `const` object __initialized_value__ allows value-initializing a variable as follows:

```
T var = initialized_value;
```

This form of initialization is semantically equivalent to `T4 var4 = T4()`,
but robust against the aforementioned compiler issues.

[endsect]

[#sec:details]
[section Details]

The C++ standard \[[link sec:references 3]\] contains the definitions
of `zero-initialization` and `default-initialization`. Informally, zero-initialization
means that the object is given the initial value `0` converted to the type and
default-initialization means that POD \[[link sec:references 4]\] types are zero-initialized,
while non-POD class types are initialized with their corresponding default constructors.

A ['declaration] can contain an ['initializer], which specifies the
object's initial value. The initializer can be just '()', which states that
the object shall be value-initialized (but see below). However, if a ['declaration]
has no ['initializer] and it is of a non-`const`, non-`static` POD type, the
initial value is indeterminate: (see &sect;8.5, [dcl.init], for the
accurate definitions).

```
int x; // no initializer. x value is indeterminate.
__std_string__ s; // no initializer, s is default-constructed.

int y = int();
// y is initialized using copy-initialization
// but the temporary uses an empty set of parentheses as the initializer,
// so it is default-constructed.
// A default constructed POD type is zero-initialized,
// therefore, y == 0.

void foo ( __std_string__ ) ;
foo ( __std_string__() ) ;
// the temporary string is default constructed
// as indicated by the initializer ()
```

[#sec:valueinit]
[h5 value-initialization]

The first [@http://www.open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html Technical
Corrigendum for the C++ Standard] (TC1), whose draft was released to the public in
November 2001, introduced [@http://www.open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#178 Core
Issue 178], among many other issues.

That issue introduced the new concept of `value-initialization`, and also fixed the
wording for zero-initialization. Informally, value-initialization is similar to
default-initialization with the exception that in some cases non-static data members
and base class sub-objects are also value-initialized.

The difference is that an object that is value-initialized will not have, or at least
is less likely to have, indeterminate values for data members and base class sub-objects;
unlike the case of an object default constructed (see Core Issue 178 for a
normative description).

In order to specify value-initialization of an object we need to use the
empty-set initializer: `()`.

As before, a declaration with no initializer specifies default-initialization,
and a declaration with a non-empty initializer specifies copy (`=xxx`) or
direct (`xxx`) initialization.

```
template<class T> void eat(T);

int x ; // indeterminate initial value.

__std_string__ s; // default-initialized.

eat ( int() ) ; // value-initialized

eat ( __std_string__() ) ; // value-initialized
```

[#sec:valueinitsyn]
[h5 value-initialization syntax]

Value initialization is specified using `()`. However, the empty set of
parentheses is not permitted by the syntax of initializers because it is
parsed as the declaration of a function taking no arguments:

```
int x() ; // declares function int(*)()
```

Thus, the empty `()` must be put in some other initialization context.

One alternative is to use copy-initialization syntax:

```
int x = int();
```

This works perfectly fine for POD types. But for non-POD class types,
copy-initialization searches for a suitable constructor, which could be,
for instance, the copy-constructor. It also searches for a suitable conversion
sequence but this does not apply in this context.

For an arbitrary unknown type, using this syntax may not have the
value-initialization effect intended because we don't know if a copy
from a default constructed object is exactly the same as a default
constructed object, and the compiler is allowed, in
some cases, but never required to, optimize the copy away.

One possible generic solution is to use value-initialization of a non static
data member:

```
template<class T>
struct W
{
    // value-initialization of 'data' here.
    W() : data() {}

    T data;
};

W<int> w;
// w.data is value-initialized for any type.
```

This is the solution as it was supplied by earlier versions of the
`__value_initialized__<T>` template class. Unfortunately this approach
suffered from various compiler issues.

[#sec:compiler_issues]
[h5 Compiler issues]

Various compilers have not yet fully implemented value-initialization.
So when an object should be ['value-initialized] according to the C++ Standard,
it ['may] in practice still be left uninitialized, because of those
compiler issues. It is hard to make a general statement on what those issues
are like, because they depend on the compiler you are using, its version number,
and the type of object you would like to have value-initialized.

All compilers we have tested so far support value-initialization for arithmetic types properly.
However, various compilers may leave some types of ['aggregates] uninitialized, when they
should be value-initialized. Value-initialization of objects of a pointer-to-member type may also
go wrong on various compilers.

At the moment of writing, May 2010, the following reported issues regarding
value-initialization are still there in current compiler releases:



* [@https://connect.microsoft.com/VisualStudio/feedback/details/100744 Microsoft Visual Studio Feedback ID 100744, Value-initialization in new-expression]: Reported by Pavel Kuznetsov (MetaCommunications Engineering), 2005.
* [@http://connect.microsoft.com/VisualStudio/feedback/details/484295 Microsoft Visual Studio Feedback ID 484295, VC++ does not value-initialize members of derived classes without user-declared constructor] Reported by Sylvester Hesp, 2009.
* [@https://connect.microsoft.com/VisualStudio/feedback/details/499606 Microsoft Visual Studio Feedback ID 499606, Presence of copy constructor breaks member class initialization] Reported by Alex Vakulenko, 2009
* [@http://qc.embarcadero.com/wc/qcmain.aspx?d=83751 Embarcadero/C++Builder Report 83751, Value-initialization: arrays should have each element value-initialized] Reported by Niels Dekker (LKEB), 2010.
* [@http://qc.embarcadero.com/wc/qcmain.aspx?d=83851 Embarcadero/C++Builder Report 83851, Value-initialized temporary triggers internal backend error C1798] Reported by Niels Dekker, 2010.
* [@http://qc.embarcadero.com/wc/qcmain.aspx?d=84279 Embarcadero/C++Builder Report 84279, Internal compiler error (F1004), value-initializing member function pointer by "new T()"] Reported by Niels Dekker, 2010
* Sun CR 6947016, Sun 5.10 may fail to value-initialize an object of a non-POD aggregate. Reported to Steve Clamage by Niels Dekker, 2010.
* IBM's XL V10.1 and V11.1 may fail to value-initialize a temporary of a non-POD aggregate. Reported to Michael Wong by Niels Dekker, 2010.
* Intel support issue 589832, Attempt to value-initialize pointer-to-member triggers internal error on Intel 11.1. Reported by John Maddock, 2010.

Note that all known GCC issues regarding value-initialization are fixed with GCC version 4.4, including
[@http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30111 GCC Bug 30111]. Clang also has completely implemented
value-initialization, as far as we know, now that [@http://llvm.org/bugs/show_bug.cgi?id=7139 Clang Bug 7139]
is fixed.

New versions of __value_initialized__ (Boost release version 1.35 or higher) offer a workaround to these
issues: __value_initialized__ may now clear its internal data, prior to constructing the object that it
contains. It will do so for those compilers that need to have such a workaround, based on the
[@boost:/doc/html/config/doc/html/boost_config/boost_macro_reference.html#boost_config.boost_macro_reference.macros_that_describe_defects
compiler defect macro] `BOOST_NO_COMPLETE_VALUE_INITIALIZATION`.

[endsect]

[#sec:types]
[section Types and objects]

[#sec:val_init]
[section `template class value_initialized<T>`]

```
namespace boost {

template<class T>
class __value_initialized__
{

  public :

    __value_initialized__() : x() {}

    operator T const &() const { return x ; }

    operator T&() { return x ; }

    T const &data() const { return x ; }

    T& data() { return x ; }

    void swap( __value_initialized__& );

  private :

    [unspecified] x ;

} ;

template<class T>

T const& get ( __value_initialized__<T> const& x )
{
  return x.data();
}

template<class T>
T& get ( __value_initialized__<T>& x )
{
  return x.data();
}

template<class T>
void swap ( __value_initialized__<T>& lhs, __value_initialized__<T>& rhs )
{
  lhs.swap(rhs);
}

} // namespace boost
```

An object of this template class is a `T`-wrapper convertible to `'T&'` whose
wrapped object (data member of type `T`) is [link sec:valueinit value-initialized] upon default-initialization
of this wrapper class:

```
int zero = 0;
__value_initialized__<int> x;
assert( x == zero ) ;

__std_string__ def;
__value_initialized__< __std_string__ > y;
assert( y == def ) ;
```

The purpose of this wrapper is to provide a consistent syntax for value initialization
of scalar, union and class types (POD and non-POD) since the correct syntax for value
initialization varies (see [link sec:valueinitsyn value-initialization syntax]).

The wrapped object can be accessed either through the conversion operator
`T&`, the member function `data()`, or the non-member function `get()`:

```
void watch(int);

__value_initialized__<int> x;

watch(x) ; // operator T& used.
watch(x.data());
watch( get(x) ) // function get() used
```

Both `const` and non-`const` objects can be wrapped. Mutable objects can be
modified directly from within the wrapper but constant objects cannot:

When `T` is a __Swappable__ type, `__value_initialized__<T>`
is swappable as well, by calling its `swap` member function
as well as by calling `boost::swap`.

```
__value_initialized__<int> x;
static_cast<int&>(x) = 1 ; // OK
get(x) = 1 ; // OK

__value_initialized__<int const> y ;
static_cast<int&>(y) = 1 ; // ERROR: cannot cast to int&
static_cast<int const&>(y) = 1 ; // ERROR: cannot modify a const value
get(y) = 1 ; // ERROR: cannot modify a const value
```

[warning

The __value_initialized__ implementation of Boost version 1.40.0 and older
allowed ['non-const] access to the wrapped object, from a constant wrapper,
both by its conversion operator and its `data()` member function.

For example:

```
__value_initialized__<int> const x_c;
int& xr = x_c ; // OK, conversion to int& available even though x_c is itself const.
xr = 2 ;
```

The reason for this obscure behavior was that some compilers did not accept the following valid code:

```
struct X
{
  operator int&() ;
    operator int const&() const ;
  };
  X x ;
  (x == 1) ; // ERROR HERE!
```

The current version of __value_initialized__ no longer has this obscure behavior.
As compilers nowadays widely support overloading the conversion operator by having a `const`
and a `non-const` version, we have decided to fix the issue accordingly. So the current version
supports the idea of logical constness.

]

[h5 Recommended practice: The non-member get() idiom]

The obscure behavior of being able to modify a non-`const`
wrapped object from within a constant wrapper (as was supported by previous
versions of __value_initialized__) can be avoided if access to the wrapped object
is always performed with the `get()` idiom:

```
value_initialized<int> x;
get(x) = 1; // OK
value_initialized<int const> cx;
get(x) = 1; // ERROR: Cannot modify a const object

value_initialized<int> const x_c;
get(x_c) = 1; // ERROR: Cannot modify a const object

value_initialized<int const> const cx_c;
get(cx_c) = 1; // ERROR: Cannot modify a const object
```

[endsect]

[#sec:initialized]
[section `template class initialized<T>`]


```
namespace boost {

template<class T>
class __initialized__
{

  public :

    __initialized__() : x() {}

    explicit __initialized__(T const & arg) : x(arg) {}

    operator T const &() const;

    operator T&();

    T const &data() const;

    T& data();

    void swap( __initialized__& );

  private :

    [unspecified] x ;

};

template<class T>
T const& get ( __initialized__<T> const& x );

template<class T>
T& get ( __initialized__<T>& x );

template<class T>
void swap ( __initialized__<T>& lhs, __initialized__<T>& rhs );

} // namespace boost
```

The template class `boost::__initialized__<T>` supports both value-initialization
and direct-initialization, so its interface is a superset of the interface
of `__value_initialized__<T>`: Its default-constructor value-initializes the
wrapped object just like the default-constructor of `__value_initialized__<T>`,
but `boost::__initialized__<T>` also offers an extra `explicit`
constructor, which direct-initializes the wrapped object by the specified value.

`__initialized__<T>` is especially useful when the wrapped
object must be either value-initialized or direct-initialized, depending on
runtime conditions. For example, `__initialized__<T>` could
hold the value of a data member that may be value-initialized by some
constructors, and direct-initialized by others.

On the other hand, if it is known beforehand that the
object must ['always] be value-initialized, `__value_initialized__<T>`
may be preferable. And if the object must always be
direct-initialized, none of the two wrappers really needs to be used.

[endsect]

[#sec:initialized_value]
[section `initialized_value`]

```
namespace boost {
class __initialized_value_t__
{
  public :
    template <class T> operator T() const ;
};

__initialized_value_t__ const initialized_value = {} ;

} // namespace boost
```

__initialized_value__ provides a convenient way to get
an initialized value: its conversion operator provides an appropriate
['value-initialized] object for any __CopyConstructible__ type.

Suppose you need to have an initialized variable of type `T`.
You could do it as follows:

```
T var = T();
```

But as mentioned before, this form suffers from various compiler issues.
The template __value_initialized__ offers a workaround:

```
T var = get( __value_initialized__<T>() );
```

Unfortunately both forms repeat the type name, which
is rather short now (`T`), but could of course be
more like `Namespace::Template<Arg>::Type`.

Instead, one could use __initialized_value__ as follows:

```
T var = __initialized_value__;
```

[endsect]
[endsect]

[#sec:references]
[section References]

# Bjarne Stroustrup, Gabriel Dos Reis, and J. Stephen Adamczyk wrote various papers,
proposing to extend the support for brace-enclosed ['initializer lists]
in C++. This [@https://en.cppreference.com/w/cpp/language/list_initialization feature] has
now been available since C++11. This would allow a variable `var` of any __DefaultConstructible__ type
`T` to be ['value-initialized] by doing `T var = {}`. The papers are listed at Bjarne's web page,
[@http://www.research.att.com/~bs/WG21.html My C++ Standards committee papers].

# Scott Meyers, Effective C++, Third Edition, item 6, ['Explicitly disallow the use of
compiler-generated functions you do not want], [@http://www.aristeia.com/books.html Scott Meyers: Books and CDs]

# The C++ Standard, Second edition (2003), ISO/IEC 14882:2003

# POD stands for "Plain Old Data"

[endsect]

[/===============]
[xinclude tmp/value_init_reference.xml]
[/===============]

[#sec:acknowledgements]
[section Acknowledgements]

__value_initialized__ was developed by Fernando Cacciola, with help and suggestions
from David Abrahams and Darin Adler.

Special thanks to Bjorn Karlsson who carefully edited and completed this documentation.

__value_initialized__ was reimplemented by Fernando Cacciola and Niels Dekker
for Boost release version 1.35 (2008), offering a workaround to various compiler issues.

`boost::__initialized__` was very much inspired by feedback from Edward Diener and Jeffrey Hellrung.

__initialized_value__ was written by Niels Dekker, and added to Boost release version 1.36 (2008).

Developed by [@mailto:fernando_cacciola@hotmail.com Fernando Cacciola]. The latest version of
this file can be found at [@http://www.boost.org www.boost.org].

[endsect]

[endsect]