Boost
C++ Libraries
...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Boost
C++ Libraries
...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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 usually won't 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, the syntax is not allowed for a class that has an explicitly declared
constructor. (But watch out for an upcoming C++ language change,
by Bjarne Stroustrup et al [1]!)
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 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 [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 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.
The C++ standard [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 [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 §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 ()
The first Technical Corrigendum for the C++ Standard (TC1), whose draft was released to the public in November 2001, introduced Core Issue 178 (among many other issues, of course).
That issue introduced the new concept of value-initialization
(it 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 won't 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 intializer 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
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 doesn't 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.
At the moment of writing, May 2010, the following reported issues regarding value-initialization are still there in current compiler releases:
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
compiler defect macro BOOST_NO_COMPLETE_VALUE_INITIALIZATION.
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 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 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
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 didn't 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.
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
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.
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 ;
var of any DefaultConstructible type
T to be value-initialized by doing T var = {}.
The papers are listed at Bjarne's web page,
My C++ Standards committee papers 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 Fernando Cacciola, the latest version of this file can be found at www.boost.org.
Revised 30 May 2010
© Copyright Fernando Cacciola, 2002 - 2010.
Distributed under the Boost Software License, Version 1.0. See www.boost.org/LICENSE_1_0.txt