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Header <boost/utility/value_init.hpp>

Contents

Rationale
Introduction
Details
Types and objects
Acknowledgements


Rationale

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.

Introduction

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.

Details

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 ()

value-initialization

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 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 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.

compiler issues

Various compilers haven't 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's 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:

Note that all known GCC issues regarding value-initialization are fixed with GCC version 4.4, including GCC Bug 30111. Clang also has completely implemented value-initialization, as far as we know, now that 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 compiler defect macro BOOST_NO_COMPLETE_VALUE_INITIALIZATION.

Types and objects

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<T>& );

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() ;
}

} // 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

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 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.

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

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( value_initialized<T>& );

private :
unspecified x ;
} ;

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

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

} // 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 ;

References

[1] Bjarne Stroustrup, Gabriel Dos Reis, and J. Stephen Adamczyk wrote various papers, proposing to extend the support for brace-enclosed initializer lists in the next version of C++. 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, My C++ Standards committee papers
[2] Scott Meyers, Effective C++, Third Edition, item 6, Explicitly disallow the use of compiler-generated functions you do not want, Scott Meyers: Books and CDs
[3] The C++ Standard, Second edition (2003), ISO/IEC 14882:2003
[4] POD stands for "Plain Old Data"

Acknowledgements

value_initialized was developed by Fernando Cacciola, with help and suggestions from David Abrahams and Darin Adler.
Special thanks to Björn 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 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