Header <boost/utility/value_init.hpp>

Contents

Rationale
Introduction
Details
Types
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.

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;

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. Compilers usually support value-initialization for built-in types properly. But objects of user-defined types that involve aggregates may in some cases be partially, or even entirely left uninitialized, when they should be value-initialized.

We have encountered issues regarding value-initialization on compilers by Microsoft, Sun, Borland, and GNU. Here is a list of bug reports on those issues:
Microsoft Feedback ID 100744 - Value-initialization in new-expression
Reported by Pavel Kuznetsov (MetaCommunications Engineering), 2005-07-28
GCC Bug 30111 - Value-initialization of POD base class doesn't initialize members
Reported by Jonathan Wakely, 2006-12-07
GCC Bug 33916 - Default constructor fails to initialize array members
Reported by Michael Elizabeth Chastain, 2007-10-26
Borland Report 51854 - Value-initialization: POD struct should be zero-initialized
Reported by Niels Dekker (LKEB, Leiden University Medical Center), 2007-09-11

New versions of value_initialized (Boost release version 1.35 or higher) offer a workaround to these issues: value_initialized will now clear its internal data, prior to constructing the object that it contains.

Types

template class value_initialized<T>

namespace boost {

template<class T>
class value_initialized
{
public :
value_initialized() : x() {}
operator T&() const { return x ; }
T& data() const { return x ; }

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:

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:

Both the conversion operator and the data() member function are const in order to allow access to the wrapped object from a constant wrapper:

void foo(int);
value_initialized<int> const x ;
foo(x);

But notice that this conversion operator is to T& although it is itself const. As a consequence, if T is a non-const type, you can modify the wrapped object even from within a constant wrapper:

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 is that some commonly used compilers just don't accept the following valid code:

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

These compilers complain about ambiguity between the conversion operators. This complaint is incorrect, but the only workaround that I know of is to provide only one of them, which leads to the obscure behavior just explained.

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 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

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.

Developed by Fernando Cacciola, the latest version of this file can be found at www.boost.org.


Revised 15 January 2008

© Copyright Fernando Cacciola, 2002, 2008.

Distributed under the Boost Software License, Version 1.0. See www.boost.org/LICENSE_1_0.txt