Header boost/utility.hpp Documentation

# Header boost/utility.hpp

The entire contents of the header `<boost/utility.hpp>` are in `namespace boost`.

## Contents

- Class templates supporting the base-from-member idiom
- Function templates checked_delete() and checked_array_delete()
- Function templates next() and prior()
- Class noncopyable
- Function template addressof()
- Class template result_of
- Macro BOOST_BINARY
- Other utilities not part of `utility.hpp`

## Function templates checked_delete() and checked_array_delete()

See separate documentation.

## Function templates next() and prior()

Certain data types, such as the C++ Standard Library's forward and bidirectional iterators, do not provide addition and subtraction via operator+() or operator-(). This means that non-modifying computation of the next or prior value requires a temporary, even though operator++() or operator--() is provided. It also means that writing code like `itr+1` inside a template restricts the iterator category to random access iterators.

The next() and prior() functions provide a simple way around these problems:

>     template <class T>
>     T next(T x) { return ++x; }

>     template <class T, class Distance>
>     T next(T x, Distance n)
>     {
>         std::advance(x, n);
>         return x;
>     }

>     template <class T>
>     T prior(T x) { return --x; }

>     template <class T, class Distance>
>     T prior(T x, Distance n)
>     {
>         std::advance(x, -n);
>         return x;
>     }

Usage is simple:

>     const std::list<T>::iterator p = get_some_iterator();
>     const std::list<T>::iterator prev = boost::prior(p);
>     const std::list<T>::iterator next = boost::next(prev, 2);

The distance from the given iterator should be supplied as an absolute value. For example, the iterator four iterators prior to the given iterator `p` may be obtained by `prior(p, 4)`.

Contributed by Dave Abrahams. Two-argument versions by Daniel Walker.

## Class noncopyable

Class noncopyable is a base class. Derive your own class from noncopyable when you want to prohibit copy construction and copy assignment.

Some objects, particularly those which hold complex resources like files or network connections, have no sensible copy semantics. Sometimes there are possible copy semantics, but these would be of very limited usefulness and be very difficult to implement correctly. Sometimes you're implementing a class that doesn't need to be copied just yet and you don't want to take the time to write the appropriate functions. Deriving from noncopyable will prevent the otherwise implicitly-generated functions (which don't have the proper semantics) from becoming a trap for other programmers.

The traditional way to deal with these is to declare a private copy constructor and copy assignment, and then document why this is done. But deriving from noncopyable is simpler and clearer, and doesn't require additional documentation.

The program noncopyable_test.cpp can be used to verify class noncopyable works as expected. It has have been run successfully under GCC 2.95, Metrowerks CodeWarrior 5.0, and Microsoft Visual C++ 6.0 sp 3.

Contributed by Dave Abrahams.

### Example

>     // inside one of your own headers ...
>     #include <boost/utility.hpp>

>     class ResourceLadenFileSystem : boost::noncopyable {
>     ...

### Rationale

Class noncopyable has protected constructor and destructor members to emphasize that it is to be used only as a base class. Dave Abrahams notes concern about the effect on compiler optimization of adding (even trivial inline) destructor declarations. He says "Probably this concern is misplaced, because noncopyable will be used mostly for classes which own resources and thus have non-trivial destruction semantics."

## Function template addressof()

Function addressof() returns the address of an object.

>     template <typename T> inline T*                addressof(T& v);
>     template <typename T> inline const T*          addressof(const T& v);
>     template <typename T> inline volatile T*       addressof(volatile T& v);
>     template <typename T> inline const volatile T* addressof(const volatile T& v);

C++ allows programmers to replace the unary operator&() class member used to get the address of an object. Getting the real address of an object requires ugly casting tricks to avoid invoking the overloaded operator&(). Function addressof() provides a wrapper around the necessary code to make it easy to get an object's real address.

The program addressof_test.cpp can be used to verify that addressof() works as expected.

Contributed by Brad King based on ideas from discussion with Doug Gregor.

### Example

>     #include <boost/utility.hpp>

>     struct useless_type {};
>     class nonaddressable {
>       useless_type operator&() const;
>     };

>     void f() {
>       nonaddressable x;
>       nonaddressable* xp = boost::addressof(x);
>       // nonaddressable* xpe = &x; /* error */
>     }

## Class template result_of

The class template `result_of` helps determine the type of a call expression. Given an lvalue `f` of type `F` and lvalues `t1`, `t2`, ..., `tN` of types `T1`, `T2`, ..., `TN`, respectively, the type `result_of<F(T1, T2, ..., TN)>::type` defines the result type of the expression `f(t1, t2, ...,tN)`. This implementation permits the type `F` to be a function pointer, function reference, member function pointer, or class type. By default, N may be any value between 0 and 10. To change the upper limit, define the macro `BOOST_RESULT_OF_NUM_ARGS` to the maximum value for N. Class template `result_of` resides in the header `<boost/utility/result_of.hpp>`.

If your compiler supports `decltype`, then you can enable automatic result type deduction by defining the macro `BOOST_RESULT_OF_USE_DECLTYPE`, as in the following example.

>     #define BOOST_RESULT_OF_USE_DECLTYPE
>     #include <boost/utility/result_of.hpp>

>     struct functor {
>         template<class T>
>         T operator()(T x)
>         {
>             return x;
>         }
>     };

>     typedef boost::result_of<
>         functor(int)
>     >::type type;

If `decltype` is not enabled, then automatic result type deduction of function objects is not possible. Instead, `result_of` uses the following protocol to allow the programmer to specify a type. When `F` is a class type with a member type `result_type`, `result_of<F(T1, T2, ..., TN)>` is `F::result_type`. When `F` does not contain `result_type`, `result_of<F(T1, T2, ..., TN)>` is `F::result<F(T1, T2, ..., TN)>::type` when `N > 0` or `void` when `N = 0`. Note that it is the responsibility of the programmer to ensure that function objects accurately advertise their result type via this protocol, as in the following example.

>     struct functor {
>         template<class> struct result;

>         template<class F, class T>
>         struct result<F(T)> {
>             typedef T type;
>         };

>         template<class T>
>         T operator()(T x)
>         {
>             return x;
>         }
>     };

>     typedef boost::result_of<
>         functor(int)
>     >::type type;

This implementation of `result_of` requires class template partial specialization, the ability to parse function types properly, and support for SFINAE. If `result_of` is not supported by your compiler, including the header `boost/utility/result_of.hpp` will define the macro `BOOST_NO_RESULT_OF`.

For additional information about `result_of`, see the C++ Library Technical Report, N1836, or, for motivation and design rationale, the `result_of` proposal.

Contributed by Doug Gregor.

## Class templates for the Base-from-Member Idiom

See separate documentation.

## Macro BOOST_BINARY

The macro `BOOST_BINARY` is used for the representation of binary literals. It takes as an argument a binary number arranged as an arbitrary amount of 1s and 0s in groupings of length 1 to 8, with groups separated by spaces. The type of the literal yielded is determined by the same rules as those of hex and octal literals (2.13.1p1). By implementation, this macro expands directly to an octal literal during preprocessing, so there is no overhead at runtime and the result is useable in any place that an octal literal would be.

In order to directly support binary literals with suffixes, additional macros of the form BOOST_BINARY_XXX are also provided, where XXX is a standard integer suffix in all capital letters. In addition, LL and ULL suffixes may be used for representing long long and unsigned long long types in compilers which provide them as an extension.

The BOOST_BINARY family of macros resides in the header <boost/utility/binary.hpp> which is automatically included by <boost/utility.hpp>.

Contributed by Matt Calabrese.

### Example

>     void foo( int );

>     void foo( unsigned long );

>     void bar()
>     {
>       int value1 = BOOST_BINARY( 100 111000 01 1 110 );

>       unsigned long value2 = BOOST_BINARY_UL( 100 001 ); // unsigned long

>       long long value3 = BOOST_BINARY_LL( 11 000 ); // long long if supported

>       assert(    BOOST_BINARY( 10010 )
>               &  BOOST_BINARY( 11000 )
>               == BOOST_BINARY( 10000 )
>             );

>       foo( BOOST_BINARY( 1010 ) ); // calls the first foo

>       foo( BOOST_BINARY_LU( 1010 ) ); // calls the second foo
>     }

Revised 04 September, 2008

© Copyright Beman Dawes 1999-2003.

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