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

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). The implementation permits the type F to be a function pointer, function reference, member function pointer, or class type. When F is a class type with a member type result_type, result_of<F(T1, T2, ..., TN)> is F::result_type. Otherwise, result_of<F(T1, T2, ..., TN)> is F::result<F(T1, T2, ..., TN)>::type when N > 0 or void when N = 0. For additional information about result_of, see the current draft of the C++ Library TR, N1647, or the result_of proposal.

Class template result_of resides in the header <boost/utility/result_of.hpp>. 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.

This implementation of result_of requires class template partial specialization, the ability to parse function types properly, and support for SFINAE. Contributed by Doug Gregor.

Class templates for the Base-from-Member Idiom

See separate documentation.

Revised  02 May, 2004

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