[ceph.git] / ceph / src / boost / libs / bind / doc / bind / limitations.qbk

[/
 /  Copyright (c) 2001, 2002 Peter Dimov and Multi Media Ltd.
 /  Copyright (c) 2003-2008 Peter Dimov
 /
 / Distributed under the Boost Software License, Version 1.0. (See
 / accompanying file LICENSE_1_0.txt or copy at
 / http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:limitations Limitations]

As a general rule, the function objects generated by `bind` take their
arguments by reference and cannot, therefore, accept non-const temporaries or
literal constants. This is an inherent limitation of the C++ language in its
current (2003) incarnation, known as the [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm forwarding problem].
(It will be fixed in the next standard, usually called C++0x.)

The library uses signatures of the form

    template<class T> void f(T & t);

to accept arguments of arbitrary types and pass them on unmodified. As noted,
this does not work with non-const r-values.

On compilers that support partial ordering of function templates, a possible
solution is to add an overload:

    template<class T> void f(T & t);
    template<class T> void f(T const & t);

Unfortunately, this requires providing 512 overloads for nine arguments, which
is impractical. The library chooses a small subset: for up to two arguments,
it provides the const overloads in full, for arities of three and more it
provides a single additional overload with all of the arguments taken by const
reference. This covers a reasonable portion of the use cases.

[endsect]
Commit	Line	Data
7c673cae FG	1	[/
	2	/ Copyright (c) 2001, 2002 Peter Dimov and Multi Media Ltd.
	3	/ Copyright (c) 2003-2008 Peter Dimov
	4	/
	5	/ Distributed under the Boost Software License, Version 1.0. (See
	6	/ accompanying file LICENSE_1_0.txt or copy at
	7	/ http://www.boost.org/LICENSE_1_0.txt)
	8	/]
	9
	10	[section:limitations Limitations]
	11
	12	As a general rule, the function objects generated by `bind` take their
	13	arguments by reference and cannot, therefore, accept non-const temporaries or
	14	literal constants. This is an inherent limitation of the C++ language in its
	15	current (2003) incarnation, known as the [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm forwarding problem].
	16	(It will be fixed in the next standard, usually called C++0x.)
	17
	18	The library uses signatures of the form
	19
	20	template<class T> void f(T & t);
	21
	22	to accept arguments of arbitrary types and pass them on unmodified. As noted,
	23	this does not work with non-const r-values.
	24
	25	On compilers that support partial ordering of function templates, a possible
	26	solution is to add an overload:
	27
	28	template<class T> void f(T & t);
	29	template<class T> void f(T const & t);
	30
	31	Unfortunately, this requires providing 512 overloads for nine arguments, which
	32	is impractical. The library chooses a small subset: for up to two arguments,
	33	it provides the const overloads in full, for arities of three and more it
	34	provides a single additional overload with all of the arguments taken by const
	35	reference. This covers a reasonable portion of the use cases.
	36
	37	[endsect]