Improved Function Object Adapters
The header functional.hpp
provides enhancements to the function object adapters specified in the C++
Standard Library (sections 20.3.5, through to 20.3.8). The enhancements are
principally possible due to two changes:
- We use the Boost call_traits templates to avoid the
problem of references to references,
and to improve the efficiency of parameter
passing.
- We use two function object traits
class templates to avoid the need for ptr_fun with the adapters in this library.
Contents
The header contains the following function and class templates:
Function object
traits |
unary_traits
binary_traits |
Used to determine the types of function objects' and
functions' arguments. Eliminate the necessity for
ptr_fun. |
Negators |
unary_negate
binary_negate
not1
not2 |
Based on section 20.3.5 of the standard. |
Binders |
binder1st
binder2nd
bind1st
bind2nd |
Based on section 20.3.6 of the standard. |
Adapters for pointers to
functions |
pointer_to_unary_function
pointer_to_binary_function
ptr_fun |
Based on section 20.3.7 of the standard. Not required
for use with this library since the binders and negators can adapt
functions, but may be needed with third party adapters. |
Adapters for pointers to member
functions |
mem_fun_t
mem_fun1_t
const_mem_fun_t
const_mem_fun1_t
mem_fun_ref_t
mem_fun1_ref_t
const_mem_fun_ref_t
const_mem_fun1_ref_t
mem_fun
mem_fun_ref |
Based on section 20.3.8 of the standard. |
Usage
Using these adapters should be pretty much the same as using the
standard function object adapters; the only differences are that you need
to write boost:: instead of std::, and that you will get
fewer headaches.
For example, suppose you had a Person class that contained a
set_name function:
class Person
{
public:
void set_name(const std::string &name);
// ...
};
You could rename a bunch of people in a collection, c, by
writing
std::for_each(c.begin(), c.end(),
boost::bind2nd(boost::mem_fun_ref(&Person::set_name), "Fred"));
If the standard adapters had been used instead then this code would
normally fail to compile, because set_name takes a reference
argument. Refer to the comments in the binder
documentation to explain why this is so.
Compiler Compatibility
The header and test program have been
compiled with the following compilers:
Compiler |
Comments |
Borland C++Builder 4 Update 2 |
No known issues. |
Borland C++ 5.5 |
No known issues. |
g++ 2.95.2 |
No known issues. |
Microsoft Visual C++ Service Pack 3 |
Compiler lacks partial specialisation, so this library offers little
more than is provided by the standard adapters:
- The call_traits mechanism is unable to prevent
references to references, and so the adapters in this library will
be usable in fewer situations.
- The function_traits mechanism is unable to determine
the argument and result types of functions, therefore
ptr_fun continues to be required to adapt functions.
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Future Directions
This library's primary focus is to solve the problem of references to
references while maintaining as much compatibility as possible with the
standard library. This allows you to use the techniques you read about in
books and magazines with many of today's compilers.
In the longer term, even better solutions are likely:
- Several Boost members are working on expression template libraries.
These will allow a more natural syntax for combining and adapting
functions. As this is a new technology, it may be some time before it has
matured and is widely supported by major compilers but shows great
promise. In the meantime, the functional.hpp library fills the gap.
- The Standard Committee has recognised the problem of references to
references occurring during template instantiation and has moved to fix
the standard (see the C++
standard core language active issues list).
Author
Mark Rodgers
Acknowledgements
Thanks to John Maddock for
suggesting the mechanism that allowed the function objects traits to work
correctly. Jens Maurer provided
invaluable feedback during the formal review process.
Revised
02
December, 2006
Copyright © 2000 Cadenza New Zealand Ltd.
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)
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