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.

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.

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.

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