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# Function Object Traits

The header functional.hpp provides two traits class templates for functions and function objects:

Type | Contents | Description
--- | --- | ---
`template <typename T> struct unary_traits` | `function_type` | The type of the function or function object itself (i.e., `T`).
`param_type` | The type that should be used to pass the function or function object as a parameter.
`result_type` | The type returned by the function or function object.
`argument_type` | The type of the argument to the function or function object.
`template <typename T> struct binary_traits` | `function_type` | The type of the function or function object itself (i.e., `T`).
`param_type` | The type that should be used to pass the function or function object as a parameter.
`result_type` | The type returned by the function or function object.
`first_argument_type` | The type of the first argument to the function or function object.
`second_argument_type` | The type of the second argument to the function or function object.

### Usage

`unary_traits` should be instantiated with either a function taking a single parameter, or an adaptable unary function object (i.e., a class derived from `std::unary_function` or one which provides the same typedefs). (See §20.3.1 in the C++ Standard.)

`binary_traits` should be instantiated with either a function taking two parameters, or an adaptable binary function object (i.e., a class derived from `std::binary_function` or one which provides the same typedefs). (See §20.3.1 in the C++ Standard.)

The most common usage of these templates is in function object adapters, thus allowing them to adapt plain functions as well as function objects. You can do this by wherever you would normally write, for example,

>     typename Operation::argument_type

simply writing

>     typename boost::unary_traits<Operation>::argument_type

instead.

### Additional Types Defined

In addition to the standard result and argument typedefs, these traits templates define two additional types.

#### `function_type`

This is the type of the function or function object, and can be used in declarations such as

>     template <class Predicate>
>     class unary_negate : // ...
>     {
>       // ...
>       private:
>         typename unary_traits<Predicate>::function_type pred;
>     };

If this typedef were not provided, it would not be possible to declare `pred` in a way that would allow `unary_negate` to be instantiated with a function type (see the C++ Standard §14.3.1 ¶3).

#### `param_type`

This is a type suitable for passing the function or function object as a parameter to another function. For example,

>     template <class Predicate>
>     class unary_negate : // ...
>     {
>       public:
>         explicit unary_negate(typename unary_traits<Predicate>::param_type x)
>             :
>             pred(x)
>         {}
>         // ...
>     };

Function objects are passed by reference to const; function pointers are passed by value.

### Limitations

This library uses these traits within all function object adapters, theoretically rendering `ptr_fun` obsolete. However, third party adapters probably won't take advantage of this mechanism, and so `ptr_fun` may still be required. Accordingly, this library also provides improved versions of the standard function pointer adapters.

These traits templates will also not work with compilers that fail to support partial specialisation of templates. With these compilers, the traits templates can only be instantiated with adaptable function objects, thus requiring `ptr_fun` to be used, even with the function object adapters in this library.

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)