From ef9aed35993e44735c3fdfd36b744c7575a72671 Mon Sep 17 00:00:00 2001 From: Beman Dawes Date: Thu, 27 Jul 2000 14:04:40 +0000 Subject: [PATCH] Initial HTML commit [SVN r7636] --- binders.html | 132 +++++++++++++++++++++++++++++ function_traits.html | 196 +++++++++++++++++++++++++++++++++++++++++++ index.html | 184 ++++++++++++++++++++++++++++++++++++++++ mem_fun.html | 171 +++++++++++++++++++++++++++++++++++++ negators.html | 132 +++++++++++++++++++++++++++++ ptr_fun.html | 135 +++++++++++++++++++++++++++++ 6 files changed, 950 insertions(+) create mode 100644 binders.html create mode 100644 function_traits.html create mode 100644 index.html create mode 100644 mem_fun.html create mode 100644 negators.html create mode 100644 ptr_fun.html diff --git a/binders.html b/binders.html new file mode 100644 index 0000000..172f299 --- /dev/null +++ b/binders.html @@ -0,0 +1,132 @@ + + + + +Boost Function Object Adapter Library + + + + + + + + + + + + + +
c++boost.gif (8819 bytes)Home Libraries People FAQ More
+ +

Binders

+ +

The header functional.hpp provides +enhanced versions of both the binder function object adapters from the +C++ Standard Library (§20.3.6):

+ + + +

As well as the corresponding helper functions

+ + + +

The key benefit of these adapters over those in the Standard +Library is they avoid the problem of references to +references. + +

Usage

+ +

Usage is identical to the standard binders. For example,

+ +
+class Foo {
+public:
+  void bar(std::ostream &);
+  // ...
+};
+// ...
+std::vector<Foo> c;
+// ...
+std::for_each(c.begin(), c.end(), 
+              boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout));
+
+ +

References to References

+ +

Consider the usage example above

+ +
+class Foo {
+public:
+  void bar(std::ostream &);
+  // ...
+};
+// ...
+std::for_each(c.begin(), c.end(), 
+              boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout));
+
+ +

If this had been written using std::bind2nd +and std::mem_fun_ref, it would be unlikely to +compile.

+ +

The problem arises because bar takes a +reference argument. The Standard defines +std::mem_fun_ref such that it creates a function +object whose second_argument_type will be +std::ostream&.

+ +

The call to bind2nd creates a +binder2nd which the Standard defines as follows: + +

+template <class Operation>
+class binder2nd
+    : public unary_function<typename Operation::first_argument_type,
+                            typename Operation::result_type> {
+...
+public:
+  binder2nd(const Operation& x,
+            const typename Operation::second_argument_type& y);
+  ...
+
+ +

Since our operation's second_argument_type is +std::ostream&, the type of y in the +constructor would be std::ostream&&. Since you +cannot have a reference to a reference, at this point we should get a +compilation error because references to references are illegal in C++ +(but see +C++ Standard core language active issues list).

+ +

The binders in this library avoid this problem by using the Boost +call_traits templates.

+ +

Our constructor is declared + +

+binder2nd(const Operation& x,
+          typename call_traits<
+             typename binary_traits<Operation>::second_argument_type
+          >::param_type y)
+
+ +

As a result, y has a type of std::ostream&, +and our example compiles.

+ +
+

Copyright © 2000 Cadenza New Zealand Ltd. Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. This document is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose.

+ +

Revised 28 June 2000

+ + + diff --git a/function_traits.html b/function_traits.html new file mode 100644 index 0000000..59e11d9 --- /dev/null +++ b/function_traits.html @@ -0,0 +1,196 @@ + + + + +Boost Function Object Adapter Library + + + + + + + + + + + + + +
c++boost.gif (8819 bytes)Home Libraries People FAQ More
+ +

Function Object Traits

+ +

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

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
TypeContentsDescription
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. + +

+

Copyright © 2000 Cadenza New Zealand Ltd. Permission to copy, +use, modify, sell and distribute this document is granted provided +this copyright notice appears in all copies. This document is provided +"as is" without express or implied warranty, and with no claim as to +its suitability for any purpose.

+ +

Revised 28 June 2000

+ + diff --git a/index.html b/index.html new file mode 100644 index 0000000..7f1525b --- /dev/null +++ b/index.html @@ -0,0 +1,184 @@ + + + + + + +Boost Function Object Adapter Library + + + + + + + + + + + + + +
c++boost.gif (8819 bytes)HomeLibrariesPeopleFAQMore
+

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:

+
    +
  1. We use the Boost call_traits + templates to avoid the problem of references + to references, and to improve the efficiency of parameter + passing.
    2. +
  2. We use two function object traits class + templates to avoid the need for ptr_fun + with the adapters in this library.
    3. +
+

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.
Negatorsunary_negate
+ binary_negate
+ not1
+ not2		Based on section 20.3.5 of the standard.
Bindersbinder1st
+ binder2nd
+ bind1st
+ bind2nd		Based on section 20.3.6 of the standard.
Adapters for pointers to functionspointer_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 + functionsmem_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:

+ + + + + + + + + + + + + + + + + + + + + +
CompilerComments
Borland C++Builder 4 Update 2No known issues.
Borland C++ 5.5No known issues.
g++ 2.95.2No known issues.
Microsoft Visual C++ Service Pack 3Compiler 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:

+
    +
  1. 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.
    2. +
  2. 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).
    3. +
+

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. +

+

Copyright © 2000 Cadenza New Zealand Ltd. Permission to copy, use, modify, +sell and distribute this document is granted provided this copyright notice +appears in all copies. This document is provided "as is" without +express or implied warranty, and with no claim as to its suitability for any +purpose.

+

Revised 28 June 2000

+ + + + diff --git a/mem_fun.html b/mem_fun.html new file mode 100644 index 0000000..9721713 --- /dev/null +++ b/mem_fun.html @@ -0,0 +1,171 @@ + + + + +Boost Function Object Adapter Library + + + + + + + + + + + + + +
c++boost.gif (8819 bytes)Home Libraries People FAQ More
+ +

Member Function Adapters

+ +

The header functional.hpp includes +improved versions of the full range of member function adapters from +the the C++ Standard Library (§ 20.3.8):

+ + + +

as well as the corresponding overloaded helper functions

+

+ +

The following changes have been made to the adapters as specified +in the Standard:

+ + + +

first_argument_type

+ +

The standard specifies const_mem_fun1_t, for example, like this: + +

+template <class S, class T, class A> class const_mem_fun1_t
+  : public binary_function<T*, A, S> {
+public:
+  explicit const_mem_fun1_t(S (T::*p)(A) const);
+  S operator()(const T* p, A x) const;
+};
+
+ +

Note that the first argument to +binary_function is T* +despite the fact that the first argument to operator() is +actually of type const T*. + +

Does this matter? Well, consider what happens when we write + +

+struct Foo { void bar(int) const; };
+const Foo *cp = new Foo;
+std::bind1st(std::mem_fun(&Foo::bar), cp);
+
+ +

We have created a const_mem_fun1_t object +which will effectively contain the following + +

+typedef Foo* first_argument_type;
+
+ +

The bind1st will then create a +binder1st object that will use this +typedef as the type of a member which will be +initialised with cp. In other words, we will +need to initialise a Foo* member with a +const Foo* pointer! Clearly this is not +possible, so to implement this your Standard Library vendor will have +had to cast away the constness of cp, probably +within the body of bind1st. + +

This hack will not suffice with the improved binders in this library, so we have had to +provide corrected versions of the member function adapters as well. + + +

Argument Types

+ +

The standard defines mem_fun1_t, for example, like this +(§20.3.8 ¶2): + +

+template <class S, class T, class A> class mem_fun1_t
+  : public binary_function<T*, A, S> {
+public:
+  explicit mem_fun1_t(S (T::*p)(A));
+  S operator()(T* p, A x) const;
+};
+
+ +

Note that the second argument to operator() is +exactly the same type as the argument to the member function. If this +is a value type, the argument will be passed by value and copied twice. + +

However, if we were to try and eliminate this inefficiency by +instead declaring the argument as const A&, then +if A were a reference type, we would have a reference to a reference, +which is currently illegal (but see C++ +core language issue number 106) + +

So the way in which we want to declare the second argument for +operator() depends on whether or not the member +function's argument is a reference. If it is a reference, we want to +declare it simply as A; if it is a value we want +to declare it as const A&. + +

The Boost call_traits class +template contains a param_type typedef, which +uses partial specialisation to make precisely this decision. By +declaring the operator() as + +

+S operator()(T* p, typename call_traits<A>::param_type x) const
+
+ +

we achieve the desired result - we improve efficiency without +generating references to references.

+ +

Limitations

+ +

The call traits template used to realise some improvements relies +on partial specialisation, so these improvements are only available on +compilers that support that feature. With other compilers, the +argument passed to the member function (in the +mem_fun1_t family) will always be passed by +reference, thus generating the possibility of references to references. + +

+ +

Copyright © 2000 Cadenza New Zealand Ltd. Permission to copy, +use, modify, sell and distribute this document is granted provided +this copyright notice appears in all copies. This document is provided +"as is" without express or implied warranty, and with no claim as to +its suitability for any purpose.

+ +

Revised 28 June 2000

+ + + diff --git a/negators.html b/negators.html new file mode 100644 index 0000000..51891f4 --- /dev/null +++ b/negators.html @@ -0,0 +1,132 @@ + + + + +Boost Function Object Adapter Library + + + + + + + + + + + + + +
c++boost.gif (8819 bytes)Home Libraries People FAQ More
+ +

Negators

+ +

The header functional.hpp provides +enhanced versions of both the negator adapters from the C++ Standard +Library (§20.3.5):

+ + + +

As well as the corresponding helper functions

+ + + +

However, the negators in this library improve on the standard +versions in two ways: + +

+ +

Usage

+ +

Usage is identical to the standard negators. For example,

+ +
+bool bad(const Foo &foo) { ... }
+...
+std::vector<Foo> c;
+...
+std::find_if(c.begin(), c.end(), boost::not1(bad));
+
+ +

Argument Types

+ +

The C++ Standard (§20.3.5) defines unary negate +like this (binary negate is similar):

+ +
+template <class Predicate>
+  class unary_negate
+    : public unary_function<typename Predicate::argument_type,bool> {
+public:
+  explicit unary_negate(const Predicate& pred);
+  bool operator()(const typename Predicate::argument_type& x) const;
+};
+ +

Note that if the Predicate's argument_type is +a reference, the type of operator()'s argument +would be a reference to a reference. Currently this is illegal in C++ +(but see the +C++ standard core language active issues list).

+ +

However, if we instead defined operator() +to accept Predicate's argument_type unmodified, this would be +needlessly inefficient if it were a value type; the argument would be +copied twice - once when calling unary_negate's +operator(), and again when operator() +called the adapted function.

+ +

So how we want to declare the argument for +operator() depends on whether or not the +Predicate's argument_type is a reference. If it +is a reference, we want to declare it simply as +argument_type; if it is a value we want to +declare it as const argument_type&. + +

The Boost call_traits class +template contains a param_type typedef, which +uses partial specialisation to make precisely this decision. If we were +to declare operator() as

+ +
+bool operator()(typename call_traits<typename Predicate::argument_type>::param_type x) const
+
+ +

the desired result would be achieved - we would eliminate +references to references without loss of efficiency. In fact, the +actual declaration is slightly more complicated because of the use of +function object traits, but the effect remains the same.

+ +

Limitations

+ +

Both the function object traits and call traits used to realise +these improvements rely on partial specialisation, these improvements +are only available on compilers that support that feature. With other +compilers, the negators in this library behave very much like those +in the Standard - ptr_fun will be required to +adapt functions, and references to references will not be avoided. + +

+

Copyright © 2000 Cadenza New Zealand Ltd. Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. This document is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose.

+ +

Revised 28 June 2000

+ + + diff --git a/ptr_fun.html b/ptr_fun.html new file mode 100644 index 0000000..f45e879 --- /dev/null +++ b/ptr_fun.html @@ -0,0 +1,135 @@ + + + + +Boost Function Object Adapter Library + + + + + + + + + + + + + +
c++boost.gif (8819 bytes)Home Libraries People FAQ More
+ +

Function Pointer Adapters

+ +

The header functional.hpp provides +enhanced versions of both the function pointer adapters from the C++ +Standard Library (§ 20.3.7):

+ + + +

As well as the corresponding helper function template:

+ + + +

However, you should not need to use the adapters in conjunction +with the adapters in this library due to our use of function object traits. You will +however need to use them if your implementation fails to work properly +with our traits classes (due to lack if partial specialisation), or if +you wish to use a function object adapter from a third party. + +

Usage

+ +

If you need to use these adapters, usage is identical to the +standard function pointer adapters. For example,

+ +
+bool bad(std::string foo) { ... }
+...
+std::vector<std::string> c;
+...
+std::vector<std::string>::iterator it
+     = std::find_if(c.begin(), c.end(), std::not1(boost::ptr_fun(bad)));
+
+ +

Note however that this library contains enhanced negators that support function object traits, +so the line above could equally be written + +

+std::vector<std::string>::iterator it
+     = std::find_if(c.begin(), c.end(), boost::not1(bad));
+
+ +

Argument Types

+ +

The standard defines +pointer_to_unary_function like this +(§20.3.8 ¶2): + +

+template <class Arg, class Result>
+class pointer_to_unary_function : public unary_function<Arg, Result> {
+public:
+  explicit pointer_to_unary_function(Result (* f)(Arg));
+  Result operator()(Arg x) const;
+};
+
+ +

Note that the argument to operator() is +exactly the same type as the argument to the wrapped function. If this +is a value type, the argument will be passed by value and copied twice. +pointer_to_binary_function has a similar problem. + +

However, if we were to try and eliminate this inefficiency by +instead declaring the argument as const Arg&, then +if Arg were a reference type, we would have a reference to a reference, +which is currently illegal (but see C++ +core language issue number 106) + +

So the way in which we want to declare the argument for +operator() depends on whether or not the +wrapped function's argument is a reference. If it +is a reference, we want to declare it simply as +Arg; if it is a value we want to +declare it as const Arg&. + +

The Boost call_traits class +template contains a param_type typedef, which +uses partial specialisation to make precisely this decision. By +declaring the operator() as + +

+Result operator()(typename call_traits<Arg>::param_type x) const
+
+ +

we achieve the desired result - we improve efficiency without +generating references to references.

+ +

Limitations

+ +

The call traits template used to realise this improvement relies +on partial specialisation, so this improvement is only available on +compilers that support that feature. With other compilers, the +argument passed to the function will always be passed by +reference, thus generating the possibility of references to references. + +

+ +

Copyright © 2000 Cadenza New Zealand Ltd. Permission to copy, +use, modify, sell and distribute this document is granted provided +this copyright notice appears in all copies. This document is provided +"as is" without express or implied warranty, and with no claim as to +its suitability for any purpose.

+ +

Revised 28 June 2000

+ + +