core/doc/enable_if.qbk

[/
 / Copyright (c) 2003, 2004 Jaakko Jarvi
 / Copyright (c) 2008 John Maddock
 / Copyright (c) 2011, 2013 Jeremiah Willcock
 / Copyright (c) 2014 Glen Fernandes
 /
 / Use, modification, and distribution is subject to 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:enable_if enable_if]

[simplesect Authors]

* Jaakko J\u00E4rvi
* Jeremiah Willcock
* Andrew Lumsdaine

[endsimplesect]

[section Introduction]

The `enable_if` family of templates is a set of tools to allow
a function template or a class template specialization to
include or exclude itself from a set of matching functions or
specializations based on properties of its template arguments.
For example, one can define function templates that are only
enabled for, and thus only match, an arbitrary set of types
defined by a traits class. The `enable_if` templates can also
be applied to enable class template specializations.
Applications of `enable_if` are discussed in length in
[link REF1 \[1\]] and [link REF2 \[2\]].

[section Header <boost/core/enable_if.hpp>]

``
namespace boost {
    template <class Cond, class T = void> struct enable_if;
    template <class Cond, class T = void> struct disable_if;
    template <class Cond, class T> struct lazy_enable_if;
    template <class Cond, class T> struct lazy_disable_if;

    template <bool B, class T = void> struct enable_if_c;
    template <bool B, class T = void> struct disable_if_c;
    template <bool B, class T> struct lazy_enable_if_c;
    template <bool B, class T> struct lazy_disable_if_c;
}
``

[endsect]

[section Background]

Sensible operation of template function overloading in C++
relies on the /SFINAE/ (substitution-failure-is-not-an-error)
principle [link REF3 \[3\]]: if an invalid argument or return
type is formed during the instantiation of a function template,
the instantiation is removed from the overload resolution set
instead of causing a compilation error. The following example, 
taken from [link REF1 \[1\]], demonstrates why this is
important:

``
int negate(int i) { return -i; }

template <class F>
typename F::result_type negate(const F& f) { return -f(); }
``

Suppose the compiler encounters the call `negate(1)`. The first
definition is obviously a better match, but the compiler must
nevertheless consider (and instantiate the prototypes) of both
definitions to find this out. Instantiating the latter
definition with `F` as `int` would result in:

``
int::result_type negate(const int&);
``

where the return type is invalid. If this were an error, adding
an unrelated function template (that was never called) could
break otherwise valid code. Due to the SFINAE principle the
above example is not, however, erroneous. The latter definition
of `negate` is simply removed from the overload resolution set.

The `enable_if` templates are tools for controlled creation of
the SFINAE conditions.

[endsect]

[endsect]

[section The enable_if templates]

The names of the `enable_if` templates have three parts: an
optional `lazy_` tag, either `enable_if` or `disable_if`, and
an optional `_c` tag. All eight combinations of these parts
are supported. The meaning of the `lazy_` tag is described
in the section [link
core.enable_if.using_enable_if.enable_if_lazy below]. The
second part of the name indicates whether a true condition
argument should enable or disable the current overload. The
third part of the name indicates whether the condition
argument is a `bool` value (`_c` suffix), or a type containing
a static `bool` constant named `value` (no suffix). The latter
version interoperates with Boost.MPL.

The definitions of `enable_if_c` and `enable_if` are as follows
(we use `enable_if` templates  unqualified but they are in the
`boost` namespace).

``
template <bool B, class T = void>
struct enable_if_c {
    typedef T type;
};

template <class T>
struct enable_if_c<false, T> {};

template <class Cond, class T = void>
struct enable_if : public enable_if_c<Cond::value, T> {};
``

An instantiation of the `enable_if_c` template with the
parameter `B` as `true` contains a member type `type`, defined
to be `T`. If `B` is `false`, no such member is defined. Thus 
`enable_if_c<B, T>::type` is either a valid or an invalid type
expression, depending on the value of `B`. When valid,
`enable_if_c<B, T>::type` equals `T`. The `enable_if_c`
template can thus be used for controlling when functions are
considered for overload resolution and when they are not. For
example, the following function is defined for all arithmetic
types (according to the classification of the Boost
*type_traits* library):

``
template <class T>
typename enable_if_c<boost::is_arithmetic<T>::value, T>::type 
foo(T t) { return t; }
``

The `disable_if_c` template is provided as well, and has the
same functionality as `enable_if_c` except for the negated
condition. The following function is enabled for all
non-arithmetic types.

``
template <class T>
typename disable_if_c<boost::is_arithmetic<T>::value, T>::type 
bar(T t) { return t; }
``

For easier syntax in some cases and interoperation with
Boost.MPL we provide versions of the `enable_if` templates
taking any type with a `bool` member constant named `value` as
the condition argument. The MPL `bool_`, `and_`, `or_`, and
`not_` templates are likely to be useful for creating such
types. Also, the traits classes in the Boost.Type_traits
library follow this convention. For example, the above example
function `foo` can be alternatively written as:

``
template <class T>
typename enable_if<boost::is_arithmetic<T>, T>::type 
foo(T t) { return t; }
``

[endsect]

[section:using_enable_if Using enable_if]

The `enable_if` templates are defined in
`boost/utility/enable_if.hpp`, which is included by
`boost/utility.hpp`.

With respect to function templates, `enable_if` can be used in
multiple different ways:

* As the return type of an instantiatied function
* As an extra parameter of an instantiated function
* As an extra template parameter (useful only in a compiler
  that supports C++0x default arguments for function template
  parameters, see [link
  core.enable_if.using_enable_if.enable_if_0x Enabling function
  templates in C++0x] for details.

In the previous section, the return type form of `enable_if`
was shown. As an example of using the form of `enable_if` that
works via an extra function parameter, the `foo` function in
the previous section could also be written as:

``
template <class T>
T foo(T t,
    typename enable_if<boost::is_arithmetic<T> >::type* dummy = 0); 
``

Hence, an extra parameter of type `void*` is added, but it is
given  a default value to keep the parameter hidden from client
code. Note that the second template argument was not given to
`enable_if`, as the default `void` gives the desired behavior.

Which way to write the enabler is largely a matter of taste,
but for certain functions, only a subset of the options is
possible:

* Many operators have a fixed number of arguments, thus
  `enable_if` must be used either in the return type or in an
  extra template parameter.
* Functions that have a variadic parameter list must use either
  the return type form or an extra template parameter.
* Constructors do not have a return type so you must use either
  an extra function parameter or an extra template parameter.
* Constructors that have a variadic parameter list must an
  extra template parameter.
* Conversion operators can only be written with an extra
  template parameter.

[section:enable_if_0x Enabling function templates in C++0x]

In a compiler which supports C++0x default arguments for
function template parameters, you can enable and disable
function templates by adding an additional template parameter.
This approach works in all situations where you would use
either the return type form of `enable_if` or the function
parameter form, including operators, constructors, variadic
function templates, and even overloaded conversion operations.

As an example:

``
#include <boost/type_traits/is_arithmetic.hpp>
#include <boost/type_traits/is_pointer.hpp>
#include <boost/utility/enable_if.hpp>

class test
{
public:
    // A constructor that works for any argument list of size 10
    template< class... T,
        typename boost::enable_if_c< sizeof...( T ) == 10,
            int >::type = 0>
    test( T&&... );

    // A conversion operation that can convert to any arithmetic type
    template< class T,
        typename boost::enable_if< boost::is_arithmetic< T >,
            int >::type = 0>
    operator T() const;

    // A conversion operation that can convert to any pointer type
    template< class T,
        typename boost::enable_if< boost::is_pointer< T >,
            int >::type = 0>
    operator T() const;
};

int main()
{
    // Works
    test test_( 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 );

    // Fails as expected
    test fail_construction( 1, 2, 3, 4, 5 );

    // Works by calling the conversion operator enabled for arithmetic types
    int arithmetic_object = test_;

    // Works by calling the conversion operator enabled for pointer types
    int* pointer_object = test_;

    // Fails as expected
    struct {} fail_conversion = test_;
}
``

[endsect]

[section Enabling template class specializations]

Class template specializations can be enabled or disabled with
`enable_if`. One extra template parameter needs to be added for
the enabler expressions. This parameter has the default value
`void`. For example:

``
template <class T, class Enable = void> 
class A { ... };

template <class T>
class A<T, typename enable_if<is_integral<T> >::type> { ... };

template <class T>
class A<T, typename enable_if<is_float<T> >::type> { ... };
``

Instantiating `A` with any integral type matches the first
specialization, whereas any floating point type matches the
second one. All other types match the primary template.
The condition can be any compile-time boolean expression that
depends on the template arguments of the class. Note that
again, the second argument to `enable_if` is not needed; the
default (`void`) is the correct value.

The `enable_if_type` template is usable this scenario but instead of
using a type traits to enable or disable a specialization, it use a
SFINAE context to check for the existence of a dependent type inside
its parameter. For example, the following structure extracts a dependent
`value_type` from T if and only if `T::value_type` exists.

``
template <class T, class Enable = void>
class value_type_from
{
  typedef T type;
};

template <class T>
class value_type_from<T, typename enable_if_type<typename T::value_type>::type>
{
  typedef typename T::value_type type;
};
``

[endsect]

[section Overlapping enabler conditions]

Once the compiler has examined the enabling conditions and
included the function into the overload resolution set, normal
C++ overload resolution rules are used to select the best
matching function. In particular, there is no ordering between
enabling conditions. Function templates with enabling
conditions that are not mutually exclusive can lead to
ambiguities. For example:

``
template <class T>
typename enable_if<boost::is_integral<T>, void>::type 
foo(T t) {}

template <class T>
typename enable_if<boost::is_arithmetic<T>, void>::type 
foo(T t) {}
``

All integral types are also arithmetic. Therefore, say, for the
call `foo(1)`, both conditions are true and both functions are
thus in the overload resolution set. They are both equally good
matches and thus ambiguous. Of course, more than one enabling
condition can be simultaneously true as long as other arguments
disambiguate the functions.

The above discussion applies to using `enable_if` in class
template partial specializations as well.

[endsect]

[section:enable_if_lazy Lazy enable_if]

In some cases it is necessary to avoid instantiating part of a
function signature unless an enabling condition is true. For
example:

``
template <class T, class U> class mult_traits;

template <class T, class U>
typename enable_if<is_multipliable<T, U>,
    typename mult_traits<T, U>::type>::type
operator*(const T& t, const U& u) { ... }
``

Assume the class template `mult_traits` is a traits class
defining  the resulting type of a multiplication operator. The
`is_multipliable` traits class specifies for which types to
enable the operator. Whenever `is_multipliable<A, B>::value` is
`true` for some types `A` and `B`, then
`mult_traits<A, B>::type` is defined.

Now, trying to invoke (some other overload) of `operator*`
with, say, operand types `C` and `D` for which
`is_multipliable<C, D>::value` is `false` and
`mult_traits<C, D>::type` is not defined is an error on some
compilers. The SFINAE principle is not applied because the
invalid type occurs as an argument to another template. The
`lazy_enable_if` and `lazy_disable_if` templates (and their
`_c` versions) can be used in such situations:

``
template<class T, class U>
typename lazy_enable_if<is_multipliable<T, U>,
    mult_traits<T, U> >::type
operator*(const T& t, const U& u) { ... }

``The second argument of `lazy_enable_if` must be a class type
that defines a nested type named `type` whenever the first
parameter (the condition) is true.

[note Referring to one member type or static constant in a
traits class causes all of the members (type and static
constant) of that specialization to be instantiated.
Therefore, if your traits classes can sometimes contain invalid
types, you should use two distinct templates for describing the
conditions and the type mappings. In the above example,
`is_multipliable<T, U>::value` defines when
`mult_traits<T, U>::type` is valid.]

[endsect]

[section Compiler workarounds]

Some compilers flag functions as ambiguous if the only
distinguishing factor is a different condition in an enabler
(even though the functions could never be ambiguous). For
example, some compilers (e.g. GCC 3.2) diagnose the following
two functions as ambiguous:

``
template <class T>
typename enable_if<boost::is_arithmetic<T>, T>::type 
foo(T t);

template <class T>
typename disable_if<boost::is_arithmetic<T>, T>::type 
foo(T t);
``

Two workarounds can be applied:

* Use an extra dummy parameter which disambiguates the
  functions. Use a default value for it to hide the parameter
  from the caller. For example:  
  ``
  template <int> struct dummy { dummy(int) {} };
  
  template <class T>
  typename enable_if<boost::is_arithmetic<T>, T>::type 
  foo(T t, dummy<0> = 0);
  
  template <class T>
  typename disable_if<boost::is_arithmetic<T>, T>::type 
  foo(T t, dummy<1> = 0);
  ``
* Define the functions in different namespaces and bring them
  into a common namespace with `using` declarations:
  ``
  namespace A {
      template <class T>
      typename enable_if<boost::is_arithmetic<T>, T>::type 
      foo(T t);
  }
  
  namespace B {
      template <class T>
      typename disable_if<boost::is_arithmetic<T>, T>::type 
      foo(T t);
  }
  
  using A::foo;
  using B::foo;
  ``
  Note that the second workaround above cannot be used for
  member templates. On the other hand, operators do not accept
  extra arguments, which makes the first workaround unusable.
  As the net effect, neither of the workarounds are of
  assistance for templated operators that need to be defined as
  member functions (assignment and subscript operators).

[endsect]

[endsect]

[section Acknowledgements]

We are grateful to Howard Hinnant, Jason Shirk, Paul
Mensonides, and Richard Smith whose findings have
influenced the library.

[endsect]

[section References]

* [#REF1] \[1\] Jaakko J\u00E4rvi, Jeremiah Willcock, Howard
  Hinnant, and Andrew Lumsdaine. Function overloading based on
  arbitrary properties of types. /C++ Users Journal/,
  21(6):25--32, June 2003.
* [#REF2] \[2\] Jaakko J\u00E4rvi, Jeremiah Willcock, and
  Andrew Lumsdaine. Concept-controlled polymorphism. In Frank
  Pfennig and Yannis Smaragdakis, editors, /Generative
  Programming and Component Engineering/, volume 2830 of
  /LNCS/, pages 228--244. Springer Verlag, September 2003.
* [#REF3] \[3\] David Vandevoorde and Nicolai M. Josuttis.
  /C++ Templates: The Complete Guide/. Addison-Wesley, 2002.

[endsect]

[endsect]