bslmf_enableif.h

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/// @file bslmf_enableif.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslmf_enableif.h                                                   -*-C++-*-
#ifndef INCLUDED_BSLMF_ENABLEIF
#define INCLUDED_BSLMF_ENABLEIF
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslmf_enableif bslmf_enableif
/// @brief  Provide a utility to set up SFINAE conditions in type deduction.
/// @addtogroup bsl
/// @{
/// @addtogroup bslmf
/// @{
/// @addtogroup bslmf_enableif
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslmf_enableif-purpose"> Purpose</a>
/// * <a href="#bslmf_enableif-classes"> Classes </a>
/// * <a href="#bslmf_enableif-description"> Description </a>
///   * <a href="#bslmf_enableif-visual-studio-workaround"> Visual Studio Workaround </a>
///   * <a href="#bslmf_enableif-usage"> Usage </a>
///     * <a href="#bslmf_enableif-example-1-implementing-a-simple-function-with-bsl-enable_if"> Example 1: Implementing a Simple Function with bsl::enable_if </a>
///     * <a href="#bslmf_enableif-example-2-using-the-bsl-enable_if-result-type"> Example 2: Using the bsl::enable_if Result Type </a>
///     * <a href="#bslmf_enableif-example-3-controlling-constructor-selection-with-bsl-enable_if"> Example 3: Controlling Constructor Selection with bsl::enable_if </a>
///
/// # Purpose {#bslmf_enableif-purpose}
/// Provide a utility to set up SFINAE conditions in type deduction.
///
/// # Classes {#bslmf_enableif-classes}
///
/// -  bsl::enable_if: standard meta-function to drop templates from overload sets
/// -  bsl::enable_if_t: alias to the return type of the meta-function
/// -  bslmf::EnableIf: meta-function to drop templates from overload sets
///
/// # Description {#bslmf_enableif-description}
/// This component defines two meta-functions, `bsl::enable_if` and
/// `bslmf::EnableIf`, both of which may be used to conditionally remove
/// (potential) template instantiations as candidates for overload resolution by
/// causing a deduced template instantiation to fail in a way compatible with
/// the C++ SFINAE rules.
///
/// `bsl::enable_if` meets the requirements of the `enable_if` template defined
/// in the C++11 standard [meta.trans.ptr], while `bslmf::EnableIf` was devised
/// before `enable_if` was standardized.
///
/// The two meta-functions provide identical functionality.  Both meta-functions
/// provide a `typedef` `type` that is an alias to a (template parameter) type
/// if a (template parameter) condition is `true`; otherwise, `type` is not
/// provided.
///
/// Note that `bsl::enable_if` should be preferred over `bslmf::EnableIf`, and
/// in general, should be used by new components.
///
/// ## Visual Studio Workaround {#bslmf_enableif-visual-studio-workaround}
///
///
/// Because of a Visual Studio bug, described here:
///   http://connect.microsoft.com/VisualStudio/feedback/details/332179/
/// the Microsoft Visual Studio compiler may not correctly associate a function
/// declaration that uses `bsl::enable_if` with that function's definition, if
/// the definition is not inline to the declaration.  This bug affects at least
/// Visual Studio 2008 and 2010.  The workaround is to implement functions using
/// `bsl::enable_if` inline with their declaration.
///
/// ## Usage {#bslmf_enableif-usage}
///
///
/// The following snippets of code illustrate basic use of the `bsl::enable_if`
/// meta-function.  We will demonstrate how to use this utility to control
/// overload sets with three increasingly complex examples.
///
/// ### Example 1: Implementing a Simple Function with bsl::enable_if {#bslmf_enableif-example-1-implementing-a-simple-function-with-bsl-enable_if}
///
///
/// Suppose that we want to implement a simple `swap` function template to
/// exchange two arbitrary values, as if defined below:
/// @code
/// template<class t_TYPE>
/// void DummySwap(t_TYPE& a, t_TYPE& b)
///     // Exchange the values of the specified objects, 'a' and 'b'.
/// {
///     t_TYPE temp(a);
///     a = b;
///     b = temp;
/// }
/// @endcode
/// However, we want to take advantage of member-swap methods supplied by user-
/// defined types, so we define a trait that can be customized by a class
/// implementer to indicate that their class supports an optimized member-swap
/// method:
/// @code
/// template<class t_TYPE>
/// struct HasMemberSwap : bsl::false_type {
///     // This traits class indicates whether the (template parameter)
///     // 't_TYPE' has a public 'swap' method to exchange values.
/// };
/// @endcode
/// Now, we implement a generic `swap` function template that will invoke the
/// member swap operation for any type that specialized our trait.  The use of
/// `bsl::enable_if` to declare the result type causes an attempt to deduce the
/// type `t_TYPE` to fail unless the specified condition is `true`, and this
/// falls under the "Substitution Failure Is Not An Error" (SFINAE) clause of
/// the C++ standard, so the compiler will look for a more suitable overload
/// rather than fail with an error.  Note that we provide two overloaded
/// declarations that appear to differ only in their return type, which would
/// normally raise an ambiguity error.  This works, and is in fact required, in
/// this case as the "enable-if" conditions are mutually exclusive, so that only
/// one overload will ever be present in an overload set.  Also note that the
/// `type` `typedef` of `bsl::enable_if` is an alias to `void` when the
/// (template parameter) type is unspecified and the (template parameter)
/// condition value is `true`.
/// @code
/// template<class t_TYPE>
/// typename bsl::enable_if<HasMemberSwap<t_TYPE>::value>::type
/// swap(t_TYPE& a, t_TYPE& b)
/// {
///     a.swap(b);
/// }
///
/// template<class t_TYPE>
/// typename bsl::enable_if< ! HasMemberSwap<t_TYPE>::value>::type
/// swap(t_TYPE& a, t_TYPE& b)
/// {
///     t_TYPE temp(a);
///     a = b;
///     b = temp;
/// }
/// @endcode
/// Next, we define a simple container template, that supports an optimized
/// `swap` operation by merely swapping the internal pointer to the array of
/// elements rather than exchanging each element:
/// @code
/// template<class t_TYPE>
/// class MyContainer {
///     // This is a simple container implementation for demonstration purposes
///     // that is modeled after 'std::vector'.
///
///     // DATA
///     t_TYPE *d_storage;
///     size_t  d_length;
///
///     // Copy operations are declared private and not defined.
///
///     // NOT IMPLEMENTED
///     MyContainer(const MyContainer&);
///     MyContainer& operator=(const MyContainer&);
///
///   public:
///     MyContainer(const t_TYPE& value, int n);
///         // Create a 'MyContainer' object having the specified 'n' copies of
///         // the specified 'value'.  The behavior is undefined unless
///         // '0 <= n'.
///
///     ~MyContainer();
///         // Destroy this container and all of its elements, reclaiming any
///         // allocated memory.
///
///     // MANIPULATORS
///     void swap(MyContainer &other);
///         // Exchange the contents of 'this' container with those of the
///         // specified 'other'.  No memory will be allocated, and no
///         // exceptions are thrown.
///
///     // ACCESSORS
///     const t_TYPE& front() const;
///         // Return a reference providing non-modifiable access to the first
///         // element in this container.  The behavior is undefined if this
///         // container is empty.
///
///     size_t size() const;
///         // Return the number of elements held by this container.
/// };
/// @endcode
/// Then, we specialize our `HasMemberSwap` trait for this new container type.
/// @code
/// template<class t_TYPE>
/// struct HasMemberSwap<MyContainer<t_TYPE> > : bsl::true_type {
/// };
/// @endcode
/// Next, we implement the methods of this class:
/// @code
/// // CREATORS
/// template<class t_TYPE>
/// MyContainer<t_TYPE>::MyContainer(const t_TYPE& value, int n)
/// : d_storage(new t_TYPE[n])
/// , d_length(n)
/// {
///     for (int i = 0; i != n; ++i) {
///         d_storage[i] = value;
///     }
/// }
///
/// template<class t_TYPE>
/// MyContainer<t_TYPE>::~MyContainer()
/// {
///     delete[] d_storage;
/// }
///
/// // MANIPULATORS
/// template<class t_TYPE>
/// void MyContainer<t_TYPE>::swap(MyContainer& other)
/// {
///     ::swap(d_storage, other.d_storage);
///     ::swap(d_length,  other.d_length);
/// }
///
/// // ACCESSORS
/// template<class t_TYPE>
/// const t_TYPE& MyContainer<t_TYPE>::front() const
/// {
///     return d_storage[0];
/// }
///
/// template<class t_TYPE>
/// size_t MyContainer<t_TYPE>::size() const
/// {
///     return d_length;
/// }
/// @endcode
/// Finally, we can test that the member-`swap` method is called by the generic
/// `swap` function.  Note that the following code will not compile unless the
/// member-function `swap` is used, as the copy constructor and assignment
/// operator for the `MyContainer` class template are declared as `private`.
/// @code
/// void TestSwap()
/// {
///     MyContainer<int> x(3, 14);
///     MyContainer<int> y(2, 78);
///     assert(14 == x.size());
///     assert( 3 == x.front());
///     assert(78 == y.size());
///     assert( 2 == y.front());
///
///     swap(x, y);
///
///     assert(78 == x.size());
///     assert( 2 == x.front());
///     assert(14 == y.size());
///     assert( 3 == y.front());
/// }
/// @endcode
///
/// ### Example 2: Using the bsl::enable_if Result Type {#bslmf_enableif-example-2-using-the-bsl-enable_if-result-type}
///
///
/// For the next example, we will demonstrate the use of the second template
/// parameter in the `bsl::enable_if` template, which serves as the "result"
/// type if the test condition passes.  Suppose that we want to write a generic
/// function to allow us to cast between pointers of different types.  If the
/// types are polymorphic, we can use @ref dynamic_cast  to potentially cast between
/// two seemingly unrelated types.  However, if either type is not polymorphic
/// then the attempt to use @ref dynamic_cast  would be a compile-time failure, and
/// we must use @ref static_cast  instead.
/// @code
/// #ifdef BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
/// @endcode
/// Note that if the current compiler supports alias templates C++11 feature, we
/// can use `bsl::enable_if_t` alias to the "result" type of `bsl::enable_if`
/// meta-function, that avoids the `::type` suffix and `typename` prefix in the
/// declaration of the function return type.
/// @code
/// template<class t_TO, class t_FROM>
/// bsl::enable_if_t<bsl::is_polymorphic<t_FROM>::value &&
///                  bsl::is_polymorphic<t_TO  >::value, t_TO> *
/// #else
/// template<class t_TO, class t_FROM>
/// typename bsl::enable_if<bsl::is_polymorphic<t_FROM>::value &&
///                                           bsl::is_polymorphic<t_TO>::value,
///                         t_TO>::type *
/// #endif
/// smart_cast(t_FROM *from)
///     // Return a pointer to the specified 'TO' type if the specified 'from'
///     // pointer refers to an object whose complete class publicly derives,
///     // directly or indirectly, from 'TO', and a null pointer otherwise.
/// {
///     return dynamic_cast<t_TO *>(from);
/// }
///
/// #ifdef BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
/// template<class t_TO, class t_FROM>
/// bsl::enable_if_t<not(bsl::is_polymorphic<t_FROM>::value &&
///                      bsl::is_polymorphic<t_TO  >::value), t_TO> *
/// #else
/// template<class t_TO, class t_FROM>
/// typename bsl::enable_if<not(bsl::is_polymorphic<t_FROM>::value &&
///                                          bsl::is_polymorphic<t_TO>::value),
///                         t_TO>::type *
/// #endif
/// smart_cast(t_FROM *from)
///     // Return the specified 'from' pointer value cast as a pointer to type
///     // 'TO'.  The behavior is undefined unless such a conversion is valid.
/// {
///     return static_cast<t_TO *>(from);
/// }
/// @endcode
/// Next, we define a small number of classes to demonstrate that this casting
/// utility works correctly:
/// @code
/// class A {
///     // Sample non-polymorphic type
///
///   public:
///     ~A() {}
/// };
///
/// class B {
///     // Sample polymorphic base-type
///
///   public:
///     virtual ~B() {}
/// };
///
/// class C {
///     // Sample polymorphic base-type
///
///   public:
///     virtual ~C() {}
/// };
///
/// class ABC : public A, public B, public C {
///     // Most-derived example class using multiple bases in order to
///     // demonstrate cross-casting.
/// };
/// @endcode
/// Finally, we demonstrate the correct behavior of the @ref smart_cast  utility:
/// @code
/// void TestSmartCast()
/// {
///     ABC  object;
///     ABC *pABC = &object;
///     A   *pA   = &object;
///     B   *pB   = &object;
///     C   *pC   = &object;
///
///     A *pA2 = smart_cast<A>(pABC);
///     B *pB2 = smart_cast<B>(pC);
///     C *pC2 = smart_cast<C>(pB);
///
///     (void) pA;
///
///     assert(&object == pA2);
///     assert(&object == pB2);
///     assert(&object == pC2);
///
///     // These lines would fail to compile
///     // A *pA3 = smart_cast<A>(pB);
///     // C *pC3 = smart_cast<C>(pA);
/// }
/// @endcode
///
/// ### Example 3: Controlling Constructor Selection with bsl::enable_if {#bslmf_enableif-example-3-controlling-constructor-selection-with-bsl-enable_if}
///
///
/// The final example demonstrates controlling the selection of a constructor
/// template in a class with (potentially) many constructors.  We define a
/// simple container template based on `std::vector` that illustrates a problem
/// that may occur when trying to call the constructor the user expects.  For
/// this example, assume we are trying to create a `vector<int>` with `42`
/// copies of the value `13`.  When we pass the literal values `42` and `13` to
/// the compiler, the "best" candidate constructor should be the template
/// constructor that takes two arguments of the same kind, deducing that type to
/// be `int`.  Unfortunately, that constructor expects those values to be of an
/// iterator type, forming a valid range.  We need to avoid calling this
/// constructor unless the deduced type really is an iterator, otherwise a
/// compile-error will occur trying to instantiate that constructor with an
/// incompatible argument type.  We use `bsl::enable_if` to create a deduction
/// context where SFINAE can kick in.  Note that we cannot deduce the `::type`
/// result of a meta-function, and there is no result type (as with a regular
/// function) to decorate, so we add an extra dummy argument using a pointer
/// type (produced from `bsl::enable_if::type`) with a default null argument:
/// @code
/// template<class t_TYPE>
/// class MyVector {
///     // This is a simple container implementation for demonstration purposes
///     // that is modeled after 'std::vector'.
///
///     // DATA
///     t_TYPE  *d_storage;
///     size_t   d_length;
///
///     // NOT IMPLEMENTED
///     MyVector(const MyVector&);
///     MyVector& operator=(const MyVector&);
///
///   public:
///     // CREATORS
///     MyVector(const t_TYPE& value, int n);
///         // Create a 'MyVector' object having the specified 'n' copies of
///         // the specified 'value'.  The behavior is undefined unless
///         // '0 <= n'.
///
///     template<class t_FORWARD_ITERATOR>
///     MyVector(t_FORWARD_ITERATOR first, t_FORWARD_ITERATOR last,
///                 typename bsl::enable_if<
///                    bsl::is_pointer<t_FORWARD_ITERATOR>::value>::type * = 0)
///         // Create a 'MyVector' object having the same sequence of values as
///         // found in the range described by the specified iterators
///         // '[first, last)'.  The behavior is undefined unless 'first' and
///         // 'last' refer to a sequence of values of the (template parameter)
///         // type 't_TYPE' where 'first' is at a position at or before
///         // 'last'.  Note that this function is currently defined inline to
///         // work around an issue with the Microsoft Visual Studio compiler.
///     {
///         d_length = 0;
///         for (t_FORWARD_ITERATOR cursor = first; cursor != last; ++cursor) {
///              ++d_length;
///         }
///
///         d_storage = new t_TYPE[d_length];
///         for (size_t i = 0; i != d_length; ++i) {
///              d_storage[i] = *first;
///              ++first;
///         }
///     }
///
///     ~MyVector();
///         // Destroy this container and all of its elements, reclaiming any
///         // allocated memory.
///
///     // ACCESSORS
///     const t_TYPE& operator[](int index) const;
///         // Return a reference providing non-modifiable access to the
///         // element held by this container at the specified 'index'.  The
///         // behavior is undefined unless 'index < size()'.
///
///     size_t size() const;
///         // Return the number of elements held by this container.
/// };
/// @endcode
/// Note that there is no easy test for whether a type is an iterator, so we
/// assume that any attempt to call a constructor with two arguments that are
/// not fundamental (such as `int`) must be passing iterators.  Now that we have
/// defined the class template, we implement its methods:
/// @code
/// template<class t_TYPE>
/// MyVector<t_TYPE>::MyVector(const t_TYPE& value, int n)
/// : d_storage(new t_TYPE[n])
/// , d_length(n)
/// {
///     for (int i = 0; i != n; ++i) {
///         d_storage[i] = value;
///     }
/// }
///
/// template<class t_TYPE>
/// MyVector<t_TYPE>::~MyVector()
/// {
///     delete[] d_storage;
/// }
///
/// // ACCESSORS
/// template<class t_TYPE>
/// const t_TYPE& MyVector<t_TYPE>::operator[](int index) const
/// {
///     return d_storage[index];
/// }
///
/// template<class t_TYPE>
/// size_t MyVector<t_TYPE>::size() const
/// {
///     return d_length;
/// }
/// @endcode
/// Finally, we demonstrate that the correct constructors are called when
/// invoked with appropriate arguments:
/// @code
/// void TestContainerConstructor()
/// {
///     const unsigned int TEST_DATA[] = { 1, 2, 3, 4, 5 };
///
///     const MyVector<unsigned int> x(&TEST_DATA[0], &TEST_DATA[5]);
///     const MyVector<unsigned int> y(13, 42);
///
///     assert(5 == x.size());
///     for (int i = 0; i != 5; ++i) {
///         assert(TEST_DATA[i] == x[i]);
///     }
///
///     assert(42 == y.size());
///     for (int i = 0; i != 42; ++i) {
///         assert(13 == y[i]);
///     }
/// }
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslmf
 * @{
 */
/** @addtogroup bslmf_enableif
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bsls_compilerfeatures.h>
 
namespace bsl {
 
                         // ================
                         // struct enable_if
                         // ================
 
/// This `struct` template implements the `enable_if` meta-function defined
/// in the C++11 standard [meta.trans.ptr].  This `struct` template provides
/// a `typedef` `type` that is an alias to the (template parameter) `t_TYPE`
/// if the (template parameter) `t_COND` is `true`; otherwise, `type` is not
/// provided.  If `t_TYPE` is not specified, it is set to `void`.  Note that
/// this generic default template provides `type` for when `t_COND` is
/// `true`; a template specialization is provided (below) that omits `type`
/// for when `t_COND` is `false`.
template <bool t_COND, class t_TYPE = void>
struct enable_if {
 
    /// This `typedef` is an alias to the (template parameter) `t_TYPE`.
    typedef t_TYPE type;
};
 
                      // ===============================
                      // struct enable_if<false, t_TYPE>
                      // ===============================
 
/// This partial specialization of `enable_if`, for when the (template
/// parameter) `t_COND` is `false`, guarantees that no `typedef` `type` is
/// supplied.  Note that this class definition is intentionally empty.
template <class t_TYPE>
struct enable_if<false, t_TYPE> {
};
 
#ifdef BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
 
// ALIASES
 
/// `enable_if_t` is an alias to the return type of the `bsl::enable_if`
/// meta-function.  Note, that the `enable_if_t` avoids the `::type` suffix
/// and `typename` prefix when we want to use the result of the
/// meta-function in templates.
template <bool t_COND, class t_TYPE = void>
using enable_if_t = typename enable_if<t_COND, t_TYPE>::type;
 
#endif  // BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
 
}  // close namespace bsl
 
 
 
namespace bslmf {
 
                               // ===============
                               // struct EnableIf
                               // ===============
 
/// This `struct` template implements a meta-function that provides a
/// `typedef` `type` that is an alias to the (template parameter) `t_TYPE`
/// if the (template parameter) `t_COND` is `true`; otherwise, `type` is not
/// provided.  If `t_TYPE` is not specified, it is set to `void`.  Note that
/// this generic default template provides `type` for when `t_COND` is
/// `true`; a template specialization is provided (below) that omits `type`
/// for when `t_COND` is `false`.
///
/// Also note that although this `struct` is functionally identical to
/// `bsl::enable_if`, the use of `bsl::enable_if` should be preferred.
template <bool t_COND, class t_TYPE = void>
struct EnableIf {
 
    /// This `typedef` is an alias to the (template parameter) `t_TYPE`.
    typedef t_TYPE type;
};
 
/// This partial specialization of `EnableIf`, for when the (template
/// parameter) `t_COND` is `false`, guarantees that no `typedef` `type` is
/// supplied.  Note that this class definition is intentionally empty.
template <class t_TYPE>
struct EnableIf<false, t_TYPE> {
};
 
}  // close package namespace
 
 
#ifndef BDE_OPENSOURCE_PUBLICATION  // BACKWARD_COMPATIBILITY
// ============================================================================
//                           BACKWARD COMPATIBILITY
// ============================================================================
 
#ifdef bslmf_EnableIf
#undef bslmf_EnableIf
#endif
/// This alias is defined for backward compatibility.
#define bslmf_EnableIf bslmf::EnableIf
#endif  // BDE_OPENSOURCE_PUBLICATION -- BACKWARD_COMPATIBILITY
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2013 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------
 
/** @} */
/** @} */
/** @} */