bslmf_conditional.h

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/// @file bslmf_conditional.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslmf_conditional.h                                                -*-C++-*-
#ifndef INCLUDED_BSLMF_CONDITIONAL
#define INCLUDED_BSLMF_CONDITIONAL
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslmf_conditional bslmf_conditional
/// @brief  Provide a compile-time conditional type selector.
/// @addtogroup bsl
/// @{
/// @addtogroup bslmf
/// @{
/// @addtogroup bslmf_conditional
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslmf_conditional-purpose"> Purpose</a>
/// * <a href="#bslmf_conditional-classes"> Classes </a>
/// * <a href="#bslmf_conditional-description"> Description </a>
///   * <a href="#bslmf_conditional-usage"> Usage </a>
///     * <a href="#bslmf_conditional-example-1-conditionally-select-from-two-types"> Example 1: Conditionally Select From Two Types </a>
///
/// # Purpose {#bslmf_conditional-purpose}
/// Provide a compile-time conditional type selector.
///
/// # Classes {#bslmf_conditional-classes}
///
/// -  bsl::conditional: standard meta-function for conditional type selection
/// -  bsl::conditional_t: alias to the return type of the `bsl::conditional`
///
/// @see  bslmf_enableif
///
/// # Description {#bslmf_conditional-description}
/// This component defines a meta-function, `bsl::conditional`,
/// that may be used to conditionally select one of its two (template parameter)
/// types based on a `bool` (template parameter) value.
///
/// `bsl::conditional` meets the requirements of the `conditional` template
/// defined in the C++11 standard [meta.trans.other], providing a `typedef`
/// `type` that is an alias to the first (template parameter) type if the
/// (template parameter) value is `true`; otherwise, `type` is an alias to the
/// second (template parameter) type.
///
/// ## Usage {#bslmf_conditional-usage}
///
///
/// In this section we show intended use of this component.
///
/// ### Example 1: Conditionally Select From Two Types {#bslmf_conditional-example-1-conditionally-select-from-two-types}
///
///
/// Suppose that we want to select between two types based on a `bool` value.
///
/// Now, we use `bsl::conditional` to select between two types, `int` and
/// `char`, with a `bool` value.  When the `bool` is `true`, we select `int`;
/// otherwise, we select `char`.  We verify that our code behaves correctly by
/// asserting the result of the `bsl::conditional` with the expected type using
/// `bsl::is_same`:
/// @code
/// assert(true ==
///     (bsl::is_same<bsl::conditional<true,  int, char>::type, int >::value));
/// assert(true ==
///     (bsl::is_same<bsl::conditional<false, int, char>::type, char>::value));
/// @endcode
/// Finally, if the current compiler supports alias templates C++11 feature, we
/// select between two types using `bsl::conditional_t` and verify that our code
/// behaves correctly by asserting the result of the `bsl::conditional_t` with
/// the expected type using `bsl::is_same`:
/// @code
/// #ifdef BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
///   assert(true ==
///           (bsl::is_same<bsl::conditional_t<true,  int, char>, int >::value));
///   assert(true ==
///           (bsl::is_same<bsl::conditional_t<false, int, char>, char>::value));
/// #endif
/// @endcode
/// Note, that the `bsl::conditional_t` avoids the `::type` suffix and
/// `typename` prefix when we want to use the result of the `bsl::conditional`
/// meta-function in templates.
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslmf
 * @{
 */
/** @addtogroup bslmf_conditional
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bsls_compilerfeatures.h>
 
namespace bsl {
                           // ==================
                           // struct conditional
                           // ==================
 
/// This `struct` template implements the `conditional` meta-function
/// defined in the C++ standard [meta.trans.other], providing an alias,
/// `type`, that returns the result.  If the (template parameter) value
/// `t_COND` is `true`, then `type` has the same type as the (template
/// parameter) type `t_TRUE_TYPE`; otherwise, `type` has the same type as the (template parameter) type `t_FALSE_TYPE`.  
/// 
/// \note Note that   this generic
/// default template defines `type` to be an alias to `t_TRUE_TYPE` for when
/// `t_COND` is `true`.  A template specialization is provided (below)
/// handles the case for when `t_COND` is `false`.
///
/// See @ref bslmf_conditional 
template <bool t_COND, class t_TRUE_TYPE, class t_FALSE_TYPE>
struct conditional {
 
    /// This `typedef` is an alias to the (template parameter) type
    /// `t_TRUE_TYPE`.
    typedef t_TRUE_TYPE type;
};
 
/// This partial specialization of `bsl::conditional`, for when the
/// (template parameter) value `t_COND` is `false`, provides a `typedef`
/// `type` that is an alias to the (template parameter) type `t_FALSE_TYPE`.
template <class t_TRUE_TYPE, class t_FALSE_TYPE>
struct conditional<false, t_TRUE_TYPE, t_FALSE_TYPE> {
 
    /// This `typedef` is an alias to the (template parameter) type
    /// `t_FALSE_TYPE`.
    typedef t_FALSE_TYPE type;
};
 
#ifdef BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
 
// ALIASES
 
/// `conditional_t` is an alias to the return type of the `bsl::conditional`
/// meta-function.  Note, that the `conditional_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_TRUE_TYPE, class t_FALSE_TYPE>
using conditional_t =
                 typename conditional<t_COND, t_TRUE_TYPE, t_FALSE_TYPE>::type;
 
#endif  // BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
 
}  // close namespace bsl
 
#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 ----------------------------------
 
/** @} */
/** @} */
/** @} */