bslmf_integralconstant.h

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/// @file bslmf_integralconstant.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslmf_integralconstant.h                                           -*-C++-*-
#ifndef INCLUDED_BSLMF_INTEGRALCONSTANT
#define INCLUDED_BSLMF_INTEGRALCONSTANT
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslmf_integralconstant bslmf_integralconstant
/// @brief  Provide a mapping from integral constants to unique types.
/// @addtogroup bsl
/// @{
/// @addtogroup bslmf
/// @{
/// @addtogroup bslmf_integralconstant
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslmf_integralconstant-purpose"> Purpose</a>
/// * <a href="#bslmf_integralconstant-classes"> Classes </a>
/// * <a href="#bslmf_integralconstant-description"> Description </a>
///   * <a href="#bslmf_integralconstant-usage"> Usage </a>
///     * <a href="#bslmf_integralconstant-example-1-compile-time-function-dispatching"> Example 1: Compile-Time Function Dispatching </a>
///     * <a href="#bslmf_integralconstant-example-2-base-class-for-metafunctions"> Example 2: Base Class For Metafunctions </a>
///
/// # Purpose {#bslmf_integralconstant-purpose}
/// Provide a mapping from integral constants to unique types.
///
/// # Classes {#bslmf_integralconstant-classes}
///
/// - bsl::integral_constant: A type representing a specific integer value
/// -     bsl::bool_constant: An alias template for `integral_constant<bool>`
/// -        bsl::false_type: `typedef` for `integral_constant<bool, false>`
/// -         bsl::true_type: `typedef` for `integral_constant<bool, true>`
///
/// @see 
///
/// # Description {#bslmf_integralconstant-description}
/// This component describes a simple class template,
/// `bsl::integral_constant`, that is used to map an integer constant to a C++
/// type.  `integral_constant<t_TYPE, t_VALUE>` generates a unique type for each
/// distinct compile-time integral `t_TYPE` and constant integer `t_VALUE`
/// parameter.  That is, instantiations with different integer types and values
/// form distinct types, so that `integral_constant<int, 0>` is a different type
/// from `integral_constant<int, 1>`, which is also distinct from
/// `integral_constant<unsigned, 1>`, and so on.  This mapping of integer values
/// to types allows for "overloading by value", i.e., multiple functions with
/// the same name can be overloaded on the "value" of an `integral_constant`
/// argument, provided that the value is known at compile-time.  The typedefs
/// `bsl::true_type` and `bsl::false_type` map the predicate values `true` and
/// `false` to C++ types that are frequently useful for compile-time algorithms.
///
/// ## Usage {#bslmf_integralconstant-usage}
///
///
/// This section illustrates intended usage of this component
///
/// ### Example 1: Compile-Time Function Dispatching {#bslmf_integralconstant-example-1-compile-time-function-dispatching}
///
///
/// The most common use of this structure is to perform compile-time function
/// dispatching based on a compile-time calculation.  Often the calculation is
/// nothing more than a simple predicate, allowing us to select one of two
/// functions based on whether the predicate holds.  The following function,
/// `doSomething`, uses a fast implementation (e.g., using `memcpy`) if the
/// parameterized type allows for such operations, otherwise it will use a more
/// generic and slower implementation (e.g., using the copy constructor).  This
/// example uses the types `true_type` and `false_type`, which are simple
/// typedefs for `integral_constant<bool, true>` and
/// `integral_constant<bool, false>`, respectively.
/// @code
///   #include <bslmf_integralconstant.h>
///
///   template <class t_T>
///   int doSomethingImp(t_T *t, bsl::true_type)
///   {
///       // slow, generic implementation
///       // ...
///       (void) t;
///       return 11;
///   }
///
///   template <class t_T>
///   int doSomethingImp(t_T *t, bsl::false_type)
///   {
///       // fast implementation that works only for some types of 't_T'
///       // ...
///       (void) t;
///       return 55;
///   }
///
///   template <bool IsSlow, class t_T>
///   int doSomething(t_T *t)
///   {
///       // Dispatch to an implementation depending on the (compile-time)
///       // value of 'IsSlow'.
///       return doSomethingImp(t, bsl::integral_constant<bool, IsSlow>());
///   }
/// @endcode
/// For some parameter types, the fast version of `doSomethingImp` is not
/// legal.  The power of this approach is that the compiler will not attempt
/// semantic analysis on the implementation that does not match the appropriate
/// `integral_constant` argument.
/// @code
///   int main()
///   {
///       int r;
///
///       int i;
///       r = doSomething<false>(&i);   // select fast version for int
///       assert(55 == r);
///
///       double m;
///       r = doSomething<true>(&m); // select slow version for double
///       assert(11 == r);
///
///       return 0;
///   }
/// @endcode
///
/// ### Example 2: Base Class For Metafunctions {#bslmf_integralconstant-example-2-base-class-for-metafunctions}
///
///
/// Hard-coding the value of an `integral_constant` is not especially useful.
/// Rather, `integral_constant` is typically used as the base class for
/// "metafunction" classes, classes that yield the value of compile-time
/// properties, including properties that are associated with types, rather
/// than with values.  For example, the following metafunction can be used at
/// compile time to determine whether a type is a floating point type:
/// @code
///   template <class t_TYPE> struct IsFloatingPoint  : bsl::false_type { };
///   template <> struct IsFloatingPoint<float>       : bsl::true_type { };
///   template <> struct IsFloatingPoint<double>      : bsl::true_type { };
///   template <> struct IsFloatingPoint<long double> : bsl::true_type { };
/// @endcode
/// The value `IsFloatingPoint<int>::value` is false and
/// `IsFloatingPoint<double>::value` is true.  The `integral_constant` base
/// class has a member type, `type`, that refers to itself and is inherited by
/// `IsFloatingPoint`.  Thus `IsFloatingPoint<float>::type` is `true_type` and
/// `IsFloatingPoint<char>::type` is `false_type`.  `IsFloatingPoint` is an a
/// member of a common category of metafunctions known as "type traits" because
/// they express certain properties (traits) of a type.  Using this
/// metafunction, we can rewrite the `doSomething` function from first example
/// so that it does not require the user to specify the `IsSlow` template
/// argument:
/// @code
///   template <class t_T>
///   int doSomething2(t_T *t)
///   {
///       // Automatically detect whether to use slow or fast imp.
///       const bool isSlow = IsFloatingPoint<t_T>::value;
///       return doSomethingImp(t, bsl::integral_constant<bool, isSlow>());
///   }
///
///   int main()
///   {
///       int r;
///
///       int i;
///       r = doSomething2(&i); // select fast version for int
///       assert(55 == r);
///
///       double m;
///       r = doSomething2(&m); // select slow version for double
///       assert(11 == r);
///
///       return 0;
///   }
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslmf
 * @{
 */
/** @addtogroup bslmf_integralconstant
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bsls_compilerfeatures.h>
#include <bsls_keyword.h>
 
#ifdef BSLS_COMPILERFEATURES_FULL_CPP11
# include <type_traits>
#endif // BSLS_COMPILERFEATURES_FULL_CPP11
 
#ifndef BDE_DONT_ALLOW_TRANSITIVE_INCLUDES
# include <bsls_nativestd.h>
#endif // BDE_DONT_ALLOW_TRANSITIVE_INCLUDES
 
#ifdef BSLS_COMPILERFEATURES_FULL_CPP11
namespace bsl {
 
                        // ================================
                        // class template integral_constant
                        // ================================
 
template <class t_TYPE, t_TYPE t_VALUE>
struct integral_constant : ::std::integral_constant<t_TYPE, t_VALUE> {
    // PUBLIC TYPES
    using type = integral_constant;
 
  public:
    // CREATORS
 
     integral_constant() = default;
     integral_constant(const integral_constant&) = default;
     integral_constant operator=(const integral_constant&) = default;
     ~integral_constant() = default;
 
    // ACCESSORS
 
    /// Return a copy of the template argument `t_VALUE`.
    constexpr t_TYPE operator()() const noexcept;
};
 
                        // ============================
                        // alias template bool_constant
                        // ============================
 
template <bool t_VALUE>
using bool_constant = integral_constant<bool, t_VALUE>;
 
                        // ===============
                        // type false_type
                        // ===============
 
using false_type = bool_constant<false>;
 
                        // ===============
                        // type true_type
                        // ===============
 
using true_type = bool_constant<true>;
 
}  // close namespace bsl
#else   // BSLS_COMPILERFEATURES_FULL_CPP11
namespace bsl {
 
                        // ================================
                        // class template integral_constant
                        // ================================
 
/// Generate a unique type for the given `t_TYPE` and `t_VALUE`.  This
/// `struct` is used for compile-time dispatch of overloaded functions and
/// as the base class for many metafunctions.
///
/// See @ref bslmf_integralconstant 
template <class t_TYPE, t_TYPE t_VALUE>
struct integral_constant {
    // PUBLIC TYPES
    typedef t_TYPE            value_type;
    typedef integral_constant type;
 
    // PUBLIC CLASS DATA
    static const t_TYPE value = t_VALUE;
 
  public:
    // CREATORS
 
     integral_constant() = default;
     integral_constant(const integral_constant&) = default;
     integral_constant operator=(const integral_constant&) = default;
     ~integral_constant() = default;
 
    // ACCESSORS
 
    /// Return a copy of the template argument `t_VALUE`.
    operator value_type() const;
 
    /// Return a copy of the template argument `t_VALUE`.
    value_type operator()() const;
};
 
                        // ===============
                        // type false_type
                        // ===============
 
typedef integral_constant<bool, false> false_type;
 
                        // ===============
                        // type true_type
                        // ===============
 
typedef integral_constant<bool, true> true_type;
 
}  // close namespace bsl
#endif // ! defined(BSLS_COMPILERFEATURES_FULL_CPP11)
 
// ============================================================================
//                      INLINE FUNCTION DEFINITIONS
// ============================================================================
 
// ACCESSORS
#ifdef BSLS_COMPILERFEATURES_FULL_CPP11
template <class t_TYPE, t_TYPE t_VALUE>
inline constexpr
t_TYPE bsl::integral_constant<t_TYPE, t_VALUE>::operator()() const noexcept
{
    return t_VALUE;
}
 
#else   // BSLS_COMPILERFEATURES_FULL_CPP11
template <class t_TYPE, t_TYPE t_VALUE>
inline
t_TYPE bsl::integral_constant<t_TYPE, t_VALUE>::operator()() const
{
    return t_VALUE;
}
 
template <class t_TYPE, t_TYPE t_VALUE>
inline
bsl::integral_constant<t_TYPE, t_VALUE>::operator t_TYPE() const
{
    return t_VALUE;
}
 
// STATIC MEMBER VARIABLE DEFINITIONS
template <class t_TYPE, t_TYPE t_VALUE>
const t_TYPE bsl::integral_constant<t_TYPE, t_VALUE>::value;
#endif // ! defined(BSLS_COMPILERFEATURES_FULL_CPP11)
 
#endif // ! defined(INCLUDED_BSLMF_INTEGRALCONSTANT)
 
// ----------------------------------------------------------------------------
// 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */