bslmf_isconst.h

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/// @file bslmf_isconst.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslmf_isconst.h                                                    -*-C++-*-
#ifndef INCLUDED_BSLMF_ISCONST
#define INCLUDED_BSLMF_ISCONST
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslmf_isconst bslmf_isconst
/// @brief  Provide a compile-time check for `const`-qualified types.
/// @addtogroup bsl
/// @{
/// @addtogroup bslmf
/// @{
/// @addtogroup bslmf_isconst
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslmf_isconst-purpose"> Purpose</a>
/// * <a href="#bslmf_isconst-classes"> Classes </a>
/// * <a href="#bslmf_isconst-description"> Description </a>
///   * <a href="#bslmf_isconst-usage"> Usage </a>
///     * <a href="#bslmf_isconst-example-1-verify-const-types"> Example 1: Verify const Types </a>
///
/// # Purpose {#bslmf_isconst-purpose}
/// Provide a compile-time check for `const`-qualified types.
///
/// # Classes {#bslmf_isconst-classes}
///
/// -  bsl::is_const: meta-function for determining `const`-qualified types
/// -  bsl::is_const_v: the result value of the `bsl::is_const` meta-function
///
/// @see  bslmf_integralconstant
///
/// # Description {#bslmf_isconst-description}
/// This component defines a meta-function, `bsl::is_const` and a
/// template variable `bsl::is_const_v`, that represents the result value of the
/// `bsl::is_const` meta-function, that may be used to query whether a type is
/// `const`-qualified as defined in the C++ standard [basic.type.qualifier].
///
/// `bsl::is_const` meets the requirements of the `is_const` template defined in
/// the C++11 standard [meta.unary.prop].
///
/// Note that the template variable `is_const_v` is defined in the C++17
/// standard as an inline variable.  If the current compiler supports the inline
/// variable C++17 compiler feature, `bsl::is_const_v` is defined as an
/// `inline constexpr bool` variable.  Otherwise, if the compiler supports the
/// variable templates C++14 compiler feature, `bsl::is_const_v` is defined
/// as a non-inline `constexpr bool` variable.  See
/// `BSLS_COMPILERFEATURES_SUPPORT_INLINE_VARIABLES` and
/// `BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES` macros in
/// bsls_compilerfeatures component for details.
///
/// ## Usage {#bslmf_isconst-usage}
///
///
/// In this section we show intended use of this component.
///
/// ### Example 1: Verify const Types {#bslmf_isconst-example-1-verify-const-types}
///
///
/// Suppose that we want to assert whether a particular type is
/// `const`-qualified.
///
/// First, we create two `typedef`s -- a `const`-qualified type and an
/// unqualified type:
/// @code
/// typedef int        MyType;
/// typedef const int  MyConstType;
/// @endcode
/// Now, we instantiate the `bsl::is_const` template for each of the `typedef`s
/// and assert the `value` static data member of each instantiation:
/// @code
/// assert(false == bsl::is_const<MyType>::value);
/// assert(true  == bsl::is_const<MyConstType>::value);
/// @endcode
/// Note that if the current compiler supports the variable templates C++14
/// feature then we can re-write the snippet of code above using the
/// `bsl::is_const_v` variable as follows:
/// @code
/// #ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES
///   assert(false == bsl::is_const_v<MyType>);
///   assert(true  == bsl::is_const_v<MyConstType>);
/// #endif
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslmf
 * @{
 */
/** @addtogroup bslmf_isconst
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bslmf_integralconstant.h>
#include <bslmf_issame.h>
 
#include <bsls_compilerfeatures.h>
#include <bsls_keyword.h>
#include <bsls_platform.h>
 
#include <stddef.h>
 
#if (defined(BSLS_PLATFORM_CMP_MSVC) && BSLS_PLATFORM_CMP_VERSION < 1910)     \
 ||  defined(BSLS_PLATFORM_CMP_IBM)
// The Microsoft compiler does not recognize array-types as cv-qualified (when
// the element type is cv-qualified) when performing matching for partial
// template specialization, but does get the correct result when performing
// overload resolution for functions (taking arrays by reference).  Given the
// function dispatch behavior being correct, we choose to work around this
// compiler bug, rather than try to report compiler behavior, as the compiler
// itself is inconsistent depending on how the trait might be used.  This also
// corresponds to how Microsft itself implements the trait in VC2010 and later.
// Last tested against VC 2015 (Release Candidate).
# define BSLMF_ISCONST_COMPILER_DOES_NOT_DETECT_CV_QUALIFIED_ARRAY_ELEMENT 1
#endif
 
namespace bsl {
 
                         // ===============
                         // struct is_const
                         // ===============
 
/// This `struct` template implements the `is_const` meta-function defined
/// in the C++11 standard [meta.unary.cat] to determine if the (template
/// parameter) `t_TYPE` is `const`-qualified.  This `struct` derives from
/// `bsl::true_type` if the `t_TYPE` is `const`-qualified, and
/// `bsl::false_type` otherwise.  Note that this generic default template
/// derives from `bsl::false_type`.  A template specialization is provided
/// (below) that derives from `bsl::true_type`.
template <class t_TYPE>
struct is_const : false_type {
};
 
                       // =============================
                       // struct is_const<t_TYPE const>
                       // =============================
 
#if defined(BSLS_PLATFORM_CMP_SUN) && BSLS_PLATFORM_CMP_VERSION < 0x5130
template <class t_TYPE>
struct is_const<const t_TYPE>
: integral_constant<bool, !is_same<t_TYPE, const t_TYPE>::value> {
    // This partial specialization of 'is_const', for when the (template
    // parameter) 't_TYPE' is 'const'-qualified, derives from 'bsl::true_type'.
    // Note that the Solaris CC compiler misdiagnoses cv-qualified "abominable"
    // function types as being cv-qualified themselves.  The correct result is
    // obtained by delegating the result to a call through 'is_same'.
};
#else
/// This partial specialization of `is_const`, for when the (template
/// parameter) `t_TYPE` is `const`-qualified, derives from `bsl::true_type`.
template <class t_TYPE>
struct is_const<const t_TYPE> : true_type {
};
#endif
 
 
#if defined(BSLMF_ISCONST_COMPILER_DOES_NOT_DETECT_CV_QUALIFIED_ARRAY_ELEMENT)
// The Microsoft compiler does not recognize array-types as cv-qualified when
// the element type is cv-qualified when performing matching for partial
// template specialization, but does get the correct result when performing
// overload resolution for functions (taking arrays by reference).  Given the
// function dispatch behavior being correct, we choose to work around this
// compiler bug, rather than try to report compiler behavior, as the compiler
// itself is inconsistent depeoning on how the trait might be used.  This also
// corresponds to how Microsft itself implements the trait in VC2010 and later.
// Last tested against VC 2015 (Release Candidate).
 
template <class t_TYPE>
struct is_const<const t_TYPE[]> : true_type {
    // This partial specialization of 'is_const', for when the (template
    // parameter) 't_TYPE' is 'const'-qualified, derives from 'bsl::true_type'.
    // Note that this single specialization is sufficient to work around the
    // MSVC issue, even for multidimensional arrays.
};
 
template <class t_TYPE, size_t LENGTH>
struct is_const<const t_TYPE[LENGTH]> : true_type {
    // This partial specialization of 'is_const', for when the (template
    // parameter) 't_TYPE' is 'const'-qualified, derives from 'bsl::true_type'.
    // Note that this single specialization is sufficient to work around the
    // MSVC issue, even for multidimensional arrays.
};
#endif
 
#ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES
/// This template variable represents the result value of the
/// `bsl::is_const` meta-function.
template <class t_TYPE>
BSLS_KEYWORD_INLINE_VARIABLE constexpr bool is_const_v =
                                                       is_const<t_TYPE>::value;
#endif
 
}  // 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */