// bslmf_isclass.h                                                    -*-C++-*-
#ifndef INCLUDED_BSLMF_ISCLASS
#define INCLUDED_BSLMF_ISCLASS

#include <bsls_ident.h>
BSLS_IDENT("$Id: $")

//@PURPOSE: Provide a compile-time check for determining class types.
//
//@CLASSES:
//  bsl::is_class: standard meta-function for determining class types
//  bsl::is_class_v: the result value of the `bsl::is_class` meta-function
//  bslmf::IsClass: meta-function for determining class types
//
//@DESCRIPTION: This component defines two meta-functions, `bsl::is_class` and
// `BloombergLP::bslmf::IsClass` and a template variable `bsl::is_class_v`,
// that represents the result value of the `bsl::is_class` meta-function.  All
// these meta-functions may be used to query whether a type is a `class`,
// `struct`, or `union`, optionally qualified with `const` or `volatile`.
//
// `bsl::is_class` meets the requirements of the `is_class` template defined in
// the C++11 standard [meta.unary.cat], while `bslmf::IsClass` was devised
// before `is_class` was standardized.
//
// The two meta-functions are functionally equivalent.  The major difference
// between them is that the result for `bsl::is_class` is indicated by the
// class member `value`, while the result for `bslmf::IsClass` is indicated by
// the class member `value`.
//
// Note that `bsl::is_class` should be preferred over `bslmf::IsClass`, and in
// general, should be used by new components.
//
// Also note that the template variable `is_class_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_class_v` is defined as an
// `inline constexpr bool` variable.  Otherwise, if the compiler supports the
// variable templates C++14 compiler feature, `bsl::is_class_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
///-----
// In this section we show intended use of this component.
//
///Example 1: Verify Class Types
///- - - - - - - - - - - - - - -
// Suppose that we want to assert whether a set of types are class types.
//
// First, we create a class type `MyClass`:
// ```
// class MyClass
// {
// };
// ```
// Now, we instantiate the `bsl::is_class` template for both a non-class type
// and the defined type `MyClass`, and assert the `value` static data member of
// each instantiation:
// ```
// assert(false == bsl::is_class<int>::value);
// assert(true  == bsl::is_class<MyClass>::value);
// ```
// Note that if the current compiler supports the variable the templates C++14
// feature then we can re-write the snippet of code above using the
// `bsl::is_class_v` variable as follows:
// ```
// #ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES
//   assert(false == bsl::is_class_v<int>);
//   assert(true  == bsl::is_class_v<MyClass>);
// #endif
// ```

#include <bslscm_version.h>

#include <bslmf_integralconstant.h>
#include <bslmf_voidtype.h>

#include <bsls_compilerfeatures.h>
#include <bsls_keyword.h>

#ifndef BDE_DONT_ALLOW_TRANSITIVE_INCLUDES
#include <bslmf_removecv.h>
#include <cstdlib>
#endif

namespace BloombergLP {
namespace bslmf {

                             // ==================
                             // struct IsClass_Imp
                             // ==================

/// This `struct` template provides a meta-function to determine whether the
/// (template parameter) `t_TYPE` is a class type.
template <class t_TYPE, class = void>
struct IsClass_Imp : bsl::false_type {
};

/// This `struct` template provides a meta-function to determine whether the
/// (template parameter) `t_TYPE` is a class type.
template <class t_TYPE>
struct IsClass_Imp<t_TYPE, BSLMF_VOIDTYPE(int t_TYPE::*)> : bsl::true_type {
};

}  // close package namespace
}  // close enterprise namespace

namespace bsl {

                             // ===============
                             // struct is_class
                             // ===============

/// This `struct` template implements the `is_class` meta-function defined
/// in the C++11 standard [meta.unary.cat] to determine if the (template
/// parameter) `t_TYPE` is a class.  Note that for implementations without
/// native library support, this component will mis-diagnose `union` types
/// as classes.  This would be correct according to the core language
/// definition of a class type, but is an error according to the standard
/// library definition of this type trait.
template <class t_TYPE>
struct is_class : BloombergLP::bslmf::IsClass_Imp<t_TYPE>::type {
};

#ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES
/// This template variable represents the result value of the
/// `bsl::is_class` meta-function.
template <class t_TYPE>
BSLS_KEYWORD_INLINE_VARIABLE constexpr bool is_class_v =
                                                       is_class<t_TYPE>::value;
#endif

}  // close namespace bsl

namespace BloombergLP {
namespace bslmf {
                            // ==============
                            // struct IsClass
                            // ==============

/// This meta-function derives from `bsl::true_type` if the (template
/// parameter) `t_TYPE` is a class type, or a reference to a class type, and
/// from `bsl::false_type` otherwise.
///
/// Note that although this `struct` is functionally identical to
/// `bsl::is_class`, the use of `bsl::is_class` should be preferred.
template <class t_TYPE>
struct IsClass : bsl::is_class<t_TYPE>::type {
};

}  // close package namespace
}  // close enterprise namespace

#ifndef BDE_OPENSOURCE_PUBLICATION  // BACKWARD_COMPATIBILITY
// ============================================================================
//                           BACKWARD COMPATIBILITY
// ============================================================================

#ifdef bslmf_IsClass
#undef bslmf_IsClass
#endif
/// This alias is defined for backward compatibility.
#define bslmf_IsClass bslmf::IsClass
#endif  // BDE_OPENSOURCE_PUBLICATION -- BACKWARD_COMPATIBILITY

#endif

// ----------------------------------------------------------------------------
// Copyright 2019 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 ----------------------------------
