Implement empty_value

include/boost/core/empty_value.hpp

@@ -0,0 +1,113 @@
+/*
+Copyright 2018 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+*/
+#ifndef BOOST_CORE_EMPTY_VALUE_HPP
+#define BOOST_CORE_EMPTY_VALUE_HPP
+
+#include <boost/config.hpp>
+#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
+#include <utility>
+#endif
+
+#if defined(BOOST_GCC_VERSION) && (BOOST_GCC_VERSION >= 40700)
+#define BOOST_DETAIL_EMPTY_VALUE_BASE
+#elif defined(BOOST_INTEL) && defined(_MSC_VER) && (_MSC_VER >= 1800)
+#define BOOST_DETAIL_EMPTY_VALUE_BASE
+#elif defined(BOOST_MSVC) && (BOOST_MSVC >= 1800)
+#define BOOST_DETAIL_EMPTY_VALUE_BASE
+#elif defined(BOOST_CLANG) && !defined(__CUDACC__)
+#if __has_feature(is_empty) && __has_feature(is_final)
+#define BOOST_DETAIL_EMPTY_VALUE_BASE
+#endif
+#endif
+
+namespace boost {
+
+template<class T>
+struct use_empty_value_base {
+    enum {
+#if defined(BOOST_DETAIL_EMPTY_VALUE_BASE)
+        value = __is_empty(T) && !__is_final(T)
+#else
+        value = false
+#endif
+    };
+};
+
+struct empty_init_t { };
+
+template<class T, unsigned = 0, bool = use_empty_value_base<T>::value>
+class empty_value {
+public:
+    empty_value()
+        : value_() { }
+
+#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
+#if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
+    template<class... Args>
+    empty_value(empty_init_t, Args&&... args)
+        : value_(std::forward<Args>(args)...) { }
+#else
+    template<class U>
+    empty_value(empty_init_t, U&& value)
+        : value_(std::forward<U>(value)) { }
+#endif
+#else
+    template<class U>
+    empty_value(empty_init_t, const U& value)
+        : value_(value) { }
+#endif
+
+    const T& get() const BOOST_NOEXCEPT {
+        return value_;
+    }
+
+    T& get() BOOST_NOEXCEPT {
+        return value_;
+    }
+
+private:
+    T value_;
+};
+
+#if !defined(BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION)
+template<class T, unsigned N>
+class empty_value<T, N, true>
+    : T {
+public:
+    empty_value()
+        : T() { }
+
+#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
+#if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
+    template<class... Args>
+    empty_value(empty_init_t, Args&&... args)
+        : T(std::forward<Args>(args)...) { }
+#else
+    template<class U>
+    empty_value(empty_init_t, U&& value)
+        : T(std::forward<U>(value)) { }
+#endif
+#else
+    template<class U>
+    empty_value(empty_init_t, const U& value)
+        : T(value) { }
+#endif
+
+    const T& get() const BOOST_NOEXCEPT {
+        return *this;
+    }
+
+    T& get() BOOST_NOEXCEPT {
+        return *this;
+    }
+};
+#endif
+
+} /* boost */
+
+#endif

test/Jamfile.v2

@@ -123,5 +123,8 @@ run to_address_test.cpp ;
 run exchange_test.cpp ;
 run exchange_move_test.cpp ;
 
+run empty_value_test.cpp ;
+run empty_value_size_test.cpp ;
+
 use-project /boost/core/swap : ./swap ;
 build-project ./swap ;

test/empty_value_size_test.cpp

@@ -0,0 +1,60 @@
+/*
+Copyright 2018 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+*/
+#include <boost/config.hpp>
+#if !defined(BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION)
+#include <boost/core/empty_value.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+struct T1 { };
+
+struct S1
+    : boost::empty_value<T1, 0, true> { };
+
+struct T2 {
+    int value;
+};
+
+struct S2
+    : boost::empty_value<T1, 0, true>
+    , boost::empty_value<T2, 1, true> { };
+
+struct S3
+    : boost::empty_value<T1, 0, false>
+    , boost::empty_value<T2, 1, true> { };
+
+struct T3 { };
+
+struct S4
+    : boost::empty_value<T1, 0, true>
+    , boost::empty_value<T3, 1, true> { };
+
+struct S5
+    : boost::empty_value<T1, 0, false>
+    , boost::empty_value<T3, 1, false> { };
+
+struct S6
+    : boost::empty_value<T1, 0, true>
+    , boost::empty_value<T2, 1, true>
+    , boost::empty_value<T3, 2, true> { };
+
+int main()
+{
+    BOOST_TEST(sizeof(S1) == sizeof(T1));
+    BOOST_TEST(sizeof(S2) == sizeof(T2));
+    BOOST_TEST(sizeof(S3) > sizeof(T2));
+    BOOST_TEST(sizeof(S4) == sizeof(T1));
+    BOOST_TEST(sizeof(S5) > sizeof(T1));
+    BOOST_TEST(sizeof(S6) == sizeof(T2));
+    return boost::report_errors();
+}
+#else
+int main()
+{
+    return 0;
+}
+#endif

test/empty_value_test.cpp

@@ -0,0 +1,76 @@
+/*
+Copyright 2018 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+*/
+#include <boost/core/empty_value.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+struct empty {
+    operator bool() const {
+        return false;
+    }
+    operator bool() {
+        return true;
+    }
+};
+
+class type {
+public:
+    type()
+        : value_(false) { }
+    explicit type(bool value)
+        : value_(value) { }
+    operator bool() const {
+        return value_;
+    }
+private:
+    bool value_;
+};
+
+void test_bool()
+{
+    const boost::empty_value<bool> v1(boost::empty_init_t(), true);
+    BOOST_TEST(v1.get());
+    boost::empty_value<bool> v2;
+    BOOST_TEST(!v2.get());
+    v2 = v1;
+    BOOST_TEST(v2.get());
+    v2.get() = false;
+    BOOST_TEST(!v2.get());
+}
+
+void test_empty()
+{
+    empty e;
+    const boost::empty_value<empty> v1(boost::empty_init_t(), e);
+    BOOST_TEST(!v1.get());
+    boost::empty_value<empty> v2;
+    BOOST_TEST(v2.get());
+    v2 = v1;
+    BOOST_TEST(v2.get());
+    v2.get() = empty();
+    BOOST_TEST(v2.get());
+}
+
+void test_type()
+{
+    const boost::empty_value<type> v1(boost::empty_init_t(), true);
+    BOOST_TEST(v1.get());
+    boost::empty_value<type> v2;
+    BOOST_TEST(!v2.get());
+    v2 = v1;
+    BOOST_TEST(v2.get());
+    v2.get() = type();
+    BOOST_TEST(!v2.get());
+}
+
+int main()
+{
+    test_bool();
+    test_empty();
+    test_type();
+    return boost::report_errors();
+}