Extracted BOOST_EXPLICIT_OPERATOR_BOOL macro from Boost.Log.

[SVN r85543]

doc/explicit_operator_bool.qbk

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+[/
+ / Copyright (c) 2013 Andrey Semashev
+ /
+ / Distributed under the Boost Software License, Version 1.0. (See accompanying
+ / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ /]
+
+[article BOOST_EXPLICIT_OPERATOR_BOOL and BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL
+    [quickbook 1.5]
+    [authors [Semashev, Andrey]]
+    [copyright 2013 Andrey Semashev]
+    [license
+        Distributed under the Boost Software License, Version 1.0.
+        (See accompanying file LICENSE_1_0.txt or copy at
+        [@http://www.boost.org/LICENSE_1_0.txt])
+    ]
+]
+
+[/===============]
+[section Overview]
+[/===============]
+
+`BOOST_EXPLICIT_OPERATOR_BOOL()` and `BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()` are compatibility helper macros that expand to an explicit conversion operator to `bool`. For compilers not supporting explicit conversion operators introduced in C++11 the macros expand to a conversion operator that implements the [@http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Safe_bool safe bool idiom]. In case if the compiler is not able to handle safe bool idiom well the macros expand to a regular conversion operator to `bool`.
+
+[endsect]
+
+
+[/===============]
+[section Examples]
+[/===============]
+
+Both macros are intended to be placed within a user's class definition. The generated conversion operators will be implemented in terms of `operator!()` that should be defined by user in this class. In case of `BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()` the generated conversion operator will be declared `constexpr` which requires the corresponding `operator!()` to also be `constexpr`.
+
+    template< typename T >
+    class my_ptr
+    {
+        T* m_p;
+
+    public:
+        BOOST_EXPLICIT_OPERATOR_BOOL()
+
+        bool operator!() const
+        {
+            return !m_p;
+        }
+    };
+
+Now `my_ptr` can be used in conditional expressions, similarly to a regular pointer:
+
+    my_ptr< int > p;
+    if (p)
+        std::cout << "true" << std::endl;
+
+[endsect]
+
+[/===============]
+[section History]
+[/===============]
+
+[heading boost 1.55]
+
+* The macro was extracted from Boost.Log.
+
+[endsect]
+
+
+
+

include/boost/utility/explicit_operator_bool.hpp

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+/*
+ *          Copyright Andrey Semashev 2007 - 2013.
+ * Distributed under the Boost Software License, Version 1.0.
+ *    (See accompanying file LICENSE_1_0.txt or copy at
+ *          http://www.boost.org/LICENSE_1_0.txt)
+ */
+/*!
+ * \file   explicit_operator_bool.hpp
+ * \author Andrey Semashev
+ * \date   08.03.2009
+ *
+ * This header defines a compatibility macro that implements an unspecified
+ * \c bool operator idiom, which is superseded with explicit conversion operators in
+ * C++11.
+ */
+
+#ifndef BOOST_UTILITY_EXPLICIT_OPERATOR_BOOL_HPP_INCLUDED_
+#define BOOST_UTILITY_EXPLICIT_OPERATOR_BOOL_HPP_INCLUDED_
+
+#include <boost/config.hpp>
+
+#ifdef BOOST_HAS_PRAGMA_ONCE
+#pragma once
+#endif
+
+#if !defined(BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS)
+
+/*!
+ * \brief The macro defines an explicit operator of conversion to \c bool
+ *
+ * The macro should be used inside the definition of a class that has to
+ * support the conversion. The class should also implement <tt>operator!</tt>,
+ * in terms of which the conversion operator will be implemented.
+ */
+#define BOOST_EXPLICIT_OPERATOR_BOOL()\
+    BOOST_FORCEINLINE explicit operator bool () const\
+    {\
+        return !this->operator! ();\
+    }
+
+/*!
+ * \brief The macro defines a constexpr explicit operator of conversion to \c bool
+ *
+ * The macro should be used inside the definition of a class that has to
+ * support the conversion. The class should also implement <tt>operator!</tt>,
+ * in terms of which the conversion operator will be implemented.
+ */
+#define BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()\
+    BOOST_FORCEINLINE BOOST_CONSTEXPR explicit operator bool () const\
+    {\
+        return !this->operator! ();\
+    }
+
+#else // !defined(BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS)
+
+#if (defined(__SUNPRO_CC) && (__SUNPRO_CC <= 0x530)) && !defined(BOOST_NO_COMPILER_CONFIG)
+// Sun C++ 5.3 can't handle the safe_bool idiom, so don't use it
+#define BOOST_NO_UNSPECIFIED_BOOL
+#endif // (defined(__SUNPRO_CC) && (__SUNPRO_CC <= 0x530)) && !defined(BOOST_NO_COMPILER_CONFIG)
+
+#if !defined(BOOST_NO_UNSPECIFIED_BOOL)
+
+namespace boost {
+
+namespace detail {
+
+#if !defined(_MSC_VER)
+
+    struct unspecified_bool
+    {
+        // NOTE TO THE USER: If you see this in error messages then you tried
+        // to apply an unsupported operator on the object that supports
+        // explicit conversion to bool.
+        struct OPERATORS_NOT_ALLOWED;
+        static void true_value(OPERATORS_NOT_ALLOWED*) {}
+    };
+    typedef void (*unspecified_bool_type)(unspecified_bool::OPERATORS_NOT_ALLOWED*);
+
+#else
+
+    // MSVC is too eager to convert pointer to function to void* even though it shouldn't
+    struct unspecified_bool
+    {
+        // NOTE TO THE USER: If you see this in error messages then you tried
+        // to apply an unsupported operator on the object that supports
+        // explicit conversion to bool.
+        struct OPERATORS_NOT_ALLOWED;
+        void true_value(OPERATORS_NOT_ALLOWED*) {}
+    };
+    typedef void (unspecified_bool::*unspecified_bool_type)(unspecified_bool::OPERATORS_NOT_ALLOWED*);
+
+#endif
+
+} // namespace detail
+
+} // namespace boost
+
+#define BOOST_EXPLICIT_OPERATOR_BOOL()\
+    BOOST_FORCEINLINE operator boost::detail::unspecified_bool_type () const\
+    {\
+        return (!this->operator! () ? &boost::detail::unspecified_bool::true_value : (boost::detail::unspecified_bool_type)0);\
+    }
+
+#define BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()\
+    BOOST_FORCEINLINE BOOST_CONSTEXPR operator boost::detail::unspecified_bool_type () const\
+    {\
+        return (!this->operator! () ? &boost::detail::unspecified_bool::true_value : (boost::detail::unspecified_bool_type)0);\
+    }
+
+#else // !defined(BOOST_NO_UNSPECIFIED_BOOL)
+
+#define BOOST_EXPLICIT_OPERATOR_BOOL()\
+    BOOST_FORCEINLINE operator bool () const\
+    {\
+        return !this->operator! ();\
+    }
+
+#define BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()\
+    BOOST_FORCEINLINE BOOST_CONSTEXPR operator bool () const\
+    {\
+        return !this->operator! ();\
+    }
+
+#endif // !defined(BOOST_NO_UNSPECIFIED_BOOL)
+
+#endif // !defined(BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS)
+
+#endif // BOOST_UTILITY_EXPLICIT_OPERATOR_BOOL_HPP_INCLUDED_

test/explicit_operator_bool.cpp

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+/*
+ *          Copyright Andrey Semashev 2007 - 2013.
+ * Distributed under the Boost Software License, Version 1.0.
+ *    (See accompanying file LICENSE_1_0.txt or copy at
+ *          http://www.boost.org/LICENSE_1_0.txt)
+ */
+/*!
+ * \file   explicit_operator_bool_compile.cpp
+ * \author Andrey Semashev
+ * \date   17.07.2010
+ *
+ * \brief  This test checks that explicit operator bool can be used in
+ *         the valid contexts.
+ */
+
+#define BOOST_TEST_MODULE explicit_operator_bool_compile
+
+#include <boost/utility/explicit_operator_bool.hpp>
+
+namespace {
+
+    // A test object that has the operator of explicit conversion to bool
+    struct checkable1
+    {
+        BOOST_EXPLICIT_OPERATOR_BOOL()
+        bool operator! () const
+        {
+            return false;
+        }
+    };
+
+    struct checkable2
+    {
+        BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()
+        BOOST_CONSTEXPR bool operator! () const
+        {
+            return false;
+        }
+    };
+
+} // namespace
+
+int main(int, char*[])
+{
+    checkable1 val1;
+    if (val1)
+    {
+        checkable2 val2;
+        if (val2)
+            return 0;
+    }
+
+    return 1;
+}

test/explicit_operator_bool_compile_fail_conv_int.cpp

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+/*
+ *          Copyright Andrey Semashev 2007 - 2013.
+ * Distributed under the Boost Software License, Version 1.0.
+ *    (See accompanying file LICENSE_1_0.txt or copy at
+ *          http://www.boost.org/LICENSE_1_0.txt)
+ */
+/*!
+ * \file   explicit_operator_bool_compile_fail_conv_int.cpp
+ * \author Andrey Semashev
+ * \date   17.07.2010
+ *
+ * \brief  This test checks that explicit operator bool cannot be used in
+ *         an unintended context.
+ */
+
+#define BOOST_TEST_MODULE explicit_operator_bool_compile_fail_conv_int
+
+#include <boost/utility/explicit_operator_bool.hpp>
+
+namespace {
+
+    // A test object that has the operator of explicit conversion to bool
+    struct checkable
+    {
+        BOOST_EXPLICIT_OPERATOR_BOOL()
+        bool operator! () const
+        {
+            return false;
+        }
+    };
+
+} // namespace
+
+int main(int, char*[])
+{
+    checkable val;
+    int n = val;
+
+    return 0;
+}

test/explicit_operator_bool_compile_fail_conv_pvoid.cpp

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+/*
+ *          Copyright Andrey Semashev 2007 - 2013.
+ * Distributed under the Boost Software License, Version 1.0.
+ *    (See accompanying file LICENSE_1_0.txt or copy at
+ *          http://www.boost.org/LICENSE_1_0.txt)
+ */
+/*!
+ * \file   explicit_operator_bool_compile_fail_conv_pvoid.cpp
+ * \author Andrey Semashev
+ * \date   17.07.2010
+ *
+ * \brief  This test checks that explicit operator bool cannot be used in
+ *         an unintended context.
+ */
+
+#define BOOST_TEST_MODULE explicit_operator_bool_compile_fail_conv_pvoid
+
+#include <boost/utility/explicit_operator_bool.hpp>
+
+namespace {
+
+    // A test object that has the operator of explicit conversion to bool
+    struct checkable
+    {
+        BOOST_EXPLICIT_OPERATOR_BOOL()
+        bool operator! () const
+        {
+            return false;
+        }
+    };
+
+} // namespace
+
+int main(int, char*[])
+{
+    checkable val;
+    void* p = val;
+
+    return 0;
+}

test/explicit_operator_bool_compile_fail_delete.cpp

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+/*
+ *          Copyright Andrey Semashev 2007 - 2013.
+ * Distributed under the Boost Software License, Version 1.0.
+ *    (See accompanying file LICENSE_1_0.txt or copy at
+ *          http://www.boost.org/LICENSE_1_0.txt)
+ */
+/*!
+ * \file   explicit_operator_bool_compile_fail_delete.cpp
+ * \author Andrey Semashev
+ * \date   17.07.2010
+ *
+ * \brief  This test checks that explicit operator bool cannot be used in
+ *         an unintended context.
+ */
+
+#define BOOST_TEST_MODULE util_explicit_operator_bool_delete
+
+#include <boost/utility/explicit_operator_bool.hpp>
+
+namespace {
+
+    // A test object that has the operator of explicit conversion to bool
+    struct checkable
+    {
+        BOOST_EXPLICIT_OPERATOR_BOOL()
+        bool operator! () const
+        {
+            return false;
+        }
+    };
+
+} // namespace
+
+int main(int, char*[])
+{
+    checkable val;
+    delete val;
+
+    return 0;
+}

test/explicit_operator_bool_compile_fail_shift.cpp

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+/*
+ *          Copyright Andrey Semashev 2007 - 2013.
+ * Distributed under the Boost Software License, Version 1.0.
+ *    (See accompanying file LICENSE_1_0.txt or copy at
+ *          http://www.boost.org/LICENSE_1_0.txt)
+ */
+/*!
+ * \file   explicit_operator_bool_compile_fail_shift.cpp
+ * \author Andrey Semashev
+ * \date   17.07.2010
+ *
+ * \brief  This test checks that explicit operator bool cannot be used in
+ *         an unintended context.
+ */
+
+#define BOOST_TEST_MODULE explicit_operator_bool_compile_fail_shift
+
+#include <boost/utility/explicit_operator_bool.hpp>
+
+namespace {
+
+    // A test object that has the operator of explicit conversion to bool
+    struct checkable
+    {
+        BOOST_EXPLICIT_OPERATOR_BOOL()
+        bool operator! () const
+        {
+            return false;
+        }
+    };
+
+} // namespace
+
+int main(int, char*[])
+{
+    checkable val;
+    val << 2;
+
+    return 0;
+}