1 /* 2 * Copyright (c) 2011, 2013, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #ifndef SHARE_VM_UTILITIES_TRAITS_SELECT_BASE_CLASS_HPP 26 #define SHARE_VM_UTILITIES_TRAITS_SELECT_BASE_CLASS_HPP 27 28 // This trait is used for injecting optional classes in inheritance hierarchies 29 // if they have been defined. This trait may not be used in such a way that 30 // in some instantiations in the same compilation unit, the optional class is 31 // defined and in others it is not as that can cause inconsistent behaviour. 32 // The ::type of this trait will be the Optional type if it was defined 33 // and is a subtype of Required, otherwise it will be Required. 34 template <class Optional, class Required> 35 class SelectBaseClass { 36 typedef char yes[1]; 37 typedef char no[2]; 38 39 template<typename T> 40 static yes &check(Optional*, T); 41 static no &check(Required*, int); 42 43 struct OptionalCheckHost { 44 operator Required*() const; 45 operator Optional*(); 46 }; 47 48 template<bool optional_valid, typename R, typename O> 49 struct OptionalCheck { typedef O type; }; 50 51 template<typename R, typename O> 52 struct OptionalCheck<false, R, O> { typedef R type; }; 53 public: 54 typedef typename OptionalCheck<sizeof(check(OptionalCheckHost(), int())) == sizeof(yes), Required, Optional>::type type; 55 }; 56 57 #endif // SHARE_VM_UTILITIES_TRAITS_SELECT_BASE_CLASS_HPP 58