include/GaudiKernel/compose.h

0001 /***********************************************************************************\
0002 * (c) Copyright 1998-2019 CERN for the benefit of the LHCb and ATLAS collaborations *
0003 *                                                                                   *
0004 * This software is distributed under the terms of the Apache version 2 licence,     *
0005 * copied verbatim in the file "LICENSE".                                            *
0006 *                                                                                   *
0007 * In applying this licence, CERN does not waive the privileges and immunities       *
0008 * granted to it by virtue of its status as an Intergovernmental Organization        *
0009 * or submit itself to any jurisdiction.                                             *
0010 \***********************************************************************************/
0011 #ifndef GAUDIKERNEL_COMPOSE_H
0012 #define GAUDIKERNEL_COMPOSE_H
0013
0014 #include <utility> // std::forward, std::move (objects)
0015
0016 namespace Gaudi {
0017
0018   namespace details {
0019
0020     // C++17 version: https://godbolt.org/g/YdcvGg
0021     template <typename... lambda_ts>
0022     struct overloaded_t : lambda_ts... {
0023       using lambda_ts::operator()...;
0024     };
0025     template <typename... lambda_ts>
0026     overloaded_t( lambda_ts... ) -> overloaded_t<lambda_ts...>;
0027
0028   } // namespace details
0029
0030   //
0031   // Create an object with an overloaded call operator by 'composing'/'joining'
0032   // a set of callables (such as lambdas)
0033   //
0034   // see eg. the example at http://en.cppreference.com/w/cpp/utility/variant/visit
0035   // for an example of why this is usefull
0036   //
0037   template <typename... lambda_ts>
0038   auto overload( lambda_ts&&... lambdas ) {
0039     return details::overloaded_t{ std::forward<lambda_ts>( lambdas )... };
0040   }
0041 } // namespace Gaudi
0042
0043 // for backwards compatibility
0044 // [[deprecated("please use Gaudi::overload instead of compose")]]
0045 template <typename... lambda_ts>
0046 auto compose( lambda_ts&&... lambdas ) {
0047   return Gaudi::overload( std::forward<lambda_ts>( lambdas )... );
0048 }
0049
0050 #endif