Test for building with different C++ standard versions

Instead of specifying C++14 as the standard in the main CMakeLists
file use whatever version the user specifies falling back to whatever
the compiler uses by default if not specified.

.github/workflows/unittest.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
  unittest:
- name: "${{matrix.generator}} ${{matrix.toolset}} Boost ${{matrix.boost_version}} ${{matrix.build_type}} ${{matrix.name_args}}"
+ name: "${{matrix.generator}} ${{matrix.toolset}} Boost ${{matrix.boost_version}} ${{matrix.build_type}} C++${{matrix.standard}} ${{matrix.name_args}}"
  runs-on: ${{matrix.os}}
  defaults:
  run:
@@ -16,6 +16,7 @@ jobs:
  os: [windows-2019, windows-2022]
  toolset: [v141, v142, v143, ClangCL]
  build_type: [Debug, Release]
+ standard: [14, 17, 20]
  generator: ["Visual Studio 16 2019", "Visual Studio 17 2022"]
  config_args: [""]
  build_args: [""]
@@ -123,6 +124,7 @@ jobs:
  run: |
  cmake -T "${{matrix.toolset}}" \
  -G "${{matrix.generator}}" \
+ -DCMAKE_CXX_STANDARD=${{matrix.standard}} \
  ${{matrix.config_args}} \
  ${BOOST_COMPILER_ARG} \
  "${GITHUB_WORKSPACE}"

CMakeLists.txt

@@ -19,6 +19,12 @@ elseif(CMAKE_SIZEOF_VOID_P EQUAL 4)
  message(STATUS "Building for 32 bit platform")
 endif()
 
+if(NOT "${CMAKE_CXX_STANDARD}")
+ set(CMAKE_CXX_STANDARD 14)
+endif()
+
+message(STATUS "C++ standard set to ${CMAKE_CXX_STANDARD}")
+
 option(ENABLE_TESTING "Enable Test Builds" ${WIN32})
 option(ENABLE_EXAMPLES "Enable Examples Builds" ${WIN32})
 option(ENABLE_DOCUMENTATION "Enable Documentation Builds" ${UNIX})
@@ -33,8 +39,6 @@ target_include_directories(${PROJECT_NAME}
  $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
 
-target_compile_features(${PROJECT_NAME} INTERFACE cxx_std_14)
-
 if(WIN32)
  set(Boost_USE_STATIC_LIBS ON)
  set(Boost_USE_MULTITHREADED ON)