CMake config: More modern way of defining the C++ version to use

We declare that we need to use C++17, but this can be overridden on
the cmake command line with "-DCMAKE_CXX_STANDARD=20" or so.

Fixes #1010.

Author: joto

.github/workflows/ci.yml

@@ -92,7 +92,6 @@ jobs:
       LUAJIT_OPTION: OFF
       POSTGRESQL_VERSION: 10
       POSTGIS_VERSION: 3
-      CPP_VERSION: 17
       BUILD_TYPE: Debug
 
     steps:
@@ -111,7 +110,6 @@ jobs:
       LUAJIT_OPTION: OFF
       POSTGRESQL_VERSION: 11
       POSTGIS_VERSION: 2.5
-      CPP_VERSION: 17
       BUILD_TYPE: Debug
 
     steps:
@@ -129,7 +127,6 @@ jobs:
       LUAJIT_OPTION: ON
       POSTGRESQL_VERSION: 12
       POSTGIS_VERSION: 2.5
-      CPP_VERSION: 17
       BUILD_TYPE: Debug
 
     steps:
@@ -148,7 +145,6 @@ jobs:
       LUAJIT_OPTION: ON
       POSTGRESQL_VERSION: 13
       POSTGIS_VERSION: 3
-      CPP_VERSION: 17
       BUILD_TYPE: Debug
 
     steps:
@@ -166,7 +162,6 @@ jobs:
       LUAJIT_OPTION: OFF
       POSTGRESQL_VERSION: 13
       POSTGIS_VERSION: 3
-      CPP_VERSION: 17
       USE_PROJ_LIB: 6
       BUILD_TYPE: Debug
 
@@ -185,7 +180,6 @@ jobs:
       LUAJIT_OPTION: OFF
       POSTGRESQL_VERSION: 13
       POSTGIS_VERSION: 3
-      CPP_VERSION: 17
       USE_PROJ_LIB: off
       BUILD_TYPE: Debug
 
@@ -204,7 +198,6 @@ jobs:
       LUAJIT_OPTION: OFF
       POSTGRESQL_VERSION: 14
       POSTGIS_VERSION: 3
-      CPP_VERSION: 17
       USE_PROJ_LIB: 6
       BUILD_TYPE: Debug
 
@@ -223,7 +216,6 @@ jobs:
       LUAJIT_OPTION: ON
       POSTGRESQL_VERSION: 13
       POSTGIS_VERSION: 2.5
-      CPP_VERSION: 17
       BUILD_TYPE: Release
 
     steps:
@@ -239,7 +231,6 @@ jobs:
       CXX: g++-10
       POSTGRESQL_VERSION: 13
       POSTGIS_VERSION: 2.5
-      CPP_VERSION: 17
       BUILD_TYPE: Release
 
     steps:
@@ -257,7 +248,6 @@ jobs:
       LUAJIT_OPTION: ON
       POSTGRESQL_VERSION: 14
       POSTGIS_VERSION: 3
-      CPP_VERSION: 17
       BUILD_TYPE: Debug
 
     steps:
@@ -275,7 +265,6 @@ jobs:
       LUAJIT_OPTION: OFF
       POSTGRESQL_VERSION: 14
       POSTGIS_VERSION: 3
-      CPP_VERSION: 17
       USE_PROJ_LIB: 6
       BUILD_TYPE: Debug
 
@@ -294,7 +283,6 @@ jobs:
       LUAJIT_OPTION: OFF
       POSTGRESQL_VERSION: 14
       POSTGIS_VERSION: 3
-      CPP_VERSION: 17
       USE_PROJ_LIB: off
       BUILD_TYPE: Debug
 
@@ -313,7 +301,6 @@ jobs:
       LUAJIT_OPTION: OFF
       POSTGRESQL_VERSION: 14
       POSTGIS_VERSION: 3
-      CPP_VERSION: 17
       USE_PROJ_LIB: 6
       BUILD_TYPE: Debug
 
@@ -333,7 +320,6 @@ jobs:
       LUAJIT_OPTION: ON
       POSTGRESQL_VERSION: 14
       POSTGIS_VERSION: 3
-      CPP_VERSION: 17
       BUILD_TYPE: Release
 
     steps:
@@ -350,7 +336,6 @@ jobs:
       EXTRA_FLAGS: -Wno-stringop-overread
       POSTGRESQL_VERSION: 14
       POSTGIS_VERSION: 3
-      CPP_VERSION: 17
       BUILD_TYPE: Release
 
     steps:

CMakeLists.txt

@@ -1,5 +1,5 @@
 
-cmake_minimum_required(VERSION 3.5.0)
+cmake_minimum_required(VERSION 3.8.0)
 
 project(osm2pgsql VERSION 1.8.1 LANGUAGES CXX C)
 
@@ -37,12 +37,8 @@ if (NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE RelWithDebInfo)
 endif()
 
-if (NOT "${CMAKE_CXX_STANDARD}")
-    set(CMAKE_CXX_STANDARD 17)
-endif()
-message(STATUS "Building in C++${CMAKE_CXX_STANDARD} mode")
+# We don't want to use special compiler extensions because we want portability
 set(CMAKE_CXX_EXTENSIONS OFF)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 if (MSVC)
     add_definitions(-D_CRT_SECURE_NO_WARNINGS -DNOMINMAX)

src/CMakeLists.txt

@@ -1,6 +1,10 @@
 
 add_library(osm2pgsql_lib STATIC)
 
+# Set the minimum required C++ version for the library and hence for all
+# binaries that use it.
+target_compile_features(osm2pgsql_lib PUBLIC cxx_std_17)
+
 target_sources(osm2pgsql_lib PRIVATE
   db-copy.cpp
   debug-output.cpp

tests/CMakeLists.txt

@@ -36,6 +36,7 @@ add_definitions(-DTESTDATA_DIR=\"${osm2pgsql_SOURCE_DIR}/tests/data/\")
 add_definitions(-DOSM2PGSQLDATA_DIR=\"${osm2pgsql_SOURCE_DIR}/\")
 
 add_library(catch_main_lib STATIC catch-main.cpp)
+target_compile_features(catch_main_lib PUBLIC cxx_std_17)
 
 set_test(test-check-input LABELS NoDB)
 set_test(test-db-copy-thread)