cmake: switch to c++-20 by default for the core library

I finally accept the incurred performance penalty from the position
independent DataMatrix detection by default.

This is finally fixing zxing-cpp#386. It also (sufficiently) fixes zxing-cpp#822 and thereby
supersedes zxing-cpp#836.

.github/workflows/ci.yml

@@ -74,7 +74,7 @@ jobs:
     - name: Configure
       run: >
         cmake -S . -B build -DCMAKE_BUILD_TYPE=${{env.BUILD_TYPE}} -DBUILD_SHARED_LIBS=ON
-        -DZXING_READERS=ON -DZXING_WRITERS=NEW -DZXING_EXPERIMENTAL_API=ON -DCMAKE_CXX_STANDARD=20
+        -DZXING_READERS=ON -DZXING_WRITERS=NEW -DZXING_EXPERIMENTAL_API=ON
         -DZXING_BLACKBOX_TESTS=ON -DZXING_UNIT_TESTS=OFF -DZXING_PYTHON_MODULE=OFF -DZXING_C_API=ON
 
     - name: Build

CMakeLists.txt

@@ -71,7 +71,9 @@ if(NOT ZXING_DEPENDENCIES IN_LIST ZXING_DEPENDENCIES_LIST)
 endif()
 
 if (NOT DEFINED CMAKE_CXX_STANDARD)
-    set (CMAKE_CXX_STANDARD 17)
+    set (CMAKE_CXX_STANDARD 20)
+    # Allow the fallback to earlier versions if the compiler does not support it.
+    set(CMAKE_CXX_STANDARD_REQUIRED OFF)
 endif()
 if (NOT DEFINED CMAKE_CXX_EXTENSIONS)
     set (CMAKE_CXX_EXTENSIONS OFF)

README.md

@@ -18,7 +18,7 @@ Thanks a lot for your contribution!
 
 ## Features
 
-* Written in pure C++17 (/C++20), no third-party dependencies (for the library itself)
+* Written in pure C++20 (/C++17), no third-party dependencies (for the library itself)
 * Thread safe
 * Wrappers/Bindings for:
   * [Android](wrappers/android/README.md)
@@ -47,7 +47,7 @@ Thanks a lot for your contribution!
 [Note:]
  * DataBar used to be called RSS.
  * DataBar, DX Film Edge, MaxiCode, Micro QR Code and rMQR Code are not supported for writing.
- * Building with C++20 (see [CMakeLists.txt](https://github.com/zxing-cpp/zxing-cpp/blob/d4b0f502775857f257d13efd25fb840ece1bca3e/CMakeLists.txt#L45)) changes the behaviour of the library: it then supports multi-symbol and position independent detection for DataMatrix. This comes at a noticable performace cost. C++20 is enabled by default for the Android, iOS, Python and WASM wrappers.
+ * Building with only C++17 (see [CMakeLists.txt](https://github.com/zxing-cpp/zxing-cpp/blob/d4b0f502775857f257d13efd25fb840ece1bca3e/CMakeLists.txt#L45)) changes the behaviour of the library: it then lacks supports multi-symbol and position independent detection for DataMatrix.
 
 ## Getting Started