[core] update to c++20 (organicmaps#7299)

* Update to C++20

Signed-off-by: Harry Bond <[email protected]>

* Update CMakeLists.txt

Signed-off-by: Alexander Borsuk <[email protected]>

---------

Signed-off-by: Harry Bond <[email protected]>
Signed-off-by: Alexander Borsuk <[email protected]>
Co-authored-by: Alexander Borsuk <[email protected]>

.clang-format

@@ -21,7 +21,7 @@ IndentCaseLabels: false
 NamespaceIndentation: None
 PointerAlignment: Middle
 SortIncludes: true
-Standard: c++17
+Standard: c++20
 IncludeBlocks: Regroup
 IncludeCategories:
 # Tests --------------------------------------------------------------------------------------------

CMakeLists.txt

@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 3.18)
 project(omim C CXX)
 
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_C_STANDARD 11)
 # Our code does not rely on gnu extensions.
 set(CMAKE_CXX_EXTENSIONS OFF)
@@ -26,7 +26,7 @@ if (APPLE AND NOT ("${CMAKE_SYSTEM_NAME}" STREQUAL Android))
   set(CMAKE_OBJC_VISIBILITY_PRESET hidden)
   enable_language(OBJCXX)
   set(CMAKE_OBJCXX_EXTENSIONS OFF)
-  set(CMAKE_OBJCXX_STANDARD 17)
+  set(CMAKE_OBJCXX_STANDARD 20)
   set(CMAKE_OBJCXX_FLAGS -fobjc-arc)
   set(CMAKE_OBJCXX_VISIBILITY_PRESET hidden)
 endif()
@@ -166,9 +166,9 @@ if (NJOBS)
   set(CMAKE_JOB_POOL_PRECOMPILE_HEADER custom)
 endif()
 
-# GCC 8.1 is required to support <charconv> header inclusion in base/string_utils.hpp, otherwise 7.0 is sufficient
-if (CMAKE_CXX_COMPILER_ID MATCHES "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 8.1)
-  message(FATAL_ERROR "Minimum supported g++ version is 8.1 yours is ${CMAKE_CXX_COMPILER_VERSION}")
+# GCC 10.0 is required to support <charconv> header inclusion in base/string_utils.hpp
+if (CMAKE_CXX_COMPILER_ID MATCHES "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 10.0)
+  message(FATAL_ERROR "Minimum supported g++ version is 10.0, yours is ${CMAKE_CXX_COMPILER_VERSION}")
 endif()
 
 if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")

cmake/OmimHelpers.cmake

@@ -173,8 +173,8 @@ function(add_precompiled_headers header pch_target_name)
   export_directory_flags("${pch_flags_file}")
   set(compiler_flags "@${pch_flags_file}")
 
-  # CMAKE_CXX_STANDARD 17 flags:
-  set(c_standard_flags "-std=c++17")
+  # CMAKE_CXX_STANDARD 20 flags:
+  set(c_standard_flags "-std=c++20")
   get_filename_component(pch_file_name ${header} NAME)
 
   add_pic_pch_target(${header} ${pch_target_name} ${pch_file_name} lib "-fPIC")

docs/CPP_STYLE.md

@@ -4,15 +4,15 @@ In general, [Google's coding standard](https://google.github.io/styleguide/cppgu
 
 Below are our specific (but not all!) exceptions to the Google's coding standard:
 
-- All C++ code should conform to the C++17 standard.
+- All C++ code should conform to the C++20 standard.
 - We use `.cpp` and `.hpp` files, not `.cc` and `.h` (`.c` and `.h` are used for C code), in UTF-8 encoding.
 - File names are lowercase with underscores, like `file_reader.cpp`.
 - We use `#pragma once` instead of the `#define` Guard in header files.
 - Includes are sorted and grouped by directory, there should be newlines between different directories.
 - Order of directories in includes: "current_dir/current_file.hpp", includes from other dirs sorted by dependencies (e.g. indexer, then coding, then base), "defines.hpp", C++ standard library headers, boost headers, 3party.
 - We ARE using C++ exceptions.
-- We are using all features of C++17 except the filesystem which is not fully supported on all platforms.
-- We try to limit the usage of boost libraries which require linking (and prefer C++17 types over their boost counterparts).
+- We are using all features of C++17 and C++20 except std::filesystem, std::to_chars & std::from_chars which are not fully supported on all platforms.
+- We try to limit the usage of boost libraries which require linking (and prefer C++20 types over their boost counterparts).
 
 Naming and formatting
 
@@ -195,7 +195,7 @@ v = w * (x + z);
 
 - If you see outdated code which can be improved, DO IT NOW (but in a separate pull request or commit)!
 - Your code should work at least on [mac|linux|android][x86|x86_64], [ios|android][x86|armv7|arm64] architectures
-- Your code should compile with C++17 compiler
+- Your code should compile with C++20 compiler
 - Avoid using any new 3party library if it is not fully tested and supported on all our platforms
 - Cover your code with unit tests. See examples for existing libraries
 - Check Base and Coding libraries for most of the basic functions