Fix: set CMAKE_CXX_STANDARD=20 only if not defined

Prevent static local in constructor to compile with C++17 too

Prevent use of spaceship operator if not available

NOTE: This project needs at least C++20!

Author: ClausKlein

.github/workflows/ci_tests.yml

@@ -49,22 +49,14 @@ jobs:
                     }
                   ]
                 },
-                { "cxxversions": ["c++23", "c++20", "c++17"],
+                { "cxxversions": ["c++23", "c++20"],
                   "tests": [{ "stdlibs": ["libstdc++"], "tests": ["Release.Default"]}]
                 }
               ]
             },
             { "versions": ["14", "13"],
               "tests": [
-                { "cxxversions": ["c++26", "c++23", "c++20", "c++17"],
-                  "tests": [{ "stdlibs": ["libstdc++"], "tests": ["Release.Default"]}]
-                }
-              ]
-            },
-            {
-              "versions": ["12", "11"],
-              "tests": [
-                { "cxxversions": ["c++23", "c++20", "c++17"],
+                { "cxxversions": ["c++26", "c++23", "c++20"],
                   "tests": [{ "stdlibs": ["libstdc++"], "tests": ["Release.Default"]}]
                 }
               ]
@@ -83,7 +75,7 @@ jobs:
                     }
                   ]
                 },
-                { "cxxversions": ["c++23", "c++20", "c++17"],
+                { "cxxversions": ["c++23", "c++20"],
                   "tests": [
                     {"stdlibs": ["libstdc++", "libc++"], "tests": ["Release.Default"]}
                   ]
@@ -92,7 +84,7 @@ jobs:
             },
             { "versions": ["20", "19", "18"],
               "tests": [
-                { "cxxversions": ["c++26", "c++23", "c++20", "c++17"],
+                { "cxxversions": ["c++26", "c++23", "c++20"],
                   "tests": [
                     {"stdlibs": ["libstdc++", "libc++"], "tests": ["Release.Default"]}
                   ]
@@ -101,10 +93,10 @@ jobs:
             },
             { "versions": ["17"],
               "tests": [
-                { "cxxversions": ["c++26", "c++23", "c++20", "c++17"],
+                { "cxxversions": ["c++26", "c++23", "c++20"],
                   "tests": [{"stdlibs": ["libc++"], "tests": ["Release.Default"]}]
                 },
-                { "cxxversions": ["c++20", "c++17"],
+                { "cxxversions": ["c++20"],
                   "tests": [{"stdlibs": ["libstdc++"], "tests": ["Release.Default"]}]
                 }
               ]
@@ -113,7 +105,7 @@ jobs:
           "appleclang": [
             { "versions": ["latest"],
               "tests": [
-                { "cxxversions": ["c++26", "c++23", "c++20", "c++17"],
+                { "cxxversions": ["c++26", "c++23", "c++20"],
                   "tests": [{ "stdlibs": ["libc++"], "tests": ["Release.Default"]}]
                 }
               ]

CMakeLists.txt

@@ -1,8 +1,10 @@
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-cmake_minimum_required(VERSION 3.25)
+cmake_minimum_required(VERSION 3.25...4.2)
 
-set(CMAKE_CXX_STANDARD 23)
+if(NOT DEFINED CMAKE_CXX_STANDARD)
+    set(CMAKE_CXX_STANDARD 20) # NEEDED! for std::type_identity, std::format, std::char8_t, ...!
+endif()
 
 project(
     beman.cstring_view # CMake Project Name, which is also the name of the top-level

CMakePresets.json

@@ -7,7 +7,6 @@
       "generator": "Ninja",
       "binaryDir": "${sourceDir}/build/${presetName}",
       "cacheVariables": {
-        "CMAKE_CXX_STANDARD": "20",
         "CMAKE_EXPORT_COMPILE_COMMANDS": "ON",
         "CMAKE_PROJECT_TOP_LEVEL_INCLUDES": "./infra/cmake/use-fetch-content.cmake"
       }

examples/example.cpp

@@ -7,6 +7,7 @@
 using namespace std::literals;
 using namespace beman::literals;
 
+#if __cpp_impl_three_way_comparison >= 201907L
 std::string_view to_string(std::strong_ordering order) {
     if (order == std::strong_ordering::equal) {
         return "equal";
@@ -21,6 +22,7 @@ std::string_view to_string(std::strong_ordering order) {
         return "internal error";
     }
 }
+#endif
 
 int main() {
     std::string         s  = "hello world";
@@ -39,7 +41,9 @@ int main() {
     std::cout << ("hello"_csv != "goodbye"sv) << "\n";
     std::cout << ("hello"_csv != "goodbye"_csv) << "\n";
     std::cout << (z0 == z1) << "\n";
+#if __cpp_impl_three_way_comparison >= 201907L
     std::cout << to_string(z0 <=> z1) << "\n";
+#endif
     std::cout << z0[z0.size()] * 1 << "\n";
     std::cout << z0.c_str() << "\n";
     std::cout << "\"" << empty << "\"\n";
@@ -61,7 +65,9 @@ int main() {
     std::cout << (L"hello"_csv != L"goodbye"sv) << "\n";
     std::cout << (L"hello"_csv != L"goodbye"_csv) << "\n";
     std::cout << (wz0 == wz1) << "\n";
+#if __cpp_impl_three_way_comparison >= 201907L
     std::cout << to_string(wz0 <=> wz1) << "\n";
+#endif
     std::cout << wz0[wz0.size()] * 1 << "\n";
     std::wcout << std::format(L"{}\n", wz0.c_str());
     std::wcout << std::format(L"\"{}\"\n", wempty);

include/beman/cstring_view/cstring_view.hpp

@@ -7,6 +7,7 @@
 #include <format>
 #include <ranges>
 #include <stdexcept>
+#include <type_traits> // for std::type_identity_t
 #include <string_view>
 #include <string>
 
@@ -31,9 +32,13 @@ constexpr bool operator==(basic_cstring_view<charT, traits>
                           std::type_identity_t<basic_cstring_view<charT, traits>> y) noexcept;
 
 template <class charT, class traits>
+
+#if __cpp_impl_three_way_comparison >= 201907L
 constexpr auto operator<=>(basic_cstring_view<charT, traits>                       x,
                            std::type_identity_t<basic_cstring_view<charT, traits>> y) noexcept;
 
+#endif
+
 // [cstring.view.io], inserters and extractors
 template <class charT, class traits>
 std::basic_ostream<charT, traits>& operator<<(std::basic_ostream<charT, traits>& os,
@@ -110,7 +115,7 @@ class basic_cstring_view {
 
     // [cstring.view.cons], construction and assignment
     constexpr basic_cstring_view() noexcept : size_() {
-        static const charT empty_string[1]{};
+        // XXX static const charT empty_string[1]{};
         data_ = std::data(empty_string);
     }
     constexpr basic_cstring_view(const basic_cstring_view&) noexcept            = default;
@@ -313,6 +318,8 @@ class basic_cstring_view {
   private:
     const_pointer data_; // exposition only
     size_type     size_; // exposition only
+
+    static constexpr charT empty_string[1]{}; // NOLINT
 };
 
 inline namespace literals {