Require C++11 to build CMake itself

CMake can now compile as C++11 on all supported platforms.  Check that
std::unique_ptr is available and fail early if missing.  This will allow
us to use C++11 more broadly in CMake's implementation (previously it
was restricted to the serve mode implementation).

Co-Author: Daniel Pfeifer <daniel@pfeifer-mail.de>

CMakeLists.txt

@@ -80,6 +80,10 @@ if(NOT CMake_TEST_EXTERNAL_CMAKE)
 
   # check for available C++ features
   include(${CMake_SOURCE_DIR}/Source/Checks/cm_cxx_features.cmake)
+
+  if(NOT CMake_HAVE_CXX_UNIQUE_PTR)
+    message(FATAL_ERROR "The C++ compiler does not support C++11 (e.g. std::unique_ptr).")
+  endif()
 endif()
 
 # set the internal encoding of CMake to UTF-8

Help/dev/source.rst

@@ -23,20 +23,18 @@ format only a subset of files, such as those that are locally modified.
 C++ Subset Permitted
 ====================
 
-CMake supports compiling as C++98 in addition to C++11 and C++14.
-In order to support building on older toolchains some constructs
-need to be handled with care:
+CMake requires compiling as C++11 or above.  However, in order to support
+building on older toolchains some constructs need to be handled with care:
 
-* Use ``CM_AUTO_PTR`` instead of ``std::auto_ptr``.
+* Do not use ``CM_AUTO_PTR`` or ``std::auto_ptr``.
 
-  The ``std::auto_ptr`` template is deprecated in C++11.  We want to use it
-  so we can build on C++98 compilers but we do not want to turn off compiler
-  warnings about deprecated interfaces in general.  Use the ``CM_AUTO_PTR``
-  macro instead.
+  The ``std::auto_ptr`` template is deprecated in C++11.  The ``CM_AUTO_PTR``
+  macro remains leftover from C++98 support until its uses can be ported to
+  ``std::unique_ptr``.  Do not add new uses of the macro.
 
 * Use ``CM_EQ_DELETE;`` instead of ``= delete;``.
 
-  Defining functions as *deleted* is not supported in C++98.  Using
+  Older C++11 compilers do not support deleting functions.  Using
   ``CM_EQ_DELETE`` will delete the functions if the compiler supports it and
   give them no implementation otherwise.  Calling such a function will lead
   to compiler errors if the compiler supports *deleted* functions and linker

Help/release/dev/require-c++11.rst

+require-c++11
+-------------
+
+* Support for building CMake itself with C++98 compilers was dropped.
+  CMake is now implemented using C++11.

README.rst

@@ -51,7 +51,7 @@ Building CMake from Scratch
 UNIX/Mac OSX/MinGW/MSYS/Cygwin
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-You need to have a compiler and a make installed.
+You need to have a C++ compiler (supporting C++11) and a ``make`` installed.
 Run the ``bootstrap`` script you find in the source directory of CMake.
 You can use the ``--help`` option to see the supported options.
 You may use the ``--prefix=<install_prefix>`` option to specify a custom

bootstrap

@@ -1004,58 +1004,45 @@ fi
 # Check if C++ compiler works
 TMPFILE=`cmake_tmp_file`
 echo '
-#if defined(TEST1)
-# include <iostream>
-#else
-# include <iostream.h>
-#endif
+#include <iostream>
+#include <memory>
 
 #if __cplusplus >= 201103L && defined(__SUNPRO_CC) && __SUNPRO_CC < 0x5140
 #error "SunPro <= 5.13 C++ 11 mode not supported due to bug in move semantics."
 #endif
 
-class NeedCXX
+class Class
 {
 public:
-  NeedCXX() { this->Foo = 1; }
-  int GetFoo() { return this->Foo; }
+  int Get() const { return this->Member; }
 private:
-  int Foo;
+  int Member = 1;
 };
 int main()
 {
-  NeedCXX c;
-#ifdef TEST3
-  cout << c.GetFoo() << endl;
-#else
-  std::cout << c.GetFoo() << std::endl;
-#endif
+  auto const c = std::unique_ptr<Class>(new Class);
+  std::cout << c->Get() << std::endl;
   return 0;
 }
 ' > "${TMPFILE}.cxx"
-for a in ${cmake_cxx_compilers}; do
-  for b in 1 2 3; do
-    if [ -z "${cmake_cxx_compiler}" ] && \
-      cmake_try_run "${a}" "${cmake_cxx_flags} -DTEST${b}" "${TMPFILE}.cxx" >> cmake_bootstrap.log 2>&1; then
-      cmake_cxx_compiler="${a}"
-    fi
-  done
-done
-for std in 14 11 98; do
+for std in 17 14 11; do
   try_flags="`cmake_extract_standard_flags \"${cmake_toolchain}\" CXX \"${std}\"`"
-  for flag in $try_flags; do
-    echo "Checking for wheter ${cmake_cxx_flags} supports ${flag}" >> cmake_bootstrap.log 2>&1
-    if cmake_try_run "${cmake_cxx_compiler}" "${cmake_cxx_flags} ${flag} -DTEST1" \
-      "${TMPFILE}.cxx" >> cmake_bootstrap.log 2>&1; then
-      cmake_cxx_flags="${cmake_cxx_flags} ${flag} "
-      break 2
-    fi
+  for compiler in ${cmake_cxx_compilers}; do
+    for flag in '' $try_flags; do
+      echo "Checking whether '${compiler} ${cmake_cxx_flags} ${flag}' works." >> cmake_bootstrap.log 2>&1
+      if cmake_try_run "${compiler}" "${cmake_cxx_flags} ${flag}" \
+        "${TMPFILE}.cxx" >> cmake_bootstrap.log 2>&1; then
+        cmake_cxx_compiler="${compiler}"
+        cmake_cxx_flags="${cmake_cxx_flags} ${flag} "
+        break 3
+      fi
+    done
   done
 done
 rm -f "${TMPFILE}.cxx"
 
 if [ -z "${cmake_cxx_compiler}" ]; then
-  cmake_error 7 "Cannot find appropriate C++ compiler on this system.
+cmake_error 7 "Cannot find a C++ compiler supporting C++11 on this system.
 Please specify one using environment variable CXX.
 See cmake_bootstrap.log for compilers attempted."
 fi