+dnl -------------------------------------------------------------
+dnl We have so many templates in deal.II that sometimes we need
+dnl to make it clear with which types a template parameter can
+dnl be instantiated. There is a neat trick to do this: SFINAE
+dnl (substitution failure is not an error). The idea is this: the
+dnl C++ standard prescribes that a template function is only
+dnl considered in a call, if all parts of its signature can be
+dnl instantiated with the template parameter replaced by the
+dnl respective types/values in this particular call. Example:
+dnl template <typename T>
+dnl typename T::type foo(T) {...};
+dnl ...
+dnl foo(1);
+dnl
+dnl The compiler should detect that in this call, the template
+dnl parameter T must be identified with the type "int". However,
+dnl the return type T::type does not exist. The trick now is
+dnl that this is not considered an error: this template is simply
+dnl not considered, the compiler keeps on looking for another
+dnl possible function foo.
+dnl
+dnl That allows for a neat trick to rule out a template for certain
+dnl template arguments without changing the function signature at
+dnl all: Make the return type un-instantiable if the template
+dnl type presently considered for instantiation does not qualify.
+dnl An example of this is shown below.
+dnl
+dnl Unfortunately, older compilers do not support this trick: they
+dnl issue an error if the return type cannot be installed. In this
+dnl case, we back out and just drop the constraint and allow for
+dnl all template types. In that case, you'll simply get compiler
+dnl error when it tries to compile this template (in case it's in
+dnl the .h file), or linker errors for missing functions in case
+dnl it's in the .cc file and explicitly instantiated.
+dnl
+dnl Usage: DEAL_II_CHECK_SFINAE_BUG
+dnl
+dnl -------------------------------------------------------------
+AC_DEFUN(DEAL_II_CHECK_SFINAE_BUG, dnl
+[
+ AC_MSG_CHECKING(for SFINAE bug)
+ AC_LANG(C++)
+ CXXFLAGS="$CXXFLAGSG"
+ AC_TRY_COMPILE(
+ [
+ template <typename> struct int_or_double { static const bool value = false;};
+ template <> struct int_or_double<int> { static const bool value = true; };
+ template <> struct int_or_double<double> { static const bool value = true; };
+
+ template <bool, typename> struct constraint_and_return_value {};
+
+ template <typename T> struct constraint_and_return_value<true,T>
+ {
+ typedef T type;
+ };
+
+ // deduction for T=char should file, since return type cannot be
+ // instantiated...
+ template <typename T>
+ typename constraint_and_return_value<int_or_double<T>::value,void>::type
+ f (T);
+
+ // ...however, this is not an error. rather, the compiler should
+ // instead choose the following function:
+ void f(int);
+ ],
+ [
+ f('c');
+ ],
+ [
+ AC_MSG_RESULT(no)
+ ],
+ [
+ AC_MSG_RESULT(yes. disabling template constraints)
+ AC_DEFINE(DEAL_II_SFINAE_BUG, 1,
+ [Defined if the compiler does not support the
+ substitution-failure-is-not-an-error paradigm.
+ For the details, look at aclocal.m4 in the
+ top-level directory.])
+ ])
+])
+
+
+
dnl -------------------------------------------------------------
dnl The boost::shared_ptr class has a templated assignment operator
#! /bin/sh
-# From configure.in Revision: 1.141.2.1 .
+# From configure.in Revision: 1.142 .
# Guess values for system-dependent variables and create Makefiles.
# Generated by GNU Autoconf 2.57.
#
_ACEOF
+fi
+rm -f conftest.$ac_objext conftest.$ac_ext
+
+
+ echo "$as_me:$LINENO: checking for SFINAE bug" >&5
+echo $ECHO_N "checking for SFINAE bug... $ECHO_C" >&6
+ ac_ext=cc
+ac_cpp='$CXXCPP $CPPFLAGS'
+ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
+
+ CXXFLAGS="$CXXFLAGSG"
+ cat >conftest.$ac_ext <<_ACEOF
+#line $LINENO "configure"
+/* confdefs.h. */
+_ACEOF
+cat confdefs.h >>conftest.$ac_ext
+cat >>conftest.$ac_ext <<_ACEOF
+/* end confdefs.h. */
+
+ template <typename> struct int_or_double { static const bool value = false;};
+ template <> struct int_or_double<int> { static const bool value = true; };
+ template <> struct int_or_double<double> { static const bool value = true; };
+
+ template <bool, typename> struct constraint_and_return_value {};
+
+ template <typename T> struct constraint_and_return_value<true,T>
+ {
+ typedef T type;
+ };
+
+ // deduction for T=char should file, since return type cannot be
+ // instantiated...
+ template <typename T>
+ typename constraint_and_return_value<int_or_double<T>::value,void>::type
+ f (T);
+
+ // ...however, this is not an error. rather, the compiler should
+ // instead choose the following function:
+ void f(int);
+
+int
+main ()
+{
+
+ f('c');
+
+ ;
+ return 0;
+}
+_ACEOF
+rm -f conftest.$ac_objext
+if { (eval echo "$as_me:$LINENO: \"$ac_compile\"") >&5
+ (eval $ac_compile) 2>&5
+ ac_status=$?
+ echo "$as_me:$LINENO: \$? = $ac_status" >&5
+ (exit $ac_status); } &&
+ { ac_try='test -s conftest.$ac_objext'
+ { (eval echo "$as_me:$LINENO: \"$ac_try\"") >&5
+ (eval $ac_try) 2>&5
+ ac_status=$?
+ echo "$as_me:$LINENO: \$? = $ac_status" >&5
+ (exit $ac_status); }; }; then
+
+ echo "$as_me:$LINENO: result: no" >&5
+echo "${ECHO_T}no" >&6
+
+else
+ echo "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+
+ echo "$as_me:$LINENO: result: yes. disabling template constraints" >&5
+echo "${ECHO_T}yes. disabling template constraints" >&6
+
+cat >>confdefs.h <<\_ACEOF
+#define DEAL_II_SFINAE_BUG 1
+_ACEOF
+
+
fi
rm -f conftest.$ac_objext conftest.$ac_ext
projects / dealii-svn.git / commitdiff