|
770 | 770 | The definition of a constexpr function shall satisfy the following |
771 | 771 | requirements: |
772 | 772 | \begin{itemize} |
773 | | -\item |
774 | | -its return type (if any) shall be a literal type; |
775 | | - |
776 | | -\item |
777 | | -each of its parameter types shall be a literal type; |
778 | | - |
779 | 773 | \item |
780 | 774 | it shall not be a coroutine\iref{dcl.fct.def.coroutine}; |
781 | 775 |
|
|
816 | 810 | \end{codeblock} |
817 | 811 | \end{example} |
818 | 812 |
|
819 | | -\pnum |
820 | | -\indextext{specifier!\idxcode{constexpr}!constructor}% |
821 | | -The definition of a constexpr constructor |
822 | | -whose \grammarterm{function-body} is not \tcode{= delete} |
823 | | -shall additionally satisfy the following requirements: |
824 | | -\begin{itemize} |
825 | | - |
826 | | -\item |
827 | | -for a non-delegating constructor, every constructor selected to initialize non-static |
828 | | -data members and base class subobjects shall be a constexpr constructor; |
829 | | - |
830 | | -\item |
831 | | -for a delegating constructor, the target constructor shall be a constexpr |
832 | | -constructor. |
833 | | -\end{itemize} |
834 | | - |
835 | | -\begin{example} |
836 | | -\begin{codeblock} |
837 | | -struct Length { |
838 | | - constexpr explicit Length(int i = 0) : val(i) { } |
839 | | -private: |
840 | | - int val; |
841 | | -}; |
842 | | -\end{codeblock} |
843 | | -\end{example} |
844 | | - |
845 | | -\pnum |
846 | | -The definition of a constexpr destructor |
847 | | -whose \grammarterm{function-body} is not \tcode{= delete} |
848 | | -shall additionally satisfy the following requirement: |
849 | | -\begin{itemize} |
850 | | -\item |
851 | | - for every subobject of class type or |
852 | | - (possibly multi-dimensional) array thereof, |
853 | | - that class type shall have a constexpr destructor. |
854 | | -\end{itemize} |
855 | | - |
856 | | -\pnum |
857 | | -For a constexpr function or constexpr constructor |
858 | | -that is neither defaulted nor a template, |
859 | | -if no argument values exist such that |
860 | | -an invocation of the function or constructor could be an evaluated subexpression of a core |
861 | | -constant expression\iref{expr.const}, or, |
862 | | -for a constructor, an evaluated subexpression of |
863 | | -the initialization full-expression of some constant-initialized object\iref{basic.start.static}, |
864 | | -the program is ill-formed, no diagnostic required. |
865 | | -\begin{example} |
866 | | -\begin{codeblock} |
867 | | -constexpr int f(bool b) |
868 | | - { return b ? throw 0 : 0; } // OK |
869 | | -constexpr int f() { return f(true); } // ill-formed, no diagnostic required |
870 | | - |
871 | | -struct B { |
872 | | - constexpr B(int x) : i(0) { } // \tcode{x} is unused |
873 | | - int i; |
874 | | -}; |
875 | | - |
876 | | -int global; |
877 | | - |
878 | | -struct D : B { |
879 | | - constexpr D() : B(global) { } // ill-formed, no diagnostic required |
880 | | - // lvalue-to-rvalue conversion on non-constant \tcode{global} |
881 | | -}; |
882 | | - |
883 | | -constexpr int f(int x) { |
884 | | - static int n = x; |
885 | | - return n + x; // ill-formed, no diagnostic required |
886 | | - // all calls reach the static variable declaration |
887 | | -} |
888 | | -\end{codeblock} |
889 | | -\end{example} |
890 | | - |
891 | 813 | \pnum |
892 | 814 | If the instantiated template specialization of a constexpr function |
893 | 815 | template |
|
896 | 818 | function, |
897 | 819 | that specialization is still a constexpr function, |
898 | 820 | even though a call to such a function cannot appear in a constant |
899 | | -expression. If no specialization of the template would satisfy the |
900 | | -requirements for a constexpr function |
901 | | -when considered as a non-template function, the template is |
902 | | -ill-formed, no diagnostic required. |
| 821 | +expression. |
903 | 822 |
|
904 | 823 | \pnum |
905 | 824 | An invocation of a constexpr function in a given context |
|
919 | 838 | This can indirectly cause calls to \tcode{std::is_constant_evaluated} |
920 | 839 | within an invocation of the function to produce a different value. |
921 | 840 | \end{note} |
| 841 | +\begin{note} |
| 842 | +It is possible to write a constexpr function for which |
| 843 | +no invocation satisfies the requirements of a core constant expression. |
| 844 | +\end{note} |
922 | 845 |
|
923 | 846 | \pnum |
924 | 847 | The \keyword{constexpr} and \keyword{consteval} specifiers have no |
|
6249 | 6172 | \end{itemize} |
6250 | 6173 |
|
6251 | 6174 | \pnum |
6252 | | -An explicitly-defaulted function that is not defined as deleted may be declared |
6253 | | -\keyword{constexpr} or \keyword{consteval} only |
6254 | | -if it is constexpr-compatible\iref{special,class.compare.default}. |
6255 | 6175 | A function explicitly defaulted on its first declaration |
6256 | 6176 | is implicitly inline\iref{dcl.inline}, |
6257 | | -and is implicitly constexpr\iref{dcl.constexpr} if it is constexpr-compatible. |
| 6177 | +and is implicitly constexpr\iref{dcl.constexpr} |
| 6178 | +if it satisfies the requirements for a constexpr function. |
6258 | 6179 |
|
6259 | 6180 | \pnum |
6260 | 6181 | \begin{example} |
6261 | 6182 | \begin{codeblock} |
6262 | 6183 | struct S { |
6263 | | - constexpr S() = default; // error: implicit \tcode{S()} is not \keyword{constexpr} |
6264 | 6184 | S(int a = 0) = default; // error: default argument |
6265 | 6185 | void operator=(const S&) = default; // error: non-matching return type |
6266 | 6186 | ~S() noexcept(false) = default; // OK, despite mismatched exception specification |
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