I know that dozens of bugs have been opened for this over the years, but please
read this through.
If you have a C++ program missing a return statement from a function that is
supposed to return a value, g++ will compile it happily with no errors (or even
a warning, unless -Wreturn-type or -Wall is used). Trying to use the return
value from the function will most likely cause a segmentation fault. Dozens of
bugs have been opened for this over the years (e.g. bug 11474), all rejected as
invalid based on the following statement from section 6.6.3 of the C++
standard:
Flowing off the end of a function is equivalent to a return with no value;
this results in undefined behavior in a value-returning function.
However, the very same paragraph begins with:
A return statement without an expression can be used only in functions
that do not return a value, that is, a function with the return type void,
a constructor (12.1), or a destructor (12.4).
In my opinion, the standard contradicts itself. Either flowing off the end of a
function is "equivalent to a return with no value" (i.e. invalid according to
the beginning of the paragraph) or it is "undefined behaviour". It can't be
both.
Appendix C is informative rather than normative, but can help us understand the
intent of the standard. Section C.1.4 says:
Change: It is now invalid to return (explicitly or implicitly) from a
function which is declared to return a value without actually returning a
value.
Rationale: The caller and callee may assume fairly elaborate return-value
mechanisms for the return of class objects. If some flow paths execute a
return without specifying any value, the implementation must embody many more
complications. Besides, promising to return a value of a given type, and then
not returning such a value, has always been recognized to be a questionable
practice, tolerated only because very-old C had no distinction between void
functions and int functions.
The number of bugs opened for this issue over the years indicates how
surprising the current behaviour is. Since it is not always practical to enable
-Werror on large codebases, I think this should be an error by default.
--
Summary: "warning: no return statement in function returning non-
void" should be an error
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: david at rothlis dot net
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43943
