I am generally using clang
to develop code, using all reasonable warnings I can (-Wall -Wextra [-Wpedantic]
). One of the nice things about this setup is that the compiler checks for the consistency of the switch
stataments in relation to the enumeration used. For example in this code:
enum class E{e1, e2};
int fun(E e){
switch(e){
case E::e1: return 11;
case E::e2: return 22; // if I forget this line, clang warns
}
}
clang
would complain (warn) if: I omit either the e1
or the e2
case, and there is no-default case.
<source>:4:12: warning: enumeration value 'e2' not handled in switch [-Wswitch]
switch(e){
This behavior is great because
default
case for which I wouldn't have a good thing to do.int
, it could be a type without a default constructor for example.(Note that I am using an enum class
so I assume only valid cases, as an invalid case can only be generated by a nasty cast on the callers end.)
Now the bad news: Unfortunately this breaks down quickly when switching to other compilers. In GCC and Intel (icc) the above code warns (using the same flags) that I am not returning from a non-void function.
<source>: In function 'int fun(E)':
<source>:11:1: warning: control reaches end of non-void function [-Wreturn-type]
11 | }
| ^
Compiler returned: 0
The only solution I found for this working to both have a default
case and return a non-sensical value.
int fun(E e){
switch(e){
case E::e1: return 11;
case E::e2: return 22;
default: return {}; // or int{} // needed by GCC and icc
}
}
This is bad because of the reasons I stated above (and not even getting to the case where the return type has no default constructor).
But it is also bad because I can forget again one of the enum cases and now clang
will not complain because there is a default case.
So what I ended up doing is to have this ugly code that works on these compilers and warns when it can for the right reasons.
enum E{e1, e2};
int fun(E e){
switch(e){
case E::e1: return 11;
case E::e2: return 22;
#ifndef __clang__
default: return {};
#endif
}
}
or
int fun(E e){
switch(e){
case E::e1: return 11;
case E::e2: return 22;
}
#ifndef __clang__
return {};
#endif
}
Is there a better way to do this?
This is the example: https://godbolt.org/z/h5_HAs
In the case on non-default constructible classes I am really out of good options completely:
A fun(E e){
switch(e){
case E::e1: return A{11};
case E::e2: return A{22};
}
#ifndef __clang__
return reinterpret_cast<A const&>(e); // :P, because return A{} could be invalid
#endif
}
https://godbolt.org/z/3WC5v8
It is important to note that, given your initial definition of fun
, it is entirely legal C++ to do the following:
fun(static_cast<E>(2));
Any enumeration type can assume any value within the number of bits of its representation. The representation for a type with an explicit underlying type (enum class
always has an underlying type; int
by default) is the entirety of that underlying type. Therefore, an enum class
by default can assume the value of any int
.
This is not undefined behavior in C++.
As such, GCC is well within its rights to assume that fun
may get any value within the range of its underlying type, rather than only one of its enumerators.
Standard C++ doesn't really have an answer for this. In an ideal world, C++ would have a contract system where you can declare up-front that fun
requires that the parameter e
be one of the enumerators. With that knowledge, GCC would know that the switch will take all control paths. Of course, even if C++20 had contracts (which is being retooled for C++23), there still isn't a way to test if an enum value only has values equal to one of its enumerators.
In a slightly less ideal world, C++ would have a way to explicitly tell the compiler that a piece of code is expected to be unreachable, and therefore the compiler can ignore the possibility of execution getting there. Unfortunately, that feature didn't make C++20 either.
So for the time being, you're stuck with compiler-specific alternatives.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With