Code before first case in switch statement - c++

Question

Please note that this is a question about C++ language, not about how real or useful is the example I'm giving to illustrate it.
Imagine we have an enum in a namespace (or namespaces):
namespace SomeVeryLargeNamespaceExample {
enum class E {
One,
Two,
};
}
Now, we want to use it in the expression of a switch statement. Today I found that it is possible to add the using SomeVeryLargeNamespaceExample::E inside the switch statement, before the first case, reducing the code cluttering in the cases:
switch (e) {
using SomeVeryLargeNamespaceExample::E;
case E::One:
std::cout << "One\n";
break;
case E::Two:
std::cout << "Two\n";
break;
}
At first glance I thought it was some kind of "do this before any case statement" feature that I never learned about, but some expressions, such as a function call, are not executed (actually, gcc generates a warning statement will never be executed [-Wswitch-unreachable]). On the other hand, a variable declaration is possible (but not initialization).
My question is, what other statements are possible? Is this a feature or just a consequence of how switch is designed?
Note: I found this other question, but it is specific for C (so it doesn't mention the namespaces case, for example), and I'm curious about C++.

Answer 1

Any valid C++ statement is possible:
int a;
void foo(int x)
{
switch (x) {
a=4;
case 0:
a=1;
break;
case 1:
a=2;
break;
}
}
This is syntactically valid C++, and gcc has no issues compiling it and producing an executable. However the initial statement can never be reached, of course. Therefore every modern C++ compiler will give you a warning message:
t.C: In function ‘void foo(int)’:
t.C:8:18: warning: statement will
never be executed [-Wswitch-unreachable]
8 | a=4;
|