According to the C++23 standard, what is the output of this program?
#include <iostream>
#include <stdexcept>
#include <type_traits>
struct S {
S() {
throw std::runtime_error("");
}
~S() {
throw std::runtime_error("");
}
};
int main() {
std::cout
<< (std::is_nothrow_constructible_v<S> ? 'y' : 'n')
<< (std::is_nothrow_destructible_v<S> ? 'y' : 'n');
}For the constructor: What's the default noexcept for normal functions?
For the destructor: Does S have any members with potentially-throwing destructors?
ny
Exception specifiers do not depend on whether the function can actually throw in practice. So the presence of throw in the constructor and destructor does not influence the exception specifiers.
For the constructor, the usual rules for exception specifiers apply. §[except.spec]¶3:
If a declaration of a function does not have a noexcept-specifier, the declaration has a potentially throwing exception specification unless it is a destructor or a deallocation function or is defaulted on its first declaration (...).
The constructor is not a destructor or a deallocation function, nor is it defaulted, so it is potentially-throwing.
For the destructor, there are special rules.
The exception specification for an implicitly-declared destructor, or a destructor without a noexcept-specifier, is potentially-throwing if and only if any of the destructors for any of its potentially constructed subobjects has a potentially-throwing exception specification or the destructor is virtual and the destructor of any virtual base class has a potentially-throwing exception specification.
Note that this rule also applies to our user-provided destructor! We don't provide a noexcept-specifier, and S does not have any subobjects (members or bases) with potentially-throwing destructors. So unlike the constructor, the destructor is non-throwing.