well c++ is has it's weird parts, I guess
This is not necessarily better, but you could also do something like
hana::template_<std::index_sequence>(hana::value(foo)...)
... I think I haven't actually tried it.
without hana literals
Plus it still wouldn't work with raw integrals.
@ricejasonf Regarding the issue that I get above with Clang (read of non-
constexpr variable), and that you didn’t get with Clang 3.8: I can indeed confirm that Clang trunk does not give an error. I think this is a bug, and it should error out. I don’t even know how it manages to compile it, and what code it generates… The compiler is very very smart if it can compile that.
I wonder if the C++ standard says anything about that.
BTW @MaikKlein http://boostorg.github.io/hana/index.html#tutorial-appendix-constexpr has info about using arguments inside constexpr functions.
#include <iostream>
struct Noisy
{
Noisy() { std::cout << "Noisy: constructed\n"; }
Noisy(const Noisy&) { std::cout << "Noisy: copy-constructed\n"; }
Noisy(Noisy&&) { std::cout << "Noisy: move-constructed\n"; }
~Noisy() { std::cout << "Noisy: destructed\n"; }
};
struct Silent { };
template<typename T>
constexpr int hello(T const) {
return 0;
}
int main() {
constexpr int x = hello(Silent{});
constexpr int y = hello(Noisy{});
}
only fails for
y
Yeah, if I add a constructor to Silent it explodes.
Clearly,
y cannot be evaluated at compile-time
I think that is material for a bug report.
#include <iostream>
struct Noisy {
Noisy() { std::cout << "Noisy: constructed\n"; }
Noisy(const Noisy&) { std::cout << "Noisy: copy-constructed\n"; }
Noisy(Noisy&&) { std::cout << "Noisy: move-constructed\n"; }
~Noisy() { std::cout << "Noisy: destructed\n"; }
};
struct Silent {
constexpr Silent() { }
};
template <typename T>
constexpr int f(T) { return 0; }
int main() {
Noisy t{}; // not constexpr
constexpr int result = f(t);
}
Uh, well that’s pretty much a copy-paste of what you wrote above.
Anyway, do you want to fill a bug report against Clang, or shall I do it%
I’ll give a link to the report when I’m done
But that copy constructor does not do anything, i.e. it does not read from the non-constexpr object.
So I’m thinking that the behavior is correct here.
But if you add a member to
Silent, it should error out. I’m going to try.
struct Foo {
Foo() : i{0} { }
constexpr Foo(Foo const& other) : i{other.i} { }
int i;
};
template <typename T>
constexpr int f(T) { return 0; }
int main() {
Foo t{};
constexpr int result = f(t);
}