Re: Supporting C++17

[Jonathan Wakely <gtkmm kayari org>]

On 4 April 2017 at 23:08, Chris Vine wrote:
> On second thoughts, noexcept(true) might possibly change ABI in C++17,
> but it seems inconceivable that noexcept(false) would.

Right.

You can verify this fairly easily:

https://godbolt.org/g/akduU4

Notice that the type of foo (as used in the type of f1) is the same
for c++98, c++14 and c++17.

The type of bar (as it shown in the type of f2, which godbolt's
demangler doesn't even support!) changes for c++17.

foo is a noexcept(false) function, and is the same whether it uses
throw(bad_cast) or noexcept(false).

bar is a noexcept(true) function, and is the same whether it uses
throw() or noexcept(true), but is not the same for c++17 comapred with
earlier versions.

But what makes a difference is -std=c++17, not whether you use
throw(...) or noexcept(...).

>  It also seems
> weird that merely applying -std=c++17 to your code should break its
> ABI.  The more I think about it, the less clear I am what making the
> noexcept specification part of the function type in C++17 actually
> involves compiler-wise, or what it achieves.

It allows overloading on whether a function can throw, and
specializing templates on it, and deducing it from template arguments.

So if you take the address of a function you don't lose it's
"noexceptiness". Previously this static assertion could never pass:

void f() noexcept;
template<typename F> void g(F f) {
    static_assert( noexcept( f() ), "callback won't throw" );
}

int main() {
    g( &f );
}

In C++17 it compiles.