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I'm setting up a new library for personal research purpose, and i'm trying to fully understand c++ standard library tools and core functionalities. Now i have a problem understanding the noexcept operator.

I wrote some test examples involving the noexcept operator and i'm puzzled with the result of the following assertion:

...
void no_throw() noexcept;

static_assert(noexcept(no_throw), "This should be true");
static_assert(noexcept((std::declval<decltype(no_throw)>())()), "this also should be true");
...

I expected this code to compile, but the second assertion pass only if using c++17 compile flag; i did run the test with gcc8.1 and clang5.0 . I've not tested with other compilers.

It fails with c++11 or c++14 flag. Could someone explain me why?

Thank you

Valeriop84
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1 Answers1

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Your first test, don't work: This pass

void no_throw() noexcept(true) {};
void do_throw() noexcept(false) {};

static_assert(noexcept(no_throw), "");
static_assert(noexcept(do_throw), "");

Should be : 

void no_throw() noexcept(true) {};
void do_throw() noexcept(false) {};

static_assert(noexcept(no_throw()), "");
static_assert(!noexcept(do_throw()), "");

Then why only in 17 :

Until 17 : The noexcept-specification is not a part of the function type source : https://en.cppreference.com/w/cpp/language/noexcept_spec

So if you run this code in 11 : 

template<class T>
   struct TD;

TD<decltype(no_throw)> e;
TD<decltype(std::declval<decltype(no_throw)>())> e2;

You got : 

prog.cc:14:24: error: aggregate 'TD<void()> e' has incomplete type and cannot be defined
 TD<decltype(no_throw)> e;
                        ^
prog.cc:15:50: error: aggregate 'TD<void (&)()> e2' has incomplete type and cannot be defined
 TD<decltype(std::declval<decltype(no_throw)>())> e2;

And in 17 :

prog.cc:14:24: error: aggregate 'TD<void() noexcept> e' has incomplete type and cannot be defined
 TD<decltype(no_throw)> e;
                        ^
prog.cc:15:50: error: aggregate 'TD<void (&)() noexcept> e2' has incomplete type and cannot be defined
 TD<decltype(std::declval<decltype(no_throw)>())> e2;

See, in 17 the noexcept is now in the type

Martin Morterol
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  • You are right about the compiler error, but it's just a parenthesis typo. I've already fixed this in my original post; problem is i do not understand why the assertion passes without adding the () after function declaration. – Valeriop84 May 04 '19 at 12:26
  • Well if it's an overload set, you can't use `decltype(no_throw)` :/ – Martin Morterol May 04 '19 at 12:26