CODEWITHSUNDEEP
c++c++11c++14
Lorah Attkins
Lorah Attkins

Reputation: 5856

On noexcept arguments

The use of noexcept was pretty clear to me, as the modern optimized way of marking functions with the no throw exception guarantee

struct A {
    A() noexcept; 
};

In item 14 of effective modern c++ I ecountered the following syntax, referred to as conditionally noexcept

template<class T, size_t N>
void swap(T (&a)[N], T (&b)[N]) noexcept(noexcept(swap(*a, *b)));

The way I get it, is that noexcept can introduce a truth value context, but then again how can another noexcept be an argument ?

Could someone elaborate on the syntax and semantics of this use of noexcept ?

Upvotes: 5

Views: 848

Answers (2)

Pradhan
Pradhan

Reputation: 16777

The keyword noexcept can be used in two contexts :

  1. The noexcept operator which takes an expression as an argument and returns a bool indicating whether or not the expression is non-throwing.
  2. The noexcept specifier which is used to specify whether a function throws or not. This form optionally takes one bool constant expression which determines whether the function is noexcept or not.

In the code you have pasted,

noexcept    ( noexcept(swap(*a, *b)))
^^^^^^^^      ^^^^^^^^
specifier     operator

Upvotes: 5

Jarod42
Jarod42

Reputation: 218098

With:

template<class T, size_t N>
void swap(T (&a)[N], T (&b)[N]) noexcept(noexcept(swap(*a, *b)));
                                  (1)      (2)

Upvotes: 10

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