Why is basic_string::swap not noexcept?

Question

I just found out that the two swap functions for basic_string (member function and function in namespace std) are not declared with noexcept - neither in the standard library of GCC-4.8 nor in the most recent C++ draft N3690.

On the other hand, the move constructor as well as the move assignment operator are declared with noexcept . That shows that is should be possible to provide noexcept swap functions.

Question: What's the reason for not having the swap functions declared with noexcept ?

Update: The problem is that I want to use a template function within my own swap functions, that uses static_assert to check that the swap is actually noexcept , eg:

struct foo {
    bar_t bar;
    baz_t baz;
    void swap(foo& rhs) noexcept {
        swap_noexcept(bar, rhs.bar);
        swap_noexcept(baz, rhs.baz);
    }
};

However, that works only if the swap functions are declared with noexcept , and that's not the case for basic_string .

Answer 1

Paragraph 21.4.6.8 of the C++11 Standard specifies:

21.4.6.8 basic_string::swap [string::swap]

 void swap(basic_string& s); 

1 Postcondition : *this contains the same sequence of characters that was in s, s contains the same sequence of characters that was in *this.

2 Throws : Nothing .

3 Complexity : constant time.

Therefore, one must conclude the absence of noexcept is an oversight.

Another clue is given by paragraph 21.4.6.3 on the assign() member function:

 basic_string& assign(basic_string&& str) noexcept; 

Effects : The function replaces the string controlled by *this with a string of length str.size() whose elements are a copy of the string controlled by str . [ Note: A valid implementation is swap(str) . — end note ]

3 Returns : *this.

If swap(str) is supposed to be a valid implementation for assign() , and assign() is marked as unconditionally noexcept , then it makes sense to assume swap() to be noexcept as well.