c++ - Why is basic_string::swap not noexcept? -

i found out 2 swap functions basic_string (member function , function in namespace std) not declared noexcept - neither in standard library of gcc-4.8 nor in recent c++ draft n3690.

on other hand, move constructor move assignment operator declared noexcept. shows should possible provide noexcept swap functions.

question: what's reason not having swap functions declared noexcept?

update: problem want use template function within own swap functions, uses static_assert check swap noexcept, e.g.:

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

however, works if swap functions declared noexcept, , that's not case basic_string.

paragraph 21.4.6.8 of c++11 standard specifies:

21.4.6.8 basic_string::swap [string::swap]

void swap(basic_string& s); 

1 postcondition: *this contains same sequence of characters in s, s contains same sequence of characters in *this.

2 throws: nothing.

3 complexity: constant time.

therefore, 1 must conclude absence of noexcept oversight.

another clue given paragraph 21.4.6.3 on assign() member function:

basic_string& assign(basic_string&& str) noexcept; 

effects: function replaces string controlled *this string of length str.size() elements copy of string controlled str. [ note: valid implementation swap(str). — end note ]

3 returns: *this.

if swap(str) supposed valid implementation assign(), , assign() marked unconditionally noexcept, makes sense assume swap() noexcept well.