c++ - Does C++11 require allocators to be default constructible, libstdc++ and libc++ disagree? -

using modified version of howard hinnants's c++11 stack allocator documented here , here, std::basic_string , compiling gcc using libstdc++, following example (see live):

const unsigned int n = 200;  arena<n> a; short_alloc<char, n> ac(a) ;  std::basic_string<char,std::char_traits<char>,short_alloc<char, n>> empty(ac); 

gives following error(amongst others):

error: no matching function call 'short_alloc<char, 200ul>::short_alloc()'    if (__n == 0 && __a == _alloc())                        ^ 

however works without error when compiling clang , using libc++ (see live).

the stdlibc++ implementation of std::basic_string expects allocator have default constructor.

does c++11 require allocators default constructible? implementation correct?

no, c++11 not require allocator have default constructor, if @ draft c++11 standard section 17.6.3.5 [allocator.requirements] contains table 28 allocator requirements not contain requirement default constructor , later on in section minimal conforming interface provided:

[ example: following allocator class template supporting minimal interface satisfies requirements of table 28:

template <class tp> struct simpleallocator {     typedef tp value_type;     simpleallocator(ctor args );      template <class t> simpleallocator(const simpleallocator<t>& other);      tp *allocate(std::size_t n);     void deallocate(tp *p, std::size_t n); }; 

—end example ]

which not contain default constructor.

there libstdc++ bug report: basic_string assumes allocators default-constructible says:

the empty-string optimization of basic_string assumes allocators default constructible. while used case in c++98, no longer true in c++11, allocators allowed have state.

consider attached example program. compiling with

g++ -std=c++11 -c t.cpp 

produces error message, though should compile fine. problem the "_s_construct" calls "_alloc()", not exist.

note c++11 standard not require default constructors. (section 17.6.3.5, table 28). in particular, simpleallocator example section 17.6.3.5 trigger same bug, too.

and response was:

this hardly c++11 allocator requirement missing std::string, of new requirements missing, , unlikely implemented until switch non-cow string implementation.

this fixed of gcc 5.0:

fixed gcc 5 (when using new string abi)

we can confirm using gcc 5 on wandbox