c++11 std containers must construct value_type using allocator?

Frank Birbacher <bloodymir.crap@gmx.net>
Sun, 13 Jan 2013 14:24:10 CST
Hash: SHA1


I'm playing around with allocators in c++11 using clang+libcxx. I've
come across a problem in range-constructing a list from iterators over
a different value_type.

My central question: Is a std container allowed to construct any
instances of its value_type without using the construct method of the

I have the expectation that it should. Suppose the following
implementation of a std::list range constructor. If the
Iter::value_type differs from the list::value_type, may the list
construct a temporary list::value_type in the call to push_back? This
one would bypass the allocator.

template<typename T, typename Alloc>
template<typename Iter>
list<T, Alloc>::list(Iter first, Iter last, Alloc const& a)
    : alloc_instance(a)
    for(; first!=last; ++first)
        push_back(*first); //temporary without allocator

If using a list with a custom allocator that augments the construction
of the value_type (like scoped_allocator_adaptor) it follows that the
range Iter::value_type must be the same as the list::value_type. Any
convenience of conversions within the constructor are gone.

Complete example here:

#include <cstdlib>
#include <memory>
#include <scoped_allocator>
#include <string>
#include <list>

// custom allocator
// simple example using malloc/free:
template<typename T>
struct mallocfree
    /* Any allocator with state may have no
     * default constructor.
    mallocfree(char) {} // no default ctor

    // converting constructor:
    template<typename U>
    mallocfree(mallocfree<U> const& other) {}

    typedef T value_type;
    T* allocate(size_t const n)
        if(void* const p = std::malloc(sizeof(T)*n))
            return static_cast<T*>(p);
        throw std::bad_alloc();
    void deallocate(T* const p, size_t) noexcept

template<typename T>
inline constexpr bool operator == (
        mallocfree<T> const&,
        mallocfree<T> const&
{ return true; }

// Typedefs for list<string> with alloc:
template<typename T>
using my_alloc = std::scoped_allocator_adaptor<

typedef std::basic_string<

typedef std::list<

int main()
    auto inits = {"a", "b", "c"};
    // ctor fails on push_back:
    AugmentedList l(

Errors from clang 3.3 shows default ctor of mallocfree is requested
from a list constructor that uses push_back:

error: constructor for
'std::__1::__scoped_allocator_storage<mallocfree<char> >' must
explicitly initialize the base class 'mallocfree<char>' which does not
have a default constructor
     __scoped_allocator_storage() _NOEXCEPT {}
note: in instantiation of member function

::__scoped_allocator_storage' requested here

     scoped_allocator_adaptor() _NOEXCEPT {}
/opt/local/libexec/llvm-3.3/bin/../lib/c++/v1/list:1104:19: note: in
instantiation of member function 'std::__1::basic_string<char,
std::__1::scoped_allocator_adaptor<mallocfree<char> > >::basic_string'
requested here

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