unique_ptrでコンパイルエラーが出て、調べても分からないので質問させてください。

A.h

#include <memory>
class B;

class A {

    public:
        static std::unique_ptr<A> create();

    private:
        void init();
        std::unique_ptr<B> _b;

};

A.cpp

#include "A.h"
#include "B.h"
std::unique_ptr<A> A::create() {
    std::unique_ptr<A> ptr(new A);
    ptr->init();
    return ptr;
}

void A::init() {
    _b = B::create();
}

B.h

#include <memory>

class B {
    public:
    static std::unique_ptr<B> create();
};

B.cpp

#include "B.h"

std::unique_ptr<B> B::create() {
        std::unique_ptr<B> ptr(new B);
        return ptr;
}

main.cpp

#include "A.h"

int main(void) {
    auto a = A::create();
    return 0;
}

以下のようなエラーが出ます

    clang++ A.cpp B.cpp main.cpp -std=c++11
    In file included from main.cpp:1:
    In file included from ./A.h:1:
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/memory:2395:27: error: invalid application of 'sizeof' to an incomplete type 'B'
                            static_assert(sizeof(_Tp) > 0, "default_delete can not delete incomplete type");
                                                        ^~~~~~~~~~~
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/memory:2603:13: note: in instantiation of member function 'std::__1::default_delete<B>::operator()' requested here
                            __ptr_.second()(__tmp);
                            ^
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/memory:2571:46: note: in instantiation of member function 'std::__1::unique_ptr<B, std::__1::default_delete<B> >::reset' requested here
            _LIBCPP_INLINE_VISIBILITY ~unique_ptr() {reset();}
                                                                                             ^
    ./A.h:4:7: note: in instantiation of member function 'std::__1::unique_ptr<B, std::__1::default_delete<B> >::~unique_ptr' requested here
    class A {
                ^
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/memory:2603:13: note: in instantiation of member function 'std::__1::default_delete<A>::operator()' requested here
                            __ptr_.second()(__tmp);
                            ^
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/memory:2571:46: note: in instantiation of member function 'std::__1::unique_ptr<A, std::__1::default_delete<A> >::reset' requested here
            _LIBCPP_INLINE_VISIBILITY ~unique_ptr() {reset();}
                                                                                             ^
    main.cpp:4:12: note: in instantiation of member function 'std::__1::unique_ptr<A, std::__1::default_delete<A> >::~unique_ptr' requested here
        auto a = A::create();
                         ^
    ./A.h:2:7: note: forward declaration of 'B'
    class B;
                ^
    1 error generated.

A.hでBの前方宣言ではなく、include B.hをすればエラーは出なくなりますが、
こうするしかないのでしょうか？
shared_ptrに変更した場合にはエラーは出ません。

このエラーで調べると、Pimpleに関する話題しか出なくて困っています。

よろしくお願いします。