c++ - boost::serialization:具有私有(private)默认构造函数的对象在 vector 中工作，但在映射中不工作

Question

考虑以下代码:

#include <boost/serialization/nvp.hpp>
#include <boost/archive/xml_iarchive.hpp>
#include <boost/archive/xml_oarchive.hpp>

class Foo{
    friend class boost::serialization::access;
    template<class Archive>
    void serialize(Archive & ar, const unsigned int)
    {
        ar & BOOST_SERIALIZATION_NVP(i);
    }
    int i;
    Foo():i(0){}
public:
    Foo(int k):i(k){}
};

int main(int argc, char *argv[])
{
    std::vector< Foo> f;
    f.push_back(Foo(12));
    std::ofstream os("path");
    boost::archive::xml_oarchive oa(os);
    oa << boost::serialization::make_nvp("f", f);
    os.close();
    std::vector<Foo> g;
    std::ifstream is("path");
    boost::archive::xml_iarchive ia(is);
    ia >> boost::serialization::make_nvp("f", g);
}

在序列化 Foos vector 时效果很好。但是，如果我尝试序列化 Foos 的 map ，它会在私有(private)默认构造函数上失败:

std::map<std::string, Foo> f;
f.insert(std::make_pair("hello", Foo(12)));
std::ofstream os("path");
boost::archive::xml_oarchive oa(os);
oa << boost::serialization::make_nvp("f", f);
os.close();
std::map<std::string, Foo> g;
std::ifstream is("path");
boost::archive::xml_iarchive ia(is);
ia >> boost::serialization::make_nvp("f", g);

失败了

In file included from main.cpp:2:
In file included from /usr/local/include/boost/serialization/nvp.hpp:19:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/utility:70:
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_pair.h:109:18: error: field of type 'Foo' has private default constructor
      : first(), second() { }
                 ^
/usr/local/include/boost/serialization/access.hpp:132:17: note: in instantiation of member function 'std::pair<const std::basic_string<char>, Foo>::pair' requested here
        ::new(t)T;
                ^
/usr/local/include/boost/serialization/serialization.hpp:93:13: note: in instantiation of function template specialization 'boost::serialization::access::construct<std::pair<const std::basic_string<char>, Foo> >' requested here
    access::construct(t);
            ^
/usr/local/include/boost/serialization/serialization.hpp:158:9: note: in instantiation of function template specialization 'boost::serialization::load_construct_data<boost::archive::xml_iarchive, std::pair<const std::basic_string<char>, Foo> >' requested here
        load_construct_data(ar, t, v);
        ^
/usr/local/include/boost/serialization/detail/stack_constructor.hpp:58:31: note: in instantiation of function template specialization 'boost::serialization::load_construct_data_adl<boost::archive::xml_iarchive, std::pair<const std::basic_string<char>, Foo> >' requested here
        boost::serialization::load_construct_data_adl(
                              ^
/usr/local/include/boost/serialization/collections_load_imp.hpp:83:48: note: in instantiation of member function 'boost::serialization::detail::stack_construct<boost::archive::xml_iarchive, std::pair<const std::basic_string<char>, Foo> >::stack_construct' requested here
        detail::stack_construct<Archive, type> t(ar, v);
                                               ^
/usr/local/include/boost/serialization/collections_load_imp.hpp:158:16: note: (skipping 12 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
        hint = ifunc(ar, s, item_version, hint);
               ^
/usr/local/include/boost/archive/detail/common_iarchive.hpp:66:18: note: in instantiation of function template specialization 'boost::archive::load<boost::archive::xml_iarchive, std::map<std::basic_string<char>, Foo, std::less<std::basic_string<char> >, std::allocator<std::pair<const std::basic_string<char>, Foo> > > >' requested here
        archive::load(* this->This(), t);
                 ^
/usr/local/include/boost/archive/basic_xml_iarchive.hpp:86:39: note: in instantiation of function template specialization 'boost::archive::detail::common_iarchive<boost::archive::xml_iarchive>::load_override<std::map<std::basic_string<char>, Foo, std::less<std::basic_string<char> >, std::allocator<std::pair<const std::basic_string<char>, Foo> > > >' requested here
        this->detail_common_iarchive::load_override(t.value(), 0);
                                      ^
/usr/local/include/boost/archive/xml_iarchive.hpp:93:38: note: in instantiation of function template specialization 'boost::archive::basic_xml_iarchive<boost::archive::xml_iarchive>::load_override<std::map<std::basic_string<char>, Foo, std::less<std::basic_string<char> >, std::allocator<std::pair<const std::basic_string<char>, Foo> > > >' requested here
        basic_xml_iarchive<Archive>::load_override(t, 0);
                                     ^
/usr/local/include/boost/archive/detail/interface_iarchive.hpp:60:23: note: in instantiation of function template specialization 'boost::archive::xml_iarchive_impl<boost::archive::xml_iarchive>::load_override<const boost::serialization::nvp<std::map<std::basic_string<char>, Foo, std::less<std::basic_string<char> >, std::allocator<std::pair<const std::basic_string<char>, Foo> > > > >' requested here
        this->This()->load_override(t, 0);
                      ^
main.cpp:50:8: note: in instantiation of function template specialization 'boost::archive::detail::interface_iarchive<boost::archive::xml_iarchive>::operator>><const boost::serialization::nvp<std::map<std::basic_string<char>, Foo, std::less<std::basic_string<char> >, std::allocator<std::pair<const std::basic_string<char>, Foo> > > > >' requested here
    ia >> boost::serialization::make_nvp("f", g);
       ^
main.cpp:34:5: note: implicitly declared private here
    Foo():i(0){}
    ^

我正在使用 clangUbuntu clang 版本 3.4-1ubuntu3 (tags/RELEASE_34/final)(基于 LLVM 3.4)

和 Ubuntu 14.04LTS 附带的 boost 版本 1.55。

我试过如下提供 load_construct_data() 函数:

namespace boost
{
    namespace serialization
    {
        template<class Archive>
        inline void load_construct_data(Archive &archive, Foo*a, unsigned int
file_version)
        {
            ::new(a)Foo(0);
        }
    }
}

但我仍然遇到同样的错误，因为它在实例化 std::pair 时需要构造函数

Answer 1

哦。啊哈。

我刚刚用Boost 1.57.0 和map<string, Foo> 对比了一下情况.

嗯，你很幸运。您发现了另一个库版本依赖项(可能是错误)。

• 不使用它，但提供私有(private)默认构造函数，GCC 4.8.2 编译它就好了: Live On Coliru ^[1]

• 虽然 GCC 4.9.0 无法编译它(它也使用了更新版本的标准库)。 std::pair<>默认构造函数无法在那里编译，因为 Foo不是默认可构造的: Live On Coliru

解决方案

幸运的是 save_construct_data 的解决方案/load_construct_data再次挽救这一天。

但是，您需要注意元素类型实际上 不是Foo 的事实。 ，但是std::pair<T const, Foo> .

template <class Archive, typename K> inline friend void save_construct_data(Archive& ar, std::pair<K, Foo> const* v, const unsigned int) {
    std::cerr << __PRETTY_FUNCTION__ << "\n";
    ar & boost::serialization::make_nvp("first", v->first);
    ar & boost::serialization::make_nvp("second", v->second.i);
}
template <class Archive, typename K> inline friend void load_construct_data(Archive& ar, std::pair<K, Foo>* v, const unsigned int) {
    std::cerr << __PRETTY_FUNCTION__ << "\n";
    typename std::remove_cv<K>::type first;
    ar & boost::serialization::make_nvp("first", first);
    int tmp;
    ar & boost::serialization::make_nvp("second", tmp);
    new(v) std::pair<K, Foo>(first, tmp);
}

现在一切正常:

Live On Coliru

#include <boost/serialization/nvp.hpp>
#include <boost/serialization/serialization.hpp>
#include <boost/archive/xml_iarchive.hpp>
#include <boost/archive/xml_oarchive.hpp>
#include <boost/serialization/access.hpp>
#include <boost/serialization/vector.hpp>
#include <boost/serialization/map.hpp>
#include <boost/version.hpp>
#include <fstream>
#include <iostream>

class Foo {
    friend class boost::serialization::access;

    template <class Archive> void serialize(Archive &, const unsigned int) {
        std::cerr << __PRETTY_FUNCTION__ << "\n";
    }

    template <class Archive, typename K> inline friend void save_construct_data(Archive& ar, std::pair<K, Foo> const* v, const unsigned int) {
        std::cerr << __PRETTY_FUNCTION__ << "\n";
        ar & boost::serialization::make_nvp("first", v->first);
        ar & boost::serialization::make_nvp("second", v->second.i);
    }
    template <class Archive, typename K> inline friend void load_construct_data(Archive& ar, std::pair<K, Foo>* v, const unsigned int) {
        std::cerr << __PRETTY_FUNCTION__ << "\n";
        typename std::remove_cv<K>::type first;
        ar & boost::serialization::make_nvp("first", first);
        int tmp;
        ar & boost::serialization::make_nvp("second", tmp);
        new(v) std::pair<K, Foo>(first, tmp);
    }

    int i;

  public:
    Foo(int k) : i(k) {}

    friend std::ostream& operator<<(std::ostream& os, Foo const& foo) {
        return os << "Foo { " << foo.i << " }";
    }
};

namespace boost { namespace serialization {


} }

int main() {
    using Data = std::map<std::string, Foo>;
    std::cout << "Boost version: " << BOOST_VERSION << "\n";

    {
        auto f = Data { {"a", 12 }, {"b", 42} };
        //for (auto& e : f) std::cout << e.first << ", " << e.second << "\n";
        std::ofstream os("path");
        boost::archive::xml_oarchive oa(os);
        oa << boost::serialization::make_nvp("f", f);
    }

    {
        Data g;
        std::ifstream is("path");
        boost::archive::xml_iarchive ia(is);
        ia >> boost::serialization::make_nvp("f", g);

        for (auto& e : g)
            std::cout << e.first << ", " << e.second << "\n";
    }
}

打印:

Boost version: 105700
void save_construct_data(Archive&, const std::pair<K, Foo>*, unsigned int) [with Archive = boost::archive::xml_oarchive; K = const std::__cxx11::basic_string<char>]
void Foo::serialize(Archive&, unsigned int) [with Archive = boost::archive::xml_oarchive]
void save_construct_data(Archive&, const std::pair<K, Foo>*, unsigned int) [with Archive = boost::archive::xml_oarchive; K = const std::__cxx11::basic_string<char>]
void Foo::serialize(Archive&, unsigned int) [with Archive = boost::archive::xml_oarchive]
void load_construct_data(Archive&, std::pair<K, Foo>*, unsigned int) [with Archive = boost::archive::xml_iarchive; K = const std::__cxx11::basic_string<char>]
void Foo::serialize(Archive&, unsigned int) [with Archive = boost::archive::xml_iarchive]
void load_construct_data(Archive&, std::pair<K, Foo>*, unsigned int) [with Archive = boost::archive::xml_iarchive; K = const std::__cxx11::basic_string<char>]
void Foo::serialize(Archive&, unsigned int) [with Archive = boost::archive::xml_iarchive]
a, Foo { 12 }
b, Foo { 42 }

---

^[1](我无法在 Coliru 上链接它，因为那里的 boost 库已重新编译为 GCC 5.0 ABI)

注意事项

更好的通用解决方案是执行 load/save_construct_data boost::serialization 中非默认可构造类型的通用技巧命名空间。这样，人们就不必“知道” std::pair<>实现细节。他们可以实现 load/save_construct_data对于他们自己的用户类型，无论他们将其放在 vector 还是 map 中，它都会 JustWork™。

尽管一般地实现它并非易事，并且可能会干扰 Boost 序列化框架内部的一些其他机制。

我宁愿得到 Boost 序列化维护者的一些帮助，以可靠的方式做到这一点。所以，看来我今天要提交两张票。